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Characterization of Nylon-12 as a water-equivalent solid phantom material for dosimetric measurements in therapeutic photon and electron beams
Journal Pre-proof Characterization of Nylon-12 as a water-equivalent solid phantom material for dosimetric measurements in therapeutic photon and electron beams Nicholas Ade, D. van Eeden, F.C.P. du Plessis PII:
S0969-8043(19)30247-7
DOI:
https://doi.org/10.1016/j.apradiso.2019.108919
Reference:
ARI 108919
To appear in:
Applied Radiation and Isotopes
Received Date: 6 March 2019 Revised Date:
26 August 2019
Accepted Date: 2 October 2019
Please cite this article as: Ade, N., van Eeden, D., du Plessis, F.C.P., Characterization of Nylon-12 as a water-equivalent solid phantom material for dosimetric measurements in therapeutic photon and electron beams, Applied Radiation and Isotopes (2019), doi: https://doi.org/10.1016/j.apradiso.2019.108919. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Characterization of Nylon-12 as a water-equivalent solid phantom material for dosimetric measurements in therapeutic photon and electron beams Nicholas Ade∗, D. van Eeden and F.C.P. du Plessis Medical Physics Department, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa ABSTRACT The tissue- or water-equivalence of dosimetry phantoms used as substitutes for water is essential for absorbed dose measurements in radiotherapy. At our institution, a heterogeneous pelvic phantom that consists of stacked Nylon-12 layers has recently been manufactured for Gafchromic film dosimetry. However, data on the use of Nylon as tissue-mimicking media for dosimetric applications are scarce. This study characterizes the water-equivalence of Nylon-12 for dosimetric measurements in therapeutic photon and electron beams. Employing an Elekta Synergy and SL25 linear accelerator (Linac), photon beam transmission measurements for 6 MV and 15 MV, acquired in narrow beam geometry with a 0.6 cm3 Farmer-type ion chamber showed that the mass attenuation coefficient µm of Nylon-12 agrees with the values of water, water-equivalent RW3 and Perspex phantom materials within 3%. For 6 MV, the µm values were 0.0477±0.002cm2/g, 0.0490±0.003cm2/g, 0.0482±0.001 cm2/g and 0.0479±0.002cm2/gfor Nylon-12, water, RW3, and Perspex, respectively. Differences within 2% were attained between depth dose data measured in Nylon-12 slabs with Gafchromic EBT3 films and in water with a Roos ion chamber for 10×10 cm2 6, 12 and 20 MeV electron beams produced by the Elekta Synergy and SL25 Linacs. Also, a good agreement within 2% was obtained between percent depth doses computed by DOSXYZnrc Monte Carlo simulations in water, Nylon-12 and RW3 materials for photon spectra between 250 kV and 15 MV. The discrepancies between the ratios of average, restricted stopping powers of Nylon to air and water to air for photon spectra ranging from 2–45 MV are typically within 1% signifying that Nylon and water have equivalent stopping power characteristics. This study highlights that Nylon-12 can be used as a tissue-mimicking phantom material for dosimetric measurements in clinical megavoltage photon and electron beams as it exhibits good water-equivalence. Key words: Nylon-12; Water-equivalence; Gafchromic film; Dosimetry; Photons; Electrons 1. Introduction High-energy electron (6–22 MeV) and photon (60Co, 4–25 MV) beams produced by teletherapy machines are commonly used in radiation therapy of cancer. Before radiation treatment, clinical dosimetry is performed, either through independent dose distribution calculations (e.g. with Monte Carlo simulations), or measurements (e.g. with dosimeters in sophisticated phantoms). Dosimetry deals with techniques for the quantitative determination of absorbed dose in a given medium by ionizing radiation using radiation dosimeters (Podgorsak, 2005). It represents one of physical procedures aimed at improving the accuracy of radiation dose delivery in radiotherapy. ∗
Corresponding author: Tel. +27 (0)51 405 2698; E-mail address: [email protected]
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In clinical dosimetry, measurements are frequently performed inside phantoms which simulate human soft tissue. Water is employed as a reference dosimetry phantom as it closely approximates the radiation absorption and scattering characteristics of muscles and other soft tissues (Khan, 2003; IAEA TRS-398, 2000). Tissue-equivalence describes the property of a material to respond to radiation in a similar manner as the human body. Water poses some practical problems such as unease of use so several plastic materials referred to as tissue- or water-equivalent materials (Ferreira et al, 2010; Jones et al, 2003;Khan, 2003) have been developed as dosimetry phantoms for clinical practice and this has been implemented in various dosimetry protocols (AAPM TG-21, 1983; AAPM TG-51, 1999; IAEA TRS-398, 2000). Table 1 lists various phantom materials including water and water-equivalent RW3 and their dosimetric properties which include mass density ρ and mass attenuation coefficient µm.
Table 1: Some dosimetric properties which include mass density (ρ) and mass attenuation coefficient (µm) of commonly used phantom materials. Values for the µm are for a 6 MV photon beam as reported by Kumar et al. (2010). Phantom material
ρ (g/cm3)
Water Solid water (WT1) RW3 Perspex Polystyrene
1.000 1.020 1.040+ 1.190 1.060
µm (×10-2 cm2/g) 4.940 4.800 4.767 4.796 4.780
+
Hill et al. (2008) reported a density value of 1.045 g/cm3 for RW3.
Although various parameters and quantities including µm, effective atomic number, effective electron density, stopping power ratios and depth dose data may be used to evaluate the waterequivalence of phantom materials, the attenuation coefficient is the major photon interaction parameter (Erk et al., 2016) that governs photon depth dose distribution with depth beyond dose maximum. At transient charged particle equilibrium conditions the variation of photon percent depth dose (PDD) with depth d beyond the depth of dose maximum dmax can be approximated by exponential attenuation e–<µ>.(d–dmax), where <µ> is the average attenuation coefficient for the heterogeneous beam and medium which takes into account the effect of beam quality (Khan, 2003). As <µ> decreases, the beam becomes more penetrating resulting in a higher PDD at any given depth beyond the build-up region. Unlike the attenuation coefficient which is the major photon interaction parameter used to characterize the tissue- or water-equivalence of a phantom material, stopping power ratios are important for electron beams (ICRU Report 44, 1989). Based on the Spencer-Attix formulation of the Bragg-Gray theory, the absorbed dose
D
med
at point P in a medium (dosimetry phantom)
measured with a detector (det) can, in general be expressed as:
2
D
med
=
L R . ρ
med
.P
(1) pert
det
where, R is the response of the detector (usually ionisation signal) corrected for various factors; med
L ‘det’ represents the radiation sensing material of the detector such as air. The parameter ρ
is
det
1
the ratio of the average, restricted mass collision stopping power of the medium to the detector material which accounts for the change in the electron fluence at the point of measurement due to the insertion of the detector. The perturbation correction factor Ppert, corrects for perturbation in the photon and electron energy fluences at the point of measurement due to the introduction of the detector material in the medium (Khan, 2003). That is, it accounts for the spatial and angular variation of the radiation fluence in the detector and the surrounding medium (Björk et al., 2000). According to Eq. (1), calculated dose data are critically dependent upon the choice of correct stopping power ratios, and these ratios are in turn dependent upon the spectrum of photons and electrons incident upon the dosimetry phantom (AAPM, 1983). Thus as a further step undertaken to evaluate the water-equivalence of Nylon-12, the stopping power ratios of Nylon-12 including those of various plastic materials relative to water are discussed. As pointed out by Borcia and Mihailescu (2008) data in literature indicate that there is no perfect agreement between depth dose distributions for any water substitute material. As such, it is essential to scale depths measured in non-water phantoms to water-equivalent depths. Also, the reading of an ionization chamber in the non-water phantom should be converted to an appropriate reading in water (IAEA TRS-398, 2000). Figure 1 shows a heterogeneous pelvic phantom that consists of stacked Nylon-12 layers manufactured for prosthetic-induced dose perturbation studies as reported by Ade and du Plessis(2017).Nylon-12 (polyamide-12, PA-12) is a polymer having a straight-chain structure with acid amide groups and 12 carbon atoms in its monomer unit (Griehl and Ruestem, 1970). It has the formula [(CH2)11CONH]n, where n depends upon the molecular weight. Ring-opening polymerization (a form of chain-growth polymerization) is the preferred route for commercial production according to equation 2: n[(CH2)11CONH] → [(CH2)11CONH]n.
(2)
Data on Nylon as tissue-mimicking media for dosimetric applications are sparse.The aim of this work is therefore to determinethe water-equivalence of Nylon-12 in clinical megavoltage photon and electron beams. The major advantage of using Nylon-12 as a phantom material over existing alternatives including Perspex and polystyrene is that Nylon and water have equivalent stopping
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Stopping power refers to the energy loss by electrons per unit path length of amaterial (Khan, 2003) 3
power characteristics (AAPM, 1983). Nylon-12 is also readily machinable so very sophistacated phantoms can be developed using 3D printing technology. 2. Materials and methods 2.1. Phantom description and measurements The phantom shown in Fig. 1 (which is made out of Nylon-12 layers and equipped with unilateral Ti implant) was 3D printed using additive manufacturing (AM) processes (Ghaffar et al., 2018; Murphy and Atala, 2014). A design of the phantom was first created and loaded on a 3D printing machine. EOSINT P385-(Plastics) and EOSINT M 280-(Metals) AM machines from EOS Electro Optical Systems− Germany were used. The phantom was “grown” using various powders (Nylon12 for the plastic layers and Ti for the metal implant). The powder is spread on a powder bed, and a laser then sinters (melts) the first layer of powder according to the data which has been programmed onto the machine from the design. This process is repeated until the design has developed into an actual product. The water-equivalence of Nylon-12 was measured in a 6 MV and 15 MV photon, and 6−20 MeV electron beams on an Elekta SL25 and Elekta Synergy linear accelerators. Layers of Nylon-12, each measuring 1 cm thick were used as phantom slabs. Central axis (CAX) transmission factors through slabs of Nylon-12 as well as slabs of Perspex and water-equivalent RW3 materials (Karaman et al, 2015; Kumar et al, 2010) were measured in narrow beam geometry to determine the mass attenuation coefficients µm of the plastic materials for comparison. A 0.6 cm3 Farmertype ion chamber connected to a PTW UNIDOS E electrometer was used for integrated charge measurements. The ion chamber was housed inside a Perspex block with 0.8 cm build-up. The block, containing the chamber, was placed on the floor at a source-to-chamber distance (SCD) of approximately 200 cm. Transmission data were acquired with the phantom slabs placed at a source-to-phantom-surface distance (SSD) of 100 cm for a set field of 2 × 2 cm2 defined at the SSD in Fig. 2. 300 monitor units (MUs) were set up for each irradiation. The transmission factors (Tx) were measured for different thicknesses of attenuating material ranging from 1 cm to 10 cm in steps of 1 cm. The linear attenuation coefficient (µ), in addition to the mass attenuation coefficient (µm = µ/ρ) were computed (Hill et al, 2008; Gray et al, 2009) according to equation 3.
R x / Ro = exp(− µ · x) = Tx
(3)
In Eq. (3), Rx is the transmitted signal through the material of thickness x (cm) and Ro is the open beam signal. The linear attenuation coefficient µ was then determined from a least-square fit of a linearized function through the transmission data points on a semi-log of Tx vs x graph. Density ρ values of 1.00 g/cm3, 1.01 g/cm3, 1.045 g/cm3 and 1.19 g/cm3 (Karaman et al, 2015; Hill et al, 2008; Kumar et al, 2010; Borcia and Mihailescu, 2008; Griehl and Ruestem, 1970) were used to determine the µm values of water, Nylon-12, RW3 and Perspex, respectively.
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A comparison was made between the PDD data measured with Gafchromic EBT3 films in Nylon12 slabs and another measured in water with a Roos ion chamber for 6, 12 and 20 MeV electron beams produced by an Elekta Synergy (6 and 12 MeV) and SL25 Linac (20 MeV). The measurements were acquired for an applicator defined field size of 10 × 10 cm2 at 95 cm SSD. Gafchromic EBT3 films were used for this study because of their good dosimetric properties which include: near-tissue equivalence (the effective atomic number of EBT3, Zeff = 6.84, closely matches Zeff of water = 7.3); they are practically energy independent in the megavoltage photon and electron energy ranges used clinically; and they have high spatial resolution (Arjomandy et al. 2010; Fiandra et al. 2006; Casanova Borca et al. 2013; Devic et al. 2005). In addition, Gafchromic EBT3 film is self-developing and unaffected by ambient light (Saur and Frengen, 2008). Batches of Gafchromic films used in this study were calibrated for conversion of optical density to absorbed dose through similar procedures described elsewhere (Ade and du Plessis, 2017). Films were scanned at a resolution of 50 dpi using an Epson Perfection V330 Photo flat-bed document scanner. The resulting images were digitised as raw 48-bit RGB (16 bits per colour channel) and stored in tagged-image-file format (TIFF). The16-bit depth red channel values were utilised in subsequent dosimetry analysis. Films were scanned 24 hours after irradiation to allow for nearcompletion of post exposure polymerization. 2.2. Monte Carlo-based DOSXYZnrc simulations 2.2.1. Depth dose simulations in water, RW3 and Nylon-12 slabs In addition to measurements, Monte Carlo-based DOSXYZnrc simulations were undertaken to compute PDD data in water, water-equivalent RW3 and Nylon-12 materials using photon spectra of 250 kV–15 MVfor a point source with rectangular collimation (Isource =3) (Ma et al., 2010). A PEGS file was generated for the different energies and contained the cross-section data that define the interactions taking place between X-rays and secondary electrons for Nylon-12 (PEGS4, 2017). Photon and electron cut-off parameters, PCUT and ECUT, were set to 0.010 MeV and 0.512 MeV, respectively, for all simulations. For the 250 kV simulations, spin effects were implemented to ensure accurate results with adequate backscattering. Rayleigh scattering, bound Compton scattering and atomic relaxation were also implemented as recommended for lowenergy (<1 MeV) simulations. The electron-step algorithm and boundary crossing algorithm (BCA) were PRESTA-II and EXACT, respectively. The skin depth for the BCA was set to three elastic mean free paths. For the simulations using photons in the megavoltage energy range, atomic relaxations, Rayleigh scattering and bound Compton scattering were not requested. Spin effects were implemented and the electron-step algorithm and BCA were PRESTA-II and PRESTA-I, respectively. The phantom model was 40 × 40 x 40 cm3 with voxel dimensions of 2 mm in all three directions. The SSD was 100 cm. The simulation geometry is shown in Fig. 3. For all the simulations, energy spectra available from the DOSXYZnrc directory were used. These spectra are measured and/or calculated at the NRC (National Research Council Canada) or
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taken from (Kosune and Rogers, 1993; NRC, 2019). The PDD data were computed for a 10 × 10 cm2 field size. 2.2.2.
DOSXYZnrc simulations with the heterogeneous pelvic phantom
Dosimetry was performed using the pelvic phantom (Fig. 1) made out of Nylon-12 slices to investigate its water-equivalence for a clinical case. Prior to DOSXYZnrc simulations, the pelvic phantom was CT-scanned as reported elsewhere (Ade et al., 2018) so that its DICOM images could be reconstructed and imported as an input phantom file for the Monte Carlo code. The phantom was converted to an .egsphant format by using an in-house, developed IDL program similar to CTCREATE (Ma et al., 2010) found in DOSXYZnrc software package. Two scenarios were simulated: (1) the pelvic phantom with the Nylon-12 layers as seen in Fig. 1; and (2) the pelvic phantom with the Nylon-12 layers replaced by water. The simulation geometry was similar to that reported by Ade et al.(2018). A 10 × 10 cm2 10 MV lateral beam at 100 cm SSD directed from the prosthesis side of the phantom was used. The phase space file for the 10 MV beam was generated with a source model of the Elekta Synergy Linac acquired from a previous study (Oderinde and du Plessis, 2017). A schematic of the simulation geometryis illustrated in Fig 4. Various simulation parameters including PCUT, ECUT, electron-step algorithm and BCA were implemented as defined in section 2.1.1. 3. Results and discussion 3.1. Transmission data Figure 5 shows transmission factors for a 2 × 2 cm2 6 MV beam for different thicknesses of water, Nylon-12, Perspex and water-equivalent RW3 materials. Also shown on the figure are the differences between the transmission factors of each solid material and water. Presented in Table 2are the calculated values of the linear attenuation (µ) and mass attenuation coefficients (µm) by application of Eq. (3). A linear relationshipbetween the logarithm of the transmission factor and thickness was obtained with goodness of fit R-squared values that ranged from 0.9983–1.000. Table 2. Linear (µ) and mass (µm) attenuation coefficients of Nylon-12 compared to the values of RW3, Perspex and water phantom materials determined using a 2 × 2 cm2 6 MV photon beam. The data in parentheses (%∆) in the last column show the differences of the µm values of the phantom materials relative to water for the Synergy Linac. The uncertainties in the µm values were about ±0.002, ±0.001, ±0.002 and ±0.003 for Nylon-12, RW3, Perspex and water, respectively. µ (cm-1)
Phantom material
Nylon-12 RW3 Perspex Water
µm(=(µ/ρ)) ( cm2/g)
SL25
Synergy
SL25
Synergy
0.0463 0.0486 0.0557 0.0474
0.0482 0.0504 0.0570 0.0490
0.0458 0.0465 0.0468 0.0474
0.0477 0.0482 0.0479 0.0490
µm/µm(Water) (%∆) Synergy 0.973 (2.7%) 0.984 (1.6%) 0.978 (2.2%) 1.000 (0.0%)
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Also shown on Table 2 are the ratios (µm/µm(Water)) of the µm values of a plastic material (Nylon12, Perspex and RW3) to that of water with percent deviations (%∆) shown in parentheses. From Table 2, the following observations could be made: (1) the attenuation coefficients (µ and µm) show a variation with the type/model of Linac used, with higher values obtained for the Elekta Synergy Linac. The inter-linac differences vary between 2.3% and 4.1%. In a study to measure the attenuation in water of high-energy X-rays (6 and 25 MV) as a function of off-axis distance for measurements made with Philips SL25 and SL75 Linacs, Bjarngard and Shackford (1994) also reported different attenuation coefficients for the two Linac models used. The inter-linac difference was 3.2% for a 0 cm off-axis distance; (2) The µm values for Nylon-12, Perspex and water-equivalent RW3 slab materials agree with the value of water within 3% (2.7% vs 2.2% vs 1.6%, respectively); (3) The deviations between the µm values of Nylon-12 and the other plastic materials (RW3 and Perspex) are within 2.1% for both the SL25 and Synergy Linacs. The magnitude of µm is known to be dependent on factors such as the incident photon energy, the chemical structure and bonding in the absorbing material and parameters such as thickness, density and field size with µm showing a decreasing trend with increasing photon energy and field size (Erk et al., 2016; Karaman et al, 2015; Kucuk et al, 2013). For instance, Karaman et al (2015) reported µ values of 0.0491, 0.044 and 0.042 cm-1 for field sizes of 5 × 5,10 × 10and 15 × 15 cm2, respectively for RW3. For the RW3 material investigated in this study, µ values of 0.0486 and 0.0504 cm-1 were determined for the SL25 and Synergy Linacs, respectively, for a 2 x 2 cm2 field − narrow beam geometry (and the uncertainty in the data − the standard deviation of the average values of three measurements − was about 1%). This study and that of Bjarngard and Shackford (1994) have thus shown that the attenuation coefficient is dependent on the type/model of theLinac. Additionally, the µm values for water (0.0490±0.003 cm2/g), Perspex (0.0479±0.002 cm2/g) and RW3 (0.0482±0.001 cm2/g) materials determined for the 6 MV photon beam in this study are comparable to the values of 0.04940, 0.04796 and 0.04767 cm2/g, respectively reported by Kumar et al. (2010). To investigate the dependence of the attenuation coefficients on photon energy in this study, transmission data were also measured for a 2 × 2 cm2 15 MV beam on the Elekta Synergy Linac for water, Nylon-12 and RW3 slabs, the results of which are tabulated in Table 3. The presented data show that the attenuation coefficients are lower for 15 MV beam compared to the values determined for the 6 MV beam displayed in Table 2 indicating that for the same field size, the attenuation coefficients decrease with increase in photon energy. As phantom materials are characterized based on photon interaction parameters such as the mass attenuation coefficient µm (Erk et al., 2016) which is a fundamental quantity used in the calculation of photon penetration and energy deposition in biological, shielding and dosimetric materials (Manohara et al, 2007), this study has established that the differences between the µm values of water and Nylon-12 for the investigated 2 × 2 cm2 6 MV and 2 × 2 cm2 15 MV beams are < 3% (Tables 2 and 3).
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Table 3. Linear (µ) and mass (µm) attenuation coefficients of Nylon-12 compared to the values of RW3 and water phantom materials determined using a 2 × 2 cm2 15 MV photon beam delivered by an Elekta Synergy Linac. The percent differences of the µm values of the phantom materials relative to water are indicated in parentheses (%∆) in the last column of this table. The uncertainty in the data was calculated as the standard deviation of the average values of three measurements. Phantom material
µ (cm-1)
Nylon-12 RW3 Water
0.0381 0.0398 0.0388
3.2.
µm(=(µ/ρ)) ( cm2/g) 0.0377±0.001 0.0380±0.001 0.0388±0.002
µm/µm(Water)(%∆) 0.972 (2.8%) 0.982 (1.8%) 1.000 (0.0%)
Depth dose distributions
Depth dose distributions are also useful quantities or functions to evaluate the water-equivalence of phantom materials (Erk et al., 2016). An important parameter for the characterization of electron beams is the half-value depth, R50 (i.e, the depth in phantom at which the PDD becomes 50% of dose maximum) which can be obtained from the electron’s CAX depth dose distribution (AAPM, 1983). The R50 value is related to the mean incident electron energy by a factor of 2.33 MeV/cm. Figs. 6(a–c) show comparisons between electron PDD data for 10 × 10 cm2 6 MeV (a), 10 × 10 cm2 12 MeV (b), and 10 × 10 cm2 20 MeV (c) beams measured with Gafchromic EBT3 films and a Roos ion chamberin Nylon-12 slabs and in water, respectively for a 10 × 10 cm2 field at 95 cm SSD. Also shown in Figs. 6(a–c) are deviations between each electron beam PDD data measured in water and in Nylon-12 slabs. For electron beams, the dose fall off in the descending region of a CAX depth dose curve is almost linear with depth as could be seen in Figs. 6(a–c). By application of linear interpolation between dose fall off and depth, the R50 values could be extracted. Tabulated in Table 4 are the R50 values, and other beam quality parameters, measured with Gafchromic EBT3 films in Nylon-12 slabs and with a Roos ion chamber in water for the different electron energies studied in this work. Fig. 6 and Table 4 show that there is good agreement (≤ 2% on average for PDD data and < 0.5% for R50, Ēo and dmax values) between measurements in water and Nylon-12 slabs indicating that Nylon-12 exhibits good waterequivalence. Shown in Fig. 7 are depth dose profiles computed by DOSXYZnrc Monte Carlo simulations for photon spectra from 250 kV−15 MV for water, RW3 and Nylon-12 materials. The average percent dose differences between water and the plastic materials (RW3 and Nylon-12) for all the spectra are shown on the secondary vertical axis of the figure. The corresponding standard deviations for the dose differences were 0.2% and 0.3% for RW3 and Nylon-12, respectively. It is evident that the dose differences are within 2% illustrating that Nylon-12 is water-equivalent.
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Table 4. Some beam quality parameters for the studied 6, 12 and 20 MeV electron beams, measured with Gafchromic EBT3 films in Nylon-12 slabs and with a Roos ion chamber in water. The mean incident electron energy Ēo and the range R50 are related by the factor 2.33 MeV/cm, where Ēo≈ 2.33R50. The 6 and 12 MeV beams were produced by the Synergy Linac while the 20 MeV beam was generated by the SL25 Linac. In this table, dmax is the depth of dose maximum. Nominal electron dmax (cm) energy (MeV) Water Nylon-12 6 12 20
1.32 2.45 1.71
1.29 2.45 1.69
R50 values (cm) Water Nylon-12 2.47 4.55 7.96
2.48 4.54 7.95
Ēo values (MeV) Water Nylon-12 5.76 10.60 18.55
5.78 10.58 18.52
Figure 8 shows the results of DOSXYZnrc Monte Carlo-based dosimetry performed using the pelvic phantom for a 10 × 10 cm2 10 MV beam. Regions consisting of Nylon-12 or water (white), bone (blue) and the Ti prosthesis (purple) are superimposed on the plotted PDD data. The figure shows that dose differences between the two independent scenarios that consist of Nylon-12 and water respectively, were on average within 2%. In the region of interest shown on the figure (in the distal region of the prosthesis where a lesion is usually located for treatment with high-energy photon beams at depths ranging from 11−19 cm for an average adult male), dose differences for the Monte Carlo simulated depth dose data between Nylon-12 and water are within 1%. 3.3.
Stopping power characteristics
An important dosimetric quantity to consider for the water-equivalence of a phantom material as it applies to real clinical situations is its electron stopping power characteristics relative to water. Stopping power ratios influence electron depth dose distributions inside phantoms (Borcia and Mihailescu, 2008). Table 4 of the AAPM TG-21 (AAPM, 1983) lists the ratios of average, restricted stopping powers of medium to air for various materials including Nylon for photon spectra ranging from 2−45 MV. Shown in Fig. 9 are the deviations between the stopping power ratios of water and some solid materials that include Nylon, Perspex and polystyrene. The figure shows that unlike Perspex and polystyrene which show stronger deviations of about 3% and 2% respectively, the deviations between the stopping power ratios of water and Nylon are within 1% implying that Nylon and water have equivalent stopping power characteristics. Studies of radiation interactions with tissue-equivalent materials find importance in efforts that seek to evade unjustifiable radiation dose to patients (Moradi et al., 2019). As dosimetry is one crucial step in the treatment process in radiation therapy, accurate dosimetric measurements in tissue- or water-equivalent solid phantoms are important for quality assurance and the safety of patients (Schoenfeld et al., 2015). Since evidence from studies of dose-response relationships show that a small inaccuracy in dose could result in a deviation from the planned response (be it tumour or normal tissue-related) that could be of clinical concern, acceptable levels of accuracy (involving all stages in the radiotherapy process) in the radiation dose delivered to the patient vary 9
between 3.5% and 5% (Thwaites, 2013; IAEA TRS-430, 2004; ICRU, 1976; Dutreix, 1984; Mijnheer and Wambersie, 1987; IAEA HHS-31, 2016; van der Merwe et al., 2017). With a further analysis of uncertainties related to radiation treatments showing that a dosimetric accuracy of 3% is required to yield a 5% accuracy in the dose delivered to the patient (ICRU, 1976; Dutreix, 1984; Mijnheer and Wambersie, 1987), it is therefore the responsibility of the medical physicist to practically ensure that treatment planning systems generate a dose calculation accuracy that is very close to 3% (IAEA TRS-430, 2004). As reference data for radiotherapy dose calculations are often obtained from measurements or from Monte Carlo calculations, and it is essential that relative doses (usually calculated by treatment planning systems) for patient dosimetry be validated against measurements (IAEA TRS-430, 2004), the results of the dosimetric characterisation of the water-equivalence of Nylon-12 presented in this study suggests that absorbed dose measurements performed in a Nylon-12 phantom would agree with those taken in water with an uncertainty that is below 3%. This indicates that Nylon-12 can be used as a tissue-mimicking phantom material for accurate dosimetric measurements and beam calibration in external beam radiotherapy. 4. Conclusion This study has dosimetrically characterised the water-equivalence of Nylon-12 in comparison with other dosimetry phantom materials commonly used for measurements in therapeutic photon and electron beams. It was established through 6 MV and 15 MV photon beam transmission measurements that the mass attenuation coefficients of Nylon-12 compare favourably with those of water within 3%. Also, deviations within 2% were attained between depth dose distributions determined in Nylon-12 slabs and in water for 10×10 cm2 6–20 MeV electron beams and for photon spectra between 250 kV and 15 MV. In addition, the half-value depth R50, and the mean incident energies Ēo of the electron beams determined in Nylon-12 slabs were in excellent agreement (< 0.5%) with those measured in water. Furthermore, an analysis of stopping power characteristics showed that the differences between the ratios of average, restricted stopping powers of Nylon to air and water to air for photon spectra ranging from 2–45 MV are typically within 1% indicating that Nylon and water have similar stopping power characteristics. Monte Carlo calculated depth dose data in the presented pelvic phantom show dose differences within 1% between Nylon-12 and water in the region of interest distal to the prosthesis in the phantom. This study, therefore, highlights that Nylon-12 exhibits good water-equivalence and so it can be used as a tissue-mimicking novel phantom material for dosimetric measurements in various clinical high-energy photon and electron beams. Acknowledgements This work was supported by the Medical Research Council of South Africa in terms of the MRC’s Flagships Awards Project [grant number SAMRC-RFA-UFSP-01-2013/HARD]. Declarations of interest: none
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References AAPM TG-21. A protocol for the determination of absorbed dose from high energy photon and electron beams. Med. Phys.10, 1983, 741. AAPM TG-51. Protocol for clinical reference dosimetry of high-energy photon and electron beams. Med. Phys.26, 1999, 1847. Ade N., du Plessis F.C.P., Measurement of the influence of titanium hip prosthesis on therapeutic electron beam dose distributions in a novel pelvic phantom.Physica Medica, 42, 2017b, 99–107. Ade N., Oderinde O.M, du Plessis F.C.P., 2018. Monte Carlo dose in a prosthesis phantom based on exact geometry vs streak artefact contaminated CT data as benchmarked against Gafchromic film measurements. Physica Medica, 54, 94–102. Arjomandy B., Tailor R., ZhaoL., DevicS., EBT2 film as a depth-dose measurement tool for radiotherapy beams over a wide range of energies and modalities. Med. Phys.39, 2012, 912. Bjärngard B.E., Shackford H.Attenuation in high‐energy x‐ray beams. Med. Phys.21, 1994, 1069–73 Björk P., Knöös T., Nilsson P. Comparative dosimetry of diode and diamond detectors inelectron beams for intraoperative radiation therapy. Med. Phys.27, 2000, 2580. Borcia C., Mihailescu D. Are water-equivalent materials used in electron beams dosimetry really water equivalent? Rom. Journ. Phys.53, 2008, 851–863. Casanova Borca V., Pasquino M., Russo G., Grosso P., CanteD., Sciacero P., Girelli G., La Porta M.R. and Tofani S., Dosimetric characterization and use of GAFCHROMIC EBT3 film for IMRT dose verification. J. Appl. Clin. Med. Phys.14, 2013, 4111. Devic S., Seuntjens J., Sham E., Podgorsak E.B., Schmidtlein C.R., Kirov A.S. and Soares C.G., Precise radiochromic film dosimetry using a flat-bed document scanner. Med. Phys. 32, 2005, 2245–53. Dutreix A., When and how can we improve precision in radiotherapy? Radiother. Oncol., 2,1984, 275–292. EGSnrc toolkit for Monte Carlo simulation of ionizing radiation transport: nrc-cnrc/EGSnrc. National Research Council Canada, 2019. Erk I, Altinsoy N, Karaaslan Ş. I, Bora A. Determination of photon mass attenuation coefficient for some phantom materials using gate code and comparison with experimental and XCOM data. I J NuRaSaT 1, 2016, 11–13.
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Ferreira C.C., Ximenes Filho R.E.M., Vieira J.W., Tomal A., Poletti M.E., Garcia C.A.B, Maia A.F. Evaluation of tissue-equivalent materials to be used as human brain tissuesubstitute in dosimetry for diagnostic radiology. Nucl. Instrum. Methods Phys. Res. B 268, 2010, 2515–2521. Fiandra C., Ricardi U., Ragona R., Anglesio S., Giglioli F.R., Calamia E. and Lucio F., Clinical use of EBT model Gafchromic film in radiotherapy. Med. Phys.33, 2006, 4314–19. Ghaffara S.H., Corkerb J., Fana M. Additive manufacturing technology and its implementation in construction as an eco-innovative solution. Autom. Constr.93, 2018, 1–11. Gray A., Oliver L. D., Johnston P. N. The accuracy of the pencil beam convolution and anisotropic analytical algorithms inpredicting the dose effects due to attenuation from immobilization devices and large air gaps. Med. Phys.36, 2009, 3181–90. Griehl W., Ruestem D. Nylon-12 – Preparation, Properties and Applications. Ind. Eng. Chem. 62(3), 1970, 16–22. HillR.F., Brown S., Baldock C. Evaluation of thewater equivalence of solid phantoms using gamma raytransmission measurements. Radiat. Meas. 43, 2008, 1258–64. IAEA Absorbed dose determination in photon and electronBeams: an international code of practice, Technical Report Series No. 398. IAEA, Vienna, 2000. IAEA International Atomic Energy Agency, Commissioning and Quality Assurance of Computerized Planning Systems for Radiation Treatment of Cancer, Technical Reports Series No. 430, IAEA, Vienna, 2004. IAEA International Atomic Energy Agency, Accuracy Requirements and Uncertainties in Radiotherapy, IAEA Human Health Series No. 31, IAEA, Vienna, 2016. ICRU International Commission on Radiation Units and Measurements, Tissue substitutes in radiation dosimetry and measurements. ICRU Report 44. Bethesda (MD), 1989. ICRU International Commission on Radiation Units and Measurements, Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures Rep. 24, ICRU, Bethesda (MD), 1976. Jones A.K., Hintenlang D.E, Bolch W.E. Tissue-equivalent materials forconstruction of tomographic dosimetry phantoms in pediatric radiology, Med. Phys. 30, 2003, 2072–2081. Karaman O., Tanir A.S., Bolukdemir M.H. The effects of plaster on radiation doses given to patients. Turk. J. Phys. 39, 2015, 31–36.
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Khan F.M. The Physics of Radiation Therapy, 3rd ed. Philadelpia: Lippincott & Wilkins, 2003. Kosunen A., Rogers D.W. Beam quality specification for photon beam dosimetry. Med. Phys. 20, 1993, 1181–88. Kucuk1 N., Cakir M., Isitman N.A. Mass attenuation coefficients, effective atomic numbers and electron densities of some polymers. Radiat. Prot. Dosim. 153, 2013, 127–134. Kumar R., Sharma S.D., Despande S., et al. Acrylonitrile Butadiene Styrene (ABS) plastic-based low cost tissue equivalent phantom for verification dosimetry in IMRT. J Appl Clin Med Phys. 11 (1), 2010, 24–32. Ma C.M., Rogers D.W., Walters B.R., DOSXYZnrc Users Manual (Ottawa: National Research Council of Canada), 2010. Manohara S.R., Hanagodimath S.M. Effective atomic numbers for photon energy absorption of essential amino acids in the energy range 1 keV to 20 MeV. Nucl. Instrum. Methods Phys. Res. B 264, 2007, 9–14. Mijnheer B.J., Battermann J.J., Wambersie A., What degree of accuracy is required and can be achieved in photon and neutron therapy? Radiother. Oncol. 8, 1987, 237–252. Moradi F., Khandaker M.U., Al Refae T., Ramazanian H., Bradley D.A., Monte Carlo simulations and analysis of transmitted gamma ray spectra through various tissuephantoms.Appl. Radiat Isot. 146, 2019, 120–126. Murphy S.V., Atala A. 3D bioprinting of tissues and organs. Nat. Biotechnol. 32, 2014, 773–85. Oderine O.M. and du Plessis F. Technical note: A new wedge-shaped ionization chamber component module for BEAMnrc to model the integral quality monitoring system®. Radiat. Phys. Chem. 141, 2017, 346–351. PEGS4. User Manual. Available from: http://www.fnal.gov/docs/products/egs4/pegs4.html. [Internet –last accessed 18 March 2017]. Podgorsak E.B. Radiation Oncology Physics: A Handbook for Teachers and Students, IAEA, Vienna, 2005, 451–458. Saur S., Frengen J., GafChromic EBT film dosimetry with flatbed CCD scanner: a novel background correction method and full dose uncertainty analysis.Med. Phys.,35 (7), 2008, 3094– 3101. Schoenfeld A.A, Harder D., Poppe B., Chofor N. Water equivalent phantom materials for 192Ir brachytherapy. Phys. Med. Biol. 60, 2015, 9403–9420.
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Thwaites D. Accuracy required and achievable in radiotherapy dosimetry: have modern technology and techniques changed our views? Journal of Physics: Conference Series 444, 2013 012006. van Battum L.J., Hoffmans D., Piersma H., Heukelom S. Accurate dosimetry with GafChromic EBT film of a 6 MV photon beam in water: what level is achievable? Med. Phys. 35 (2), 2008, 704–716. van der Merwe D., Van Dyk J., Healy B., Zubizarreta E., IzewskaJ., Mijnheer B., Meghzifene A. Accuracy requirements and uncertainties in radiotherapy: a report of the International Atomic Energy Agency, Acta Oncologica, 56, 2017,1–6.
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Figure Captions Figure 1: The heterogeneous pelvic phantom made out of Nylon-12 layers. The phantom is equipped with hip Ti prosthesis and bony structures as described by Ade and du Plessis (2017). Figure 2: A diagrammatic setup of narrow-beam geometry for 6 and 15 MV transmission measurements through slabs of Nylon-12, Perspex and water-equivalent RW3 materials. Figure 3: Simulation geometry consisting of a point source and phantom materials including water, Nylon-12 and RW3. The SSD was set at 100 cm for a 10 × 10 cm2 field. Figure 4: Simulation geometry with the heterogeneous pelvic phantom consisting of two scenarios: (1) Nylon-12; and (2) water. Figure 5: 6 MV transmission factors as a function of thickness for various phantom materials measured using an Elekta Synergy Linac. Differences between water and Nylon-12 ( ), water and RW3 (+ +), and water and Perspex (*) are shown on the secondary vertical axis. Figures 6(a–c): Comparisons of 10 × 10 cm2 6 MeV (a), 12 MeV (b) and 20 MeV (c) electron beam depth-dose data measured in water with a Roos ion chamber and in Nylon-12 slabs with Gafchromic EBT3 films at 95 cm SSD. The 6 and 12 MeV beams were produced the Elekta Synergy Linac whilethe 20 MeV beam was generated by the SL25 Linac. Figure7: DOSXYZnrc Monte Carlo-based PDD data in water, RW3 and Nylon-12 for photon spectra ranging from 250 kV–15 MV computed for a 10 × 10 cm2 field at 100 cm SSD. The dose differences are with respect to water, and the average values are for the five calculations/spectra. Figure 8: DOSXYZnrc Monte Carlo-based 10 MV PDD data computed in the heterogeneous pelvic phantom for a 10 × 10 cm2 field at 100 cm SSD. Dosimetry was performed for two scenarios (consisting of Nylon-12 and water) to establish the water-equivalence of the phantom. Figure 9: The stopping power ratios of various plastic materials (Nylon, Perspex and Polystyrene) relative to water for photon spectra (∆ = 10 keV) ranging from 2–45 MV. The stopping power ratios are extracted from the Table 4 of the AAPM TG-21 (1983) and analysed in this study to show the relative differences between water and the various plastic materials.
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Highlights The water-equivalence of Nylon-12 as a phantom material is characterized. Electron energies (6–15 MeV) and photon spectra (250 kV–15 MV) were studied. Mass attenuation coefficient values of Nylon-12 and water differ by < 3%. Percent depth doses obtained in water and Nylon-12 slabs agreed within 2%. Nylon-12 exhibits good water-equivalence as a dosimetric material.
1
S0969-8043(19)30247-7
DOI:
https://doi.org/10.1016/j.apradiso.2019.108919
Reference:
ARI 108919
To appear in:
Applied Radiation and Isotopes
Received Date: 6 March 2019 Revised Date:
26 August 2019
Accepted Date: 2 October 2019
Please cite this article as: Ade, N., van Eeden, D., du Plessis, F.C.P., Characterization of Nylon-12 as a water-equivalent solid phantom material for dosimetric measurements in therapeutic photon and electron beams, Applied Radiation and Isotopes (2019), doi: https://doi.org/10.1016/j.apradiso.2019.108919. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Characterization of Nylon-12 as a water-equivalent solid phantom material for dosimetric measurements in therapeutic photon and electron beams Nicholas Ade∗, D. van Eeden and F.C.P. du Plessis Medical Physics Department, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa ABSTRACT The tissue- or water-equivalence of dosimetry phantoms used as substitutes for water is essential for absorbed dose measurements in radiotherapy. At our institution, a heterogeneous pelvic phantom that consists of stacked Nylon-12 layers has recently been manufactured for Gafchromic film dosimetry. However, data on the use of Nylon as tissue-mimicking media for dosimetric applications are scarce. This study characterizes the water-equivalence of Nylon-12 for dosimetric measurements in therapeutic photon and electron beams. Employing an Elekta Synergy and SL25 linear accelerator (Linac), photon beam transmission measurements for 6 MV and 15 MV, acquired in narrow beam geometry with a 0.6 cm3 Farmer-type ion chamber showed that the mass attenuation coefficient µm of Nylon-12 agrees with the values of water, water-equivalent RW3 and Perspex phantom materials within 3%. For 6 MV, the µm values were 0.0477±0.002cm2/g, 0.0490±0.003cm2/g, 0.0482±0.001 cm2/g and 0.0479±0.002cm2/gfor Nylon-12, water, RW3, and Perspex, respectively. Differences within 2% were attained between depth dose data measured in Nylon-12 slabs with Gafchromic EBT3 films and in water with a Roos ion chamber for 10×10 cm2 6, 12 and 20 MeV electron beams produced by the Elekta Synergy and SL25 Linacs. Also, a good agreement within 2% was obtained between percent depth doses computed by DOSXYZnrc Monte Carlo simulations in water, Nylon-12 and RW3 materials for photon spectra between 250 kV and 15 MV. The discrepancies between the ratios of average, restricted stopping powers of Nylon to air and water to air for photon spectra ranging from 2–45 MV are typically within 1% signifying that Nylon and water have equivalent stopping power characteristics. This study highlights that Nylon-12 can be used as a tissue-mimicking phantom material for dosimetric measurements in clinical megavoltage photon and electron beams as it exhibits good water-equivalence. Key words: Nylon-12; Water-equivalence; Gafchromic film; Dosimetry; Photons; Electrons 1. Introduction High-energy electron (6–22 MeV) and photon (60Co, 4–25 MV) beams produced by teletherapy machines are commonly used in radiation therapy of cancer. Before radiation treatment, clinical dosimetry is performed, either through independent dose distribution calculations (e.g. with Monte Carlo simulations), or measurements (e.g. with dosimeters in sophisticated phantoms). Dosimetry deals with techniques for the quantitative determination of absorbed dose in a given medium by ionizing radiation using radiation dosimeters (Podgorsak, 2005). It represents one of physical procedures aimed at improving the accuracy of radiation dose delivery in radiotherapy. ∗
Corresponding author: Tel. +27 (0)51 405 2698; E-mail address: [email protected]
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In clinical dosimetry, measurements are frequently performed inside phantoms which simulate human soft tissue. Water is employed as a reference dosimetry phantom as it closely approximates the radiation absorption and scattering characteristics of muscles and other soft tissues (Khan, 2003; IAEA TRS-398, 2000). Tissue-equivalence describes the property of a material to respond to radiation in a similar manner as the human body. Water poses some practical problems such as unease of use so several plastic materials referred to as tissue- or water-equivalent materials (Ferreira et al, 2010; Jones et al, 2003;Khan, 2003) have been developed as dosimetry phantoms for clinical practice and this has been implemented in various dosimetry protocols (AAPM TG-21, 1983; AAPM TG-51, 1999; IAEA TRS-398, 2000). Table 1 lists various phantom materials including water and water-equivalent RW3 and their dosimetric properties which include mass density ρ and mass attenuation coefficient µm.
Table 1: Some dosimetric properties which include mass density (ρ) and mass attenuation coefficient (µm) of commonly used phantom materials. Values for the µm are for a 6 MV photon beam as reported by Kumar et al. (2010). Phantom material
ρ (g/cm3)
Water Solid water (WT1) RW3 Perspex Polystyrene
1.000 1.020 1.040+ 1.190 1.060
µm (×10-2 cm2/g) 4.940 4.800 4.767 4.796 4.780
+
Hill et al. (2008) reported a density value of 1.045 g/cm3 for RW3.
Although various parameters and quantities including µm, effective atomic number, effective electron density, stopping power ratios and depth dose data may be used to evaluate the waterequivalence of phantom materials, the attenuation coefficient is the major photon interaction parameter (Erk et al., 2016) that governs photon depth dose distribution with depth beyond dose maximum. At transient charged particle equilibrium conditions the variation of photon percent depth dose (PDD) with depth d beyond the depth of dose maximum dmax can be approximated by exponential attenuation e–<µ>.(d–dmax), where <µ> is the average attenuation coefficient for the heterogeneous beam and medium which takes into account the effect of beam quality (Khan, 2003). As <µ> decreases, the beam becomes more penetrating resulting in a higher PDD at any given depth beyond the build-up region. Unlike the attenuation coefficient which is the major photon interaction parameter used to characterize the tissue- or water-equivalence of a phantom material, stopping power ratios are important for electron beams (ICRU Report 44, 1989). Based on the Spencer-Attix formulation of the Bragg-Gray theory, the absorbed dose
D
med
at point P in a medium (dosimetry phantom)
measured with a detector (det) can, in general be expressed as:
2
D
med
=
L R . ρ
med
.P
(1) pert
det
where, R is the response of the detector (usually ionisation signal) corrected for various factors; med
L ‘det’ represents the radiation sensing material of the detector such as air. The parameter ρ
is
det
1
the ratio of the average, restricted mass collision stopping power of the medium to the detector material which accounts for the change in the electron fluence at the point of measurement due to the insertion of the detector. The perturbation correction factor Ppert, corrects for perturbation in the photon and electron energy fluences at the point of measurement due to the introduction of the detector material in the medium (Khan, 2003). That is, it accounts for the spatial and angular variation of the radiation fluence in the detector and the surrounding medium (Björk et al., 2000). According to Eq. (1), calculated dose data are critically dependent upon the choice of correct stopping power ratios, and these ratios are in turn dependent upon the spectrum of photons and electrons incident upon the dosimetry phantom (AAPM, 1983). Thus as a further step undertaken to evaluate the water-equivalence of Nylon-12, the stopping power ratios of Nylon-12 including those of various plastic materials relative to water are discussed. As pointed out by Borcia and Mihailescu (2008) data in literature indicate that there is no perfect agreement between depth dose distributions for any water substitute material. As such, it is essential to scale depths measured in non-water phantoms to water-equivalent depths. Also, the reading of an ionization chamber in the non-water phantom should be converted to an appropriate reading in water (IAEA TRS-398, 2000). Figure 1 shows a heterogeneous pelvic phantom that consists of stacked Nylon-12 layers manufactured for prosthetic-induced dose perturbation studies as reported by Ade and du Plessis(2017).Nylon-12 (polyamide-12, PA-12) is a polymer having a straight-chain structure with acid amide groups and 12 carbon atoms in its monomer unit (Griehl and Ruestem, 1970). It has the formula [(CH2)11CONH]n, where n depends upon the molecular weight. Ring-opening polymerization (a form of chain-growth polymerization) is the preferred route for commercial production according to equation 2: n[(CH2)11CONH] → [(CH2)11CONH]n.
(2)
Data on Nylon as tissue-mimicking media for dosimetric applications are sparse.The aim of this work is therefore to determinethe water-equivalence of Nylon-12 in clinical megavoltage photon and electron beams. The major advantage of using Nylon-12 as a phantom material over existing alternatives including Perspex and polystyrene is that Nylon and water have equivalent stopping
1
Stopping power refers to the energy loss by electrons per unit path length of amaterial (Khan, 2003) 3
power characteristics (AAPM, 1983). Nylon-12 is also readily machinable so very sophistacated phantoms can be developed using 3D printing technology. 2. Materials and methods 2.1. Phantom description and measurements The phantom shown in Fig. 1 (which is made out of Nylon-12 layers and equipped with unilateral Ti implant) was 3D printed using additive manufacturing (AM) processes (Ghaffar et al., 2018; Murphy and Atala, 2014). A design of the phantom was first created and loaded on a 3D printing machine. EOSINT P385-(Plastics) and EOSINT M 280-(Metals) AM machines from EOS Electro Optical Systems− Germany were used. The phantom was “grown” using various powders (Nylon12 for the plastic layers and Ti for the metal implant). The powder is spread on a powder bed, and a laser then sinters (melts) the first layer of powder according to the data which has been programmed onto the machine from the design. This process is repeated until the design has developed into an actual product. The water-equivalence of Nylon-12 was measured in a 6 MV and 15 MV photon, and 6−20 MeV electron beams on an Elekta SL25 and Elekta Synergy linear accelerators. Layers of Nylon-12, each measuring 1 cm thick were used as phantom slabs. Central axis (CAX) transmission factors through slabs of Nylon-12 as well as slabs of Perspex and water-equivalent RW3 materials (Karaman et al, 2015; Kumar et al, 2010) were measured in narrow beam geometry to determine the mass attenuation coefficients µm of the plastic materials for comparison. A 0.6 cm3 Farmertype ion chamber connected to a PTW UNIDOS E electrometer was used for integrated charge measurements. The ion chamber was housed inside a Perspex block with 0.8 cm build-up. The block, containing the chamber, was placed on the floor at a source-to-chamber distance (SCD) of approximately 200 cm. Transmission data were acquired with the phantom slabs placed at a source-to-phantom-surface distance (SSD) of 100 cm for a set field of 2 × 2 cm2 defined at the SSD in Fig. 2. 300 monitor units (MUs) were set up for each irradiation. The transmission factors (Tx) were measured for different thicknesses of attenuating material ranging from 1 cm to 10 cm in steps of 1 cm. The linear attenuation coefficient (µ), in addition to the mass attenuation coefficient (µm = µ/ρ) were computed (Hill et al, 2008; Gray et al, 2009) according to equation 3.
R x / Ro = exp(− µ · x) = Tx
(3)
In Eq. (3), Rx is the transmitted signal through the material of thickness x (cm) and Ro is the open beam signal. The linear attenuation coefficient µ was then determined from a least-square fit of a linearized function through the transmission data points on a semi-log of Tx vs x graph. Density ρ values of 1.00 g/cm3, 1.01 g/cm3, 1.045 g/cm3 and 1.19 g/cm3 (Karaman et al, 2015; Hill et al, 2008; Kumar et al, 2010; Borcia and Mihailescu, 2008; Griehl and Ruestem, 1970) were used to determine the µm values of water, Nylon-12, RW3 and Perspex, respectively.
4
A comparison was made between the PDD data measured with Gafchromic EBT3 films in Nylon12 slabs and another measured in water with a Roos ion chamber for 6, 12 and 20 MeV electron beams produced by an Elekta Synergy (6 and 12 MeV) and SL25 Linac (20 MeV). The measurements were acquired for an applicator defined field size of 10 × 10 cm2 at 95 cm SSD. Gafchromic EBT3 films were used for this study because of their good dosimetric properties which include: near-tissue equivalence (the effective atomic number of EBT3, Zeff = 6.84, closely matches Zeff of water = 7.3); they are practically energy independent in the megavoltage photon and electron energy ranges used clinically; and they have high spatial resolution (Arjomandy et al. 2010; Fiandra et al. 2006; Casanova Borca et al. 2013; Devic et al. 2005). In addition, Gafchromic EBT3 film is self-developing and unaffected by ambient light (Saur and Frengen, 2008). Batches of Gafchromic films used in this study were calibrated for conversion of optical density to absorbed dose through similar procedures described elsewhere (Ade and du Plessis, 2017). Films were scanned at a resolution of 50 dpi using an Epson Perfection V330 Photo flat-bed document scanner. The resulting images were digitised as raw 48-bit RGB (16 bits per colour channel) and stored in tagged-image-file format (TIFF). The16-bit depth red channel values were utilised in subsequent dosimetry analysis. Films were scanned 24 hours after irradiation to allow for nearcompletion of post exposure polymerization. 2.2. Monte Carlo-based DOSXYZnrc simulations 2.2.1. Depth dose simulations in water, RW3 and Nylon-12 slabs In addition to measurements, Monte Carlo-based DOSXYZnrc simulations were undertaken to compute PDD data in water, water-equivalent RW3 and Nylon-12 materials using photon spectra of 250 kV–15 MVfor a point source with rectangular collimation (Isource =3) (Ma et al., 2010). A PEGS file was generated for the different energies and contained the cross-section data that define the interactions taking place between X-rays and secondary electrons for Nylon-12 (PEGS4, 2017). Photon and electron cut-off parameters, PCUT and ECUT, were set to 0.010 MeV and 0.512 MeV, respectively, for all simulations. For the 250 kV simulations, spin effects were implemented to ensure accurate results with adequate backscattering. Rayleigh scattering, bound Compton scattering and atomic relaxation were also implemented as recommended for lowenergy (<1 MeV) simulations. The electron-step algorithm and boundary crossing algorithm (BCA) were PRESTA-II and EXACT, respectively. The skin depth for the BCA was set to three elastic mean free paths. For the simulations using photons in the megavoltage energy range, atomic relaxations, Rayleigh scattering and bound Compton scattering were not requested. Spin effects were implemented and the electron-step algorithm and BCA were PRESTA-II and PRESTA-I, respectively. The phantom model was 40 × 40 x 40 cm3 with voxel dimensions of 2 mm in all three directions. The SSD was 100 cm. The simulation geometry is shown in Fig. 3. For all the simulations, energy spectra available from the DOSXYZnrc directory were used. These spectra are measured and/or calculated at the NRC (National Research Council Canada) or
5
taken from (Kosune and Rogers, 1993; NRC, 2019). The PDD data were computed for a 10 × 10 cm2 field size. 2.2.2.
DOSXYZnrc simulations with the heterogeneous pelvic phantom
Dosimetry was performed using the pelvic phantom (Fig. 1) made out of Nylon-12 slices to investigate its water-equivalence for a clinical case. Prior to DOSXYZnrc simulations, the pelvic phantom was CT-scanned as reported elsewhere (Ade et al., 2018) so that its DICOM images could be reconstructed and imported as an input phantom file for the Monte Carlo code. The phantom was converted to an .egsphant format by using an in-house, developed IDL program similar to CTCREATE (Ma et al., 2010) found in DOSXYZnrc software package. Two scenarios were simulated: (1) the pelvic phantom with the Nylon-12 layers as seen in Fig. 1; and (2) the pelvic phantom with the Nylon-12 layers replaced by water. The simulation geometry was similar to that reported by Ade et al.(2018). A 10 × 10 cm2 10 MV lateral beam at 100 cm SSD directed from the prosthesis side of the phantom was used. The phase space file for the 10 MV beam was generated with a source model of the Elekta Synergy Linac acquired from a previous study (Oderinde and du Plessis, 2017). A schematic of the simulation geometryis illustrated in Fig 4. Various simulation parameters including PCUT, ECUT, electron-step algorithm and BCA were implemented as defined in section 2.1.1. 3. Results and discussion 3.1. Transmission data Figure 5 shows transmission factors for a 2 × 2 cm2 6 MV beam for different thicknesses of water, Nylon-12, Perspex and water-equivalent RW3 materials. Also shown on the figure are the differences between the transmission factors of each solid material and water. Presented in Table 2are the calculated values of the linear attenuation (µ) and mass attenuation coefficients (µm) by application of Eq. (3). A linear relationshipbetween the logarithm of the transmission factor and thickness was obtained with goodness of fit R-squared values that ranged from 0.9983–1.000. Table 2. Linear (µ) and mass (µm) attenuation coefficients of Nylon-12 compared to the values of RW3, Perspex and water phantom materials determined using a 2 × 2 cm2 6 MV photon beam. The data in parentheses (%∆) in the last column show the differences of the µm values of the phantom materials relative to water for the Synergy Linac. The uncertainties in the µm values were about ±0.002, ±0.001, ±0.002 and ±0.003 for Nylon-12, RW3, Perspex and water, respectively. µ (cm-1)
Phantom material
Nylon-12 RW3 Perspex Water
µm(=(µ/ρ)) ( cm2/g)
SL25
Synergy
SL25
Synergy
0.0463 0.0486 0.0557 0.0474
0.0482 0.0504 0.0570 0.0490
0.0458 0.0465 0.0468 0.0474
0.0477 0.0482 0.0479 0.0490
µm/µm(Water) (%∆) Synergy 0.973 (2.7%) 0.984 (1.6%) 0.978 (2.2%) 1.000 (0.0%)
6
Also shown on Table 2 are the ratios (µm/µm(Water)) of the µm values of a plastic material (Nylon12, Perspex and RW3) to that of water with percent deviations (%∆) shown in parentheses. From Table 2, the following observations could be made: (1) the attenuation coefficients (µ and µm) show a variation with the type/model of Linac used, with higher values obtained for the Elekta Synergy Linac. The inter-linac differences vary between 2.3% and 4.1%. In a study to measure the attenuation in water of high-energy X-rays (6 and 25 MV) as a function of off-axis distance for measurements made with Philips SL25 and SL75 Linacs, Bjarngard and Shackford (1994) also reported different attenuation coefficients for the two Linac models used. The inter-linac difference was 3.2% for a 0 cm off-axis distance; (2) The µm values for Nylon-12, Perspex and water-equivalent RW3 slab materials agree with the value of water within 3% (2.7% vs 2.2% vs 1.6%, respectively); (3) The deviations between the µm values of Nylon-12 and the other plastic materials (RW3 and Perspex) are within 2.1% for both the SL25 and Synergy Linacs. The magnitude of µm is known to be dependent on factors such as the incident photon energy, the chemical structure and bonding in the absorbing material and parameters such as thickness, density and field size with µm showing a decreasing trend with increasing photon energy and field size (Erk et al., 2016; Karaman et al, 2015; Kucuk et al, 2013). For instance, Karaman et al (2015) reported µ values of 0.0491, 0.044 and 0.042 cm-1 for field sizes of 5 × 5,10 × 10and 15 × 15 cm2, respectively for RW3. For the RW3 material investigated in this study, µ values of 0.0486 and 0.0504 cm-1 were determined for the SL25 and Synergy Linacs, respectively, for a 2 x 2 cm2 field − narrow beam geometry (and the uncertainty in the data − the standard deviation of the average values of three measurements − was about 1%). This study and that of Bjarngard and Shackford (1994) have thus shown that the attenuation coefficient is dependent on the type/model of theLinac. Additionally, the µm values for water (0.0490±0.003 cm2/g), Perspex (0.0479±0.002 cm2/g) and RW3 (0.0482±0.001 cm2/g) materials determined for the 6 MV photon beam in this study are comparable to the values of 0.04940, 0.04796 and 0.04767 cm2/g, respectively reported by Kumar et al. (2010). To investigate the dependence of the attenuation coefficients on photon energy in this study, transmission data were also measured for a 2 × 2 cm2 15 MV beam on the Elekta Synergy Linac for water, Nylon-12 and RW3 slabs, the results of which are tabulated in Table 3. The presented data show that the attenuation coefficients are lower for 15 MV beam compared to the values determined for the 6 MV beam displayed in Table 2 indicating that for the same field size, the attenuation coefficients decrease with increase in photon energy. As phantom materials are characterized based on photon interaction parameters such as the mass attenuation coefficient µm (Erk et al., 2016) which is a fundamental quantity used in the calculation of photon penetration and energy deposition in biological, shielding and dosimetric materials (Manohara et al, 2007), this study has established that the differences between the µm values of water and Nylon-12 for the investigated 2 × 2 cm2 6 MV and 2 × 2 cm2 15 MV beams are < 3% (Tables 2 and 3).
7
Table 3. Linear (µ) and mass (µm) attenuation coefficients of Nylon-12 compared to the values of RW3 and water phantom materials determined using a 2 × 2 cm2 15 MV photon beam delivered by an Elekta Synergy Linac. The percent differences of the µm values of the phantom materials relative to water are indicated in parentheses (%∆) in the last column of this table. The uncertainty in the data was calculated as the standard deviation of the average values of three measurements. Phantom material
µ (cm-1)
Nylon-12 RW3 Water
0.0381 0.0398 0.0388
3.2.
µm(=(µ/ρ)) ( cm2/g) 0.0377±0.001 0.0380±0.001 0.0388±0.002
µm/µm(Water)(%∆) 0.972 (2.8%) 0.982 (1.8%) 1.000 (0.0%)
Depth dose distributions
Depth dose distributions are also useful quantities or functions to evaluate the water-equivalence of phantom materials (Erk et al., 2016). An important parameter for the characterization of electron beams is the half-value depth, R50 (i.e, the depth in phantom at which the PDD becomes 50% of dose maximum) which can be obtained from the electron’s CAX depth dose distribution (AAPM, 1983). The R50 value is related to the mean incident electron energy by a factor of 2.33 MeV/cm. Figs. 6(a–c) show comparisons between electron PDD data for 10 × 10 cm2 6 MeV (a), 10 × 10 cm2 12 MeV (b), and 10 × 10 cm2 20 MeV (c) beams measured with Gafchromic EBT3 films and a Roos ion chamberin Nylon-12 slabs and in water, respectively for a 10 × 10 cm2 field at 95 cm SSD. Also shown in Figs. 6(a–c) are deviations between each electron beam PDD data measured in water and in Nylon-12 slabs. For electron beams, the dose fall off in the descending region of a CAX depth dose curve is almost linear with depth as could be seen in Figs. 6(a–c). By application of linear interpolation between dose fall off and depth, the R50 values could be extracted. Tabulated in Table 4 are the R50 values, and other beam quality parameters, measured with Gafchromic EBT3 films in Nylon-12 slabs and with a Roos ion chamber in water for the different electron energies studied in this work. Fig. 6 and Table 4 show that there is good agreement (≤ 2% on average for PDD data and < 0.5% for R50, Ēo and dmax values) between measurements in water and Nylon-12 slabs indicating that Nylon-12 exhibits good waterequivalence. Shown in Fig. 7 are depth dose profiles computed by DOSXYZnrc Monte Carlo simulations for photon spectra from 250 kV−15 MV for water, RW3 and Nylon-12 materials. The average percent dose differences between water and the plastic materials (RW3 and Nylon-12) for all the spectra are shown on the secondary vertical axis of the figure. The corresponding standard deviations for the dose differences were 0.2% and 0.3% for RW3 and Nylon-12, respectively. It is evident that the dose differences are within 2% illustrating that Nylon-12 is water-equivalent.
8
Table 4. Some beam quality parameters for the studied 6, 12 and 20 MeV electron beams, measured with Gafchromic EBT3 films in Nylon-12 slabs and with a Roos ion chamber in water. The mean incident electron energy Ēo and the range R50 are related by the factor 2.33 MeV/cm, where Ēo≈ 2.33R50. The 6 and 12 MeV beams were produced by the Synergy Linac while the 20 MeV beam was generated by the SL25 Linac. In this table, dmax is the depth of dose maximum. Nominal electron dmax (cm) energy (MeV) Water Nylon-12 6 12 20
1.32 2.45 1.71
1.29 2.45 1.69
R50 values (cm) Water Nylon-12 2.47 4.55 7.96
2.48 4.54 7.95
Ēo values (MeV) Water Nylon-12 5.76 10.60 18.55
5.78 10.58 18.52
Figure 8 shows the results of DOSXYZnrc Monte Carlo-based dosimetry performed using the pelvic phantom for a 10 × 10 cm2 10 MV beam. Regions consisting of Nylon-12 or water (white), bone (blue) and the Ti prosthesis (purple) are superimposed on the plotted PDD data. The figure shows that dose differences between the two independent scenarios that consist of Nylon-12 and water respectively, were on average within 2%. In the region of interest shown on the figure (in the distal region of the prosthesis where a lesion is usually located for treatment with high-energy photon beams at depths ranging from 11−19 cm for an average adult male), dose differences for the Monte Carlo simulated depth dose data between Nylon-12 and water are within 1%. 3.3.
Stopping power characteristics
An important dosimetric quantity to consider for the water-equivalence of a phantom material as it applies to real clinical situations is its electron stopping power characteristics relative to water. Stopping power ratios influence electron depth dose distributions inside phantoms (Borcia and Mihailescu, 2008). Table 4 of the AAPM TG-21 (AAPM, 1983) lists the ratios of average, restricted stopping powers of medium to air for various materials including Nylon for photon spectra ranging from 2−45 MV. Shown in Fig. 9 are the deviations between the stopping power ratios of water and some solid materials that include Nylon, Perspex and polystyrene. The figure shows that unlike Perspex and polystyrene which show stronger deviations of about 3% and 2% respectively, the deviations between the stopping power ratios of water and Nylon are within 1% implying that Nylon and water have equivalent stopping power characteristics. Studies of radiation interactions with tissue-equivalent materials find importance in efforts that seek to evade unjustifiable radiation dose to patients (Moradi et al., 2019). As dosimetry is one crucial step in the treatment process in radiation therapy, accurate dosimetric measurements in tissue- or water-equivalent solid phantoms are important for quality assurance and the safety of patients (Schoenfeld et al., 2015). Since evidence from studies of dose-response relationships show that a small inaccuracy in dose could result in a deviation from the planned response (be it tumour or normal tissue-related) that could be of clinical concern, acceptable levels of accuracy (involving all stages in the radiotherapy process) in the radiation dose delivered to the patient vary 9
between 3.5% and 5% (Thwaites, 2013; IAEA TRS-430, 2004; ICRU, 1976; Dutreix, 1984; Mijnheer and Wambersie, 1987; IAEA HHS-31, 2016; van der Merwe et al., 2017). With a further analysis of uncertainties related to radiation treatments showing that a dosimetric accuracy of 3% is required to yield a 5% accuracy in the dose delivered to the patient (ICRU, 1976; Dutreix, 1984; Mijnheer and Wambersie, 1987), it is therefore the responsibility of the medical physicist to practically ensure that treatment planning systems generate a dose calculation accuracy that is very close to 3% (IAEA TRS-430, 2004). As reference data for radiotherapy dose calculations are often obtained from measurements or from Monte Carlo calculations, and it is essential that relative doses (usually calculated by treatment planning systems) for patient dosimetry be validated against measurements (IAEA TRS-430, 2004), the results of the dosimetric characterisation of the water-equivalence of Nylon-12 presented in this study suggests that absorbed dose measurements performed in a Nylon-12 phantom would agree with those taken in water with an uncertainty that is below 3%. This indicates that Nylon-12 can be used as a tissue-mimicking phantom material for accurate dosimetric measurements and beam calibration in external beam radiotherapy. 4. Conclusion This study has dosimetrically characterised the water-equivalence of Nylon-12 in comparison with other dosimetry phantom materials commonly used for measurements in therapeutic photon and electron beams. It was established through 6 MV and 15 MV photon beam transmission measurements that the mass attenuation coefficients of Nylon-12 compare favourably with those of water within 3%. Also, deviations within 2% were attained between depth dose distributions determined in Nylon-12 slabs and in water for 10×10 cm2 6–20 MeV electron beams and for photon spectra between 250 kV and 15 MV. In addition, the half-value depth R50, and the mean incident energies Ēo of the electron beams determined in Nylon-12 slabs were in excellent agreement (< 0.5%) with those measured in water. Furthermore, an analysis of stopping power characteristics showed that the differences between the ratios of average, restricted stopping powers of Nylon to air and water to air for photon spectra ranging from 2–45 MV are typically within 1% indicating that Nylon and water have similar stopping power characteristics. Monte Carlo calculated depth dose data in the presented pelvic phantom show dose differences within 1% between Nylon-12 and water in the region of interest distal to the prosthesis in the phantom. This study, therefore, highlights that Nylon-12 exhibits good water-equivalence and so it can be used as a tissue-mimicking novel phantom material for dosimetric measurements in various clinical high-energy photon and electron beams. Acknowledgements This work was supported by the Medical Research Council of South Africa in terms of the MRC’s Flagships Awards Project [grant number SAMRC-RFA-UFSP-01-2013/HARD]. Declarations of interest: none
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References AAPM TG-21. A protocol for the determination of absorbed dose from high energy photon and electron beams. Med. Phys.10, 1983, 741. AAPM TG-51. Protocol for clinical reference dosimetry of high-energy photon and electron beams. Med. Phys.26, 1999, 1847. Ade N., du Plessis F.C.P., Measurement of the influence of titanium hip prosthesis on therapeutic electron beam dose distributions in a novel pelvic phantom.Physica Medica, 42, 2017b, 99–107. Ade N., Oderinde O.M, du Plessis F.C.P., 2018. Monte Carlo dose in a prosthesis phantom based on exact geometry vs streak artefact contaminated CT data as benchmarked against Gafchromic film measurements. Physica Medica, 54, 94–102. Arjomandy B., Tailor R., ZhaoL., DevicS., EBT2 film as a depth-dose measurement tool for radiotherapy beams over a wide range of energies and modalities. Med. Phys.39, 2012, 912. Bjärngard B.E., Shackford H.Attenuation in high‐energy x‐ray beams. Med. Phys.21, 1994, 1069–73 Björk P., Knöös T., Nilsson P. Comparative dosimetry of diode and diamond detectors inelectron beams for intraoperative radiation therapy. Med. Phys.27, 2000, 2580. Borcia C., Mihailescu D. Are water-equivalent materials used in electron beams dosimetry really water equivalent? Rom. Journ. Phys.53, 2008, 851–863. Casanova Borca V., Pasquino M., Russo G., Grosso P., CanteD., Sciacero P., Girelli G., La Porta M.R. and Tofani S., Dosimetric characterization and use of GAFCHROMIC EBT3 film for IMRT dose verification. J. Appl. Clin. Med. Phys.14, 2013, 4111. Devic S., Seuntjens J., Sham E., Podgorsak E.B., Schmidtlein C.R., Kirov A.S. and Soares C.G., Precise radiochromic film dosimetry using a flat-bed document scanner. Med. Phys. 32, 2005, 2245–53. Dutreix A., When and how can we improve precision in radiotherapy? Radiother. Oncol., 2,1984, 275–292. EGSnrc toolkit for Monte Carlo simulation of ionizing radiation transport: nrc-cnrc/EGSnrc. National Research Council Canada, 2019. Erk I, Altinsoy N, Karaaslan Ş. I, Bora A. Determination of photon mass attenuation coefficient for some phantom materials using gate code and comparison with experimental and XCOM data. I J NuRaSaT 1, 2016, 11–13.
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Ferreira C.C., Ximenes Filho R.E.M., Vieira J.W., Tomal A., Poletti M.E., Garcia C.A.B, Maia A.F. Evaluation of tissue-equivalent materials to be used as human brain tissuesubstitute in dosimetry for diagnostic radiology. Nucl. Instrum. Methods Phys. Res. B 268, 2010, 2515–2521. Fiandra C., Ricardi U., Ragona R., Anglesio S., Giglioli F.R., Calamia E. and Lucio F., Clinical use of EBT model Gafchromic film in radiotherapy. Med. Phys.33, 2006, 4314–19. Ghaffara S.H., Corkerb J., Fana M. Additive manufacturing technology and its implementation in construction as an eco-innovative solution. Autom. Constr.93, 2018, 1–11. Gray A., Oliver L. D., Johnston P. N. The accuracy of the pencil beam convolution and anisotropic analytical algorithms inpredicting the dose effects due to attenuation from immobilization devices and large air gaps. Med. Phys.36, 2009, 3181–90. Griehl W., Ruestem D. Nylon-12 – Preparation, Properties and Applications. Ind. Eng. Chem. 62(3), 1970, 16–22. HillR.F., Brown S., Baldock C. Evaluation of thewater equivalence of solid phantoms using gamma raytransmission measurements. Radiat. Meas. 43, 2008, 1258–64. IAEA Absorbed dose determination in photon and electronBeams: an international code of practice, Technical Report Series No. 398. IAEA, Vienna, 2000. IAEA International Atomic Energy Agency, Commissioning and Quality Assurance of Computerized Planning Systems for Radiation Treatment of Cancer, Technical Reports Series No. 430, IAEA, Vienna, 2004. IAEA International Atomic Energy Agency, Accuracy Requirements and Uncertainties in Radiotherapy, IAEA Human Health Series No. 31, IAEA, Vienna, 2016. ICRU International Commission on Radiation Units and Measurements, Tissue substitutes in radiation dosimetry and measurements. ICRU Report 44. Bethesda (MD), 1989. ICRU International Commission on Radiation Units and Measurements, Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures Rep. 24, ICRU, Bethesda (MD), 1976. Jones A.K., Hintenlang D.E, Bolch W.E. Tissue-equivalent materials forconstruction of tomographic dosimetry phantoms in pediatric radiology, Med. Phys. 30, 2003, 2072–2081. Karaman O., Tanir A.S., Bolukdemir M.H. The effects of plaster on radiation doses given to patients. Turk. J. Phys. 39, 2015, 31–36.
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Khan F.M. The Physics of Radiation Therapy, 3rd ed. Philadelpia: Lippincott & Wilkins, 2003. Kosunen A., Rogers D.W. Beam quality specification for photon beam dosimetry. Med. Phys. 20, 1993, 1181–88. Kucuk1 N., Cakir M., Isitman N.A. Mass attenuation coefficients, effective atomic numbers and electron densities of some polymers. Radiat. Prot. Dosim. 153, 2013, 127–134. Kumar R., Sharma S.D., Despande S., et al. Acrylonitrile Butadiene Styrene (ABS) plastic-based low cost tissue equivalent phantom for verification dosimetry in IMRT. J Appl Clin Med Phys. 11 (1), 2010, 24–32. Ma C.M., Rogers D.W., Walters B.R., DOSXYZnrc Users Manual (Ottawa: National Research Council of Canada), 2010. Manohara S.R., Hanagodimath S.M. Effective atomic numbers for photon energy absorption of essential amino acids in the energy range 1 keV to 20 MeV. Nucl. Instrum. Methods Phys. Res. B 264, 2007, 9–14. Mijnheer B.J., Battermann J.J., Wambersie A., What degree of accuracy is required and can be achieved in photon and neutron therapy? Radiother. Oncol. 8, 1987, 237–252. Moradi F., Khandaker M.U., Al Refae T., Ramazanian H., Bradley D.A., Monte Carlo simulations and analysis of transmitted gamma ray spectra through various tissuephantoms.Appl. Radiat Isot. 146, 2019, 120–126. Murphy S.V., Atala A. 3D bioprinting of tissues and organs. Nat. Biotechnol. 32, 2014, 773–85. Oderine O.M. and du Plessis F. Technical note: A new wedge-shaped ionization chamber component module for BEAMnrc to model the integral quality monitoring system®. Radiat. Phys. Chem. 141, 2017, 346–351. PEGS4. User Manual. Available from: http://www.fnal.gov/docs/products/egs4/pegs4.html. [Internet –last accessed 18 March 2017]. Podgorsak E.B. Radiation Oncology Physics: A Handbook for Teachers and Students, IAEA, Vienna, 2005, 451–458. Saur S., Frengen J., GafChromic EBT film dosimetry with flatbed CCD scanner: a novel background correction method and full dose uncertainty analysis.Med. Phys.,35 (7), 2008, 3094– 3101. Schoenfeld A.A, Harder D., Poppe B., Chofor N. Water equivalent phantom materials for 192Ir brachytherapy. Phys. Med. Biol. 60, 2015, 9403–9420.
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Thwaites D. Accuracy required and achievable in radiotherapy dosimetry: have modern technology and techniques changed our views? Journal of Physics: Conference Series 444, 2013 012006. van Battum L.J., Hoffmans D., Piersma H., Heukelom S. Accurate dosimetry with GafChromic EBT film of a 6 MV photon beam in water: what level is achievable? Med. Phys. 35 (2), 2008, 704–716. van der Merwe D., Van Dyk J., Healy B., Zubizarreta E., IzewskaJ., Mijnheer B., Meghzifene A. Accuracy requirements and uncertainties in radiotherapy: a report of the International Atomic Energy Agency, Acta Oncologica, 56, 2017,1–6.
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Figure Captions Figure 1: The heterogeneous pelvic phantom made out of Nylon-12 layers. The phantom is equipped with hip Ti prosthesis and bony structures as described by Ade and du Plessis (2017). Figure 2: A diagrammatic setup of narrow-beam geometry for 6 and 15 MV transmission measurements through slabs of Nylon-12, Perspex and water-equivalent RW3 materials. Figure 3: Simulation geometry consisting of a point source and phantom materials including water, Nylon-12 and RW3. The SSD was set at 100 cm for a 10 × 10 cm2 field. Figure 4: Simulation geometry with the heterogeneous pelvic phantom consisting of two scenarios: (1) Nylon-12; and (2) water. Figure 5: 6 MV transmission factors as a function of thickness for various phantom materials measured using an Elekta Synergy Linac. Differences between water and Nylon-12 ( ), water and RW3 (+ +), and water and Perspex (*) are shown on the secondary vertical axis. Figures 6(a–c): Comparisons of 10 × 10 cm2 6 MeV (a), 12 MeV (b) and 20 MeV (c) electron beam depth-dose data measured in water with a Roos ion chamber and in Nylon-12 slabs with Gafchromic EBT3 films at 95 cm SSD. The 6 and 12 MeV beams were produced the Elekta Synergy Linac whilethe 20 MeV beam was generated by the SL25 Linac. Figure7: DOSXYZnrc Monte Carlo-based PDD data in water, RW3 and Nylon-12 for photon spectra ranging from 250 kV–15 MV computed for a 10 × 10 cm2 field at 100 cm SSD. The dose differences are with respect to water, and the average values are for the five calculations/spectra. Figure 8: DOSXYZnrc Monte Carlo-based 10 MV PDD data computed in the heterogeneous pelvic phantom for a 10 × 10 cm2 field at 100 cm SSD. Dosimetry was performed for two scenarios (consisting of Nylon-12 and water) to establish the water-equivalence of the phantom. Figure 9: The stopping power ratios of various plastic materials (Nylon, Perspex and Polystyrene) relative to water for photon spectra (∆ = 10 keV) ranging from 2–45 MV. The stopping power ratios are extracted from the Table 4 of the AAPM TG-21 (1983) and analysed in this study to show the relative differences between water and the various plastic materials.
15
% Difference
Nylon-12
100
10
80
8
60
6
40
% Difference
120
12
100
10
80
8
60
6
4
40
4
20
2
20
2
0
0
0
0
1
2
3
4 5 6 Depth in phantom (cm)
(a)
0
2
4
6
(b)
Nylon-12
Water
% Difference
120
12
100
10
80
8
60
6
40
4
20
2
0
0 0
(c)
2
4
6
8
10
12
14
16
Depth in phantom (cm)
18
Dose difference (%)
0
Relative dose (%)
12
Dose difference (%)
120
Water
8 10 12 Depth in phantom (cm)
Dose difference (%)
Water
Relative dose (%)
Relative dose (%)
Nylon-12
Highlights The water-equivalence of Nylon-12 as a phantom material is characterized. Electron energies (6–15 MeV) and photon spectra (250 kV–15 MV) were studied. Mass attenuation coefficient values of Nylon-12 and water differ by < 3%. Percent depth doses obtained in water and Nylon-12 slabs agreed within 2%. Nylon-12 exhibits good water-equivalence as a dosimetric material.
1