Radiation Measurements 96 (2017) 42e52
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Performance tests and comparison of microdosimetric measurements with four tissue-equivalent proportional counters in scanning proton therapy d _ J. Farah a, b, *, M. De Saint-Hubert c, N. Mojzeszek , S. Chiriotti c, M. Gryzinski e, O. Ploc f, a f c d F. Trompier , K. Turek , F. Vanhavere , P. Olko ^le Radioprotection de l’Homme, BP-17, 92260 Fontenay-aux-Roses, France Institut de Radioprotection et de Sûret e Nucl eaire (IRSN), Po ^pitaux Universitaires Paris-Sud, Po ^le Imagerie et M ^pital du Kremlin-Bic^ Ho edecine Nucl eaire, Ho etre, 78 Rue G en eral Leclerc, 94270 Le Kremlin-Bic^ etre, France c Belgium Nuclear Research Center (SCK-CEN), Boeretang 200, BE-2400 Mol, Belgium d Institute of Nuclear Physics (IFJ-PAN), Radzikowskiego 152, 31-342 Krakow, Poland e National Centre for Nuclear Research (NCBJ), ul. Andrzeja Sołtana 7, 05-400 Otwock, Swierk, Poland f r ce 39/64, 180 00 Praha 8, Czechia Nuclear Physics Institute (NPI), Department of Radiation Dosimetry, Na Truhla a
b
h i g h l i g h t s Intercomparison of four commercial TEPCs in standard and complex radiation fields. Methodology to convert propane to propane-TE spectra and extrapolate low-LET events. TEPCs provide critical data to monitor proton therapy's stray radiation environment. HAWKs' proton edge position, set by the manufacturer, remains controversial. Calibration and processing of TEPCs involve several challenges and open questions.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2016 Received in revised form 25 November 2016 Accepted 14 December 2016 Available online 15 December 2016
This paper compares the performance of four different Tissue-Equivalent proportional counters (TEPC) first in standard radiation fields, with gamma and neutron sources, then in the mixed and complex/ intense neutron and photon stray radiation field of a scanning proton therapy facility. The paper focuses on the dead time correction and introduces a new spectra processing methodology to enable the comparison of the four TEPCs while accounting for their different gas filling and gain, lineal energy range of the spectrum and the analysis methodology. Measurements with 137Cs and/or 60Co gamma sources demonstrate variable low-LET threshold for each TEPC while data acquired with a 252Cf neutron source show comparable response of the four TEPCs for high-LET particles. Meanwhile, in the scattered field of proton therapy, microdosimetric spectra measured at different positions and orientations around the patient show a majority of high-LET events at the smallest angle with respect to the beam axis while lowLET particles were mainly dominant at 90 from the beam axis. The introduced processing methodology led to good overlapping of microdosimetric spectra for the four systems. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Tissue-equivalent proportional counters Microdosimetry Proton therapy Stray neutrons and photons
1. Introduction
^pitaux Universitaires Paris-Sud, Po ^ le Imagerie et * Corresponding author. Ho decine Nucle aire, Ho ^pital du Kremlin-Bice ^tre, 78 Rue Ge ne ral Leclerc, 94270 Le Me ^tre, France. Kremlin-Bice E-mail address:
[email protected] (J. Farah). http://dx.doi.org/10.1016/j.radmeas.2016.12.005 1350-4487/© 2016 Elsevier Ltd. All rights reserved.
Microdosimetric measurements with Tissue-Equivalent proportional counters (TEPC) are a well-established method for sound dosimetry in complex radiation fields (Rossi and Zaider, 1994; Waker, 1995; Siebers et al., 1992; Bottollier-Depois et al., 2004). The advantage of using TEPCs compared to other dosimeters is their capability to measure the lineal energy (y) spectrum which enables
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the characterization of the Lineal Energy Transfer (LET) in mixed radiation fields. TEPCs are also known to have a quite stable neutron dose equivalent response (within 15%) for neutrons below 200 MeV (Alexeev et al., 1998) and are therefore of particular interest for applications in the medical field. In proton therapy for example where secondary neutrons of hundreds of MeV may be encountered, the microdosimetric approach enables the measurement of radiation dose deposition in tissue-equivalent material as well as the evaluation of radiation quality factors and help deriving good estimates of the Relative Biological Effectiveness (RBE) of the therapeutic beam (Loncol et al., 1994; Binns and Hough, 1997; Coutrakon et al., 1997; De Nardo et al., 2004; Agosteo et al., 2008; Yonai et al., 2010; Rollet et al., 2011). In addition, TEPCs have been used for shielding studies and to monitor the stray radiation environment inherent to both scattering and scanning proton therapy facilities (Perez-Andujar et al., 2012; Farah et al., 2014, 2015). Finally, the rather simple implementation of TEPCs is also a major asset in characterizing proton therapy's stray radiation environment when compared to cumbersome and complex Bonner Sphere spectrometry systems (BSS). However, a deep knowledge of the radiation physics and interaction mechanisms at the micro metric scale is required to make full use of TEPCs and the microdosimetric approach; this is less critical when using regular and extended-range proportional counters. Additionally, TEPCs suffer from an extreme sensitivity to measurement conditions which can strongly affect the lineal energy spectrum. A known effect due to dead time is the spectra distortion due to pile up of pulses (Aslam et al., 2011). Finally, TEPCspecific physical properties and acquisition features such as gas filling, site size, gas gain settings, etc. render the intercomparison of microdosimetric data difficult when different TEPCs are at use. All these considerations strongly impact the overall measurement uncertainties especially in the intense stray radiation field of proton therapy. This work addresses the above mentioned challenges while comparing four of the most commonly used TEPCs including three different versions of the HAWK system and one LET-SW5 TEPC, all manufactured by the Far West Technology (FWT Inc., USA). The paper first recalls the different calibration procedures, introduces a new dead time correction approach and focuses on the spectra processing methodology for the comparison of the four TEPCs. Results shown here involve measurements done in standard radiation fields using 137Cs and/or 60Co gamma sources to check the low-LET threshold of each TEPC. Measurements with 252Cf and 241 Am-Be neutron sources are also shown to visualize the position of the proton edge in the presence of high-LET particles. Finally, measurement results in the stray radiation field of a spot scanning proton beam are shown while simulating a brain tumor treatment and considering a 5-years old anthropomorphic phantom. In this case, the paper also compares TEPC H values against H*(10) data from other instruments such as the extended-range Wendi-II remcounter (Olsher et al., 2000). 2. Materials and methods 2.1. TEPC systems In this study, three HAWK systems (versions 1, 2 and 3) and one LET-SW5 TEPC were used (Cf. Table 1). These systems are all made of A-150 conducting tissue equivalent plastic, have identical spherical volume (12.7 cm-diameter) and simulate a site size of 0.2 mg/cm2, corresponding to a site diameter of 2 mm at density of 1 g/cm3. The four TEPCs are however physically different in their gas composition and the acquisition shows differences in gas gain,
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energy binning and resolution, etc. HAWK TEPCs have an integrated system for the processing of microdosimetric spectra and absorbed/ equivalent dose calculations and display while the LET-SW5 TEPC requires manual data processing. 2.2. Calibration, dead time correction and spectra analysis 2.2.1. Lineal energy calibration TEPCs measure the pulse height spectrum due to charged particles that ionize the gas. A multichannel analyzer (MCA) measures the pulse size detected and processes the signal. After processing the raw TEPC data is possible to obtain the so-called microdosimetric spectrum in which the dose distribution is usually plotted as yd(y) against log(y) (ICRU, 1983). Specific features of the microdosimetric spectrum such as the electron, proton and alpha edges can be used for calibration purposes. The proton edge corresponds to the maximum energy imparted by a recoil proton in a given cavity and is generally used for calibration of the y scale. In fact, the raw pulse height spectra are calibrated versus lineal energy (y) by adjusting the y scale to the proton edge position (yp-edge). According to the manufacturer, the proton edge should rise at 150 keV mm1 in lineal energy for a 0.2 mg cm2 mass site size cavity filled with C3H8 (HAWKs). Meanwhile, for LET-SW5 with an identical site sized cavity filled with C3H8-TE, the proton edge should be at 136 keV mm1. For HAWKs, during the initial calibration performed by the manufacturer, the gas gain is adjusted by changing the high voltage of the TE plastic chamber so that channel 100 in the spectrum corresponds to ~150 keV mm1. Then, as the gas gain voltage is fixed and becomes inaccessible for HAWK end-users, these check the calibration adequacy of their system before each measurement by verifying the position of the proton edge using an internal 244Cm source. When a shift in the proton edge position is observed, the TEPC should be sent back to FWT for refilling the gas counter and subsequent re-calibration. Alternatively, for the LET-SW5 TEPC, the microdosimetric spectra calibration adjustment can be performed by the user with an internal 244Cm source emitting 5.8 MeV alpha particles (Schrewe et al., 1988). The imparted energy, Ɛ, by this source is known (172 keV in a 0.2 mg cm2 mass site size) and is used to convert channels to y (keV mm1) using the following relationship which holds for a spherical TEPC: y ¼ 2εd where d is the 3 simulated site size. 2.2.2. Dead time correction HAWK TEPCs are well known to suffer from an extreme sensitivity at measurement conditions where a dead time value > 25% can lead to large distortions in the lineal energy spectrum (Aslam et al., 2011). It is hence necessary to compute the dead time value whenever high dose rates may be encountered as in the proton therapy field. For HAWK systems, the dead time value is calculated from the number of counts using the equation provided by FWT in the users' manual: dead time ¼ 1/[1-(N*1.07*106)] where N is in counts per minutes. HAWKs v. 2 and v. 3 automatically compute this dead time correction factor while this is not the case for HAWK v. 1. Absorbed dose and equivalent dose values are then simply multiplied by this coefficient to correct for dead time. However, as the relationship between the number of counts and the dose is not linear, especially considering the different particle types and LETs, such a correction is not fully satisfactory. A proper management of dead time would indeed require an appropriate correction of microdosimetric spectra differently for low and high-LET events. Hence in this work, the number of low-LET and high-LET counts was separately used to calculate two dead time correction factors which were then specifically used to multiply the high-gain (low-
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Table 1 Characteristics of the four TEPC systems involved in this study. HAWK v.1 Physical properties
Acquisition system
Size of detector Gas filling
12.7 cm diameter sphere Pure propane (~99%)
Gas pressure Mass site size Stopping powera (5.8 MeV alpha particles) Lineal energy of the proton edgeb Container geometry
7 Torr 0.2 mg/cm2 955.6 MeV cm2 g1
Container material Container thickness Gas gain
Energy binning
Resolution Data processing a b
HAWK v.2
HAWK v.3
LET-SW5
Propane-based TE with 55% of C3H8, 39.6% of CO2 and 5.4% of N2 6.6 Torr 839.1 MeV cm2 g1
161 keV mm1 Cylindrical (d ¼ 15.6 cm, h ¼ 17.5 cm) Stainless steel 0.0635 cm Includes 2 linear multi-channel analyzers (MCA): high gain for Low-LET particles and low gain for High-LET particles 0.1 keV mm1 for high 0.1 keV mm1 for high gain channels gain channels and 1 keV mm1 for and 1.5 keV mm1 for low gain channels low gain channels 256 channels for the high gain 1024 channels for the low gain Automated/semi-automated
136 keV mm1 Cylindrical (d ¼ 15.2 cm, h ¼ 17.1 cm) Vacuum tight, aluminum wall 0.127 cm Fully adjustable linear MCA; one gain for the whole spectrum
Same binning at high/low part; 0.07 keV mm1
8192 channels Fully manual
Source NIST website: http://physics.nist.gov/PhysRefData/Star/Text/ASTAR.html. Chiriotti et al. (2015a) using ICRU Report 49 (1993).
LET particles) and the low-gain counts (high-LET particles). Microdosimetric spectra were processed again to determine the dosimetric quantities. This analysis was made on each HAWK TEPC record (minute-by-minute analysis) which rendered the spectra processing far more laborious and time consuming. 2.2.3. Spectra analysis and computation of dosimetric quantities In this study, an in-house Matlab routine based on ICRU 36 equations (1983) and Q factor from ICRP 60 (1991) was used to process the TEPC data and obtain the microdosimetric spectrum. This routine can also be used to check the position of the proton edge using a Fermi-fitting function in which the yp-edge is determined by the marker point corresponding to the intercept of the tangent through the inflexion point with the horizontal axis (Moro et al., 2015). When analyzing microdosimetric spectra, events above the proton edge are due to the formation of recoil heavy charged particles (a, N, C, O, Be, B) which occur mainly following the interaction of high energy (>20 MeV) intranuclear cascade neutrons (INC) with the elements of the A-150 wall of TEPC (Schrewe et al., 2000). Events lower than 10 keV mm1 are generally considered to be gamma-induced (Silari et al., 2009) although, in this region, also thermal and low-energy neutrons may contribute to the signal as well as high energy protons including recoil protons by high energy neutrons (Yonai et al., 2010). TEPCs also allow the evaluation of several dosimetric quantities including absorbed dose (D), dose equivalent (H) and doseaveraged quality factor (QD) and also micro-dosimetric quantities such as the frequency-mean lineal energy (yF ) and dose-mean lineal energy (yD ). These quantities are computed using the general microdosimetric equations described in ICRU 36 (1983) which consider the chord length and the mass of the gas to determine the dose deposition. For HAWK systems, all dosimetric quantities are automatically calculated by the data processing routine provided by FWT while the previously described Matlab routine is used for the LET-SW5 system as no software is provided by the manufacturer for this TEPC. To further compare the different TEPC models, yF and yD were calculated with the Matlab routine.
2.3. Spectra processing for intercomparison Considering the differences between the HAWKs and the LETSW5 TEPC, highlighted in Table 1, three corrections were made to microdosimetric spectra acquired with 252Cf and 241Am-Be sources and in the proton therapy field to allow the comparison of microdosimetric spectra: (1) Rescaling the response of pure propane TEPCs (HAWKs) to a propane-based TE counter (like the LET-SW5 TEPC). Chiriotti et al. (2015b) showed that a TEPC filled with propane (C3H8) has almost the same response function as a C3H8-TE if the mass per area in pure propane is reduced by a factor of 0.75. Using this factor, the equivalent site size of HAWK devices (filled with C3H8) were actually 2.6 mm compared to 2 mm for the TEPC filled with C3H8-TE. As yp-edge values depend on the site size, for a 2.6 mm site size cavity, the proton edge is calculated to rise at 131 keV mm1 (Moro et al., 2015). The spectra for C3H8 were calibrated according to the new yp-edge value (starting from the manufacturer proton edge position at 150 keV mm1) to allow the intercomparison of the pure propane TEPCs (HAWKs) to a propane-based TE counter. (2) In addition, the systems have a threshold for low lineal energy events which is higher for the LET-SW5 TEPC compared to HAWKs. To avoid underestimating gamma radiation, counts at the lowest lineal energies were hence extrapolated for the LET-SW5 TEPC with a constant positive slope as described in (Varma, 1983). This enabled to set the lineal energy range for all spectra to a comparable scale. (3) For HAWKs, the system provides the number of counts in each of the 2 linear multi-channel analyzers separately. To obtain a microdosimetric spectrum, it is hence necessary to combine the counts in the low and high gain channels. Again, as the HAWK manufacturer does not provide a clear recommendation for this process, low/high gain counts are manually merged (for all versions of the system) while normalizing the number of counts in low/high gain channels by the channel width (binning). This method however
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requires the smoothing of merged spectra at the low/high gain limit to suppress undesired peaks. This smoothing was performed using a linear interpolation on the logarithmic yd(y) spectra between 18 and 45 keV mm1 for HAWK v.2 and v.3 and between 14 and 45 keV mm1 for Hawk version 1. As a consequence of this spectra processing methodology, yF values are expected to decrease for HAWKs due to the rescaling of their response from pure propane to propane-based TE. For the LET-SW5 TEPC, yF values are also expected to decrease following spectra processing due to the interpolation of low-LET values. Meanwhile, as the dose-average lineal energy is dominated by the high-LET neutron component, spectra processing is expected to have a limited impact on yD values. 2.4. Irradiation conditions and TEPC intercomparison 2.4.1. Standard gamma and neutron radiation fields First, to check the threshold of each TEPC for low-LET radiation, measurements using 137Cs and/or 60Co sources were performed at secondary standard dosimetry laboratories. Additionally, the response of the four TEPCs to high-LET neutron particles was checked at the National Centre for Nuclear Research (NCBJ, Poland) using 252Cf and 241Am-Be sources (cf. Fig. 1). As a shadow cone was not available during these experiments, the measured 252Cf and 241 Am-Be microdosimetric spectra may be contaminated with room-scattered neutrons emerging from the floor and walls. For all these measurements, the minimum TEPC integration time was long enough (>20 min, depending on the activity of the source) to maintain statistical uncertainties on H values within 1%. Microdosimetric spectra acquired with these TEPC were hence compared. 2.4.2. Complex/mixed radiation field of proton therapy TEPCs were also used to monitor proton therapy's stray radiation environment. Experiments were performed at the Bronowice Cyclotron Center (CCB) - Poland, where a Proteus C-235 cyclotron, designed by IBA (Ion Beam Applications S.A., Belgium), is installed. Protons of up to 230 MeV can hence be delivered to the clinical target volume using the Pencil Beam Scanning (PBS) technology where steering magnets are used to deflect and adjust the proton beam to the target volume. PBS allows a high degree of precision and minimizes the overall stray radiation to healthy tissue compared against conventional scattering proton therapy (Farah et al., 2015). In these experiments, a 5-years-old anthropomorphic phantom (CIRS Inc., USA) was irradiated using a proton pencil beam of 3 mm-spot-Gaussian sigma to target a 5 cm-diameter intracranial tumor located in the center of the brain. Proton energies ranged from 85.9 MeV (minimum energy) to 138.5 MeV (maximum energy layer). The beam incidence was set at 270 (right lateral field) while the phantom was set 90 with respect to beam
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axis (see Fig. 2). A total of ~123 Gy were delivered to the target volume in ~6 min of beam time. Microdosimetric measurements were carried out at several distances (with respect to isocenter) and angular positions (with respect to the beam axis) around the phantom (see Fig. 2). Each detector was aligned horizontally with the treatment isocenter at a height of ~1.5 m from the floor and almost same distance from the ceiling. The nearest wall was about 2 m away from the detectors. Room walls, floor and ceiling are expected to affect the thermal and epithermal neutron components (Sayah, 2012); however, their effect should be limited under these experimental conditions. Microdosimetric spectra and dosimetric quantities measured with the four TEPCs were compared against each other and against photon and neutron H*(10) values measured with the FH-40G detector (Thermo Scientific™, USA) associated to the extended-range Wendi-II rem-meter (Olsher et al., 2000).
2.5. Measurement uncertainties The calibration of TEPCs represents a major source of uncertainties. The alpha calibration method described in Section 2.2.1 and used for the LET-SW5 TEPC can introduce an overall uncertainty of 10% as demonstrated by Schrewe et al. (1988). Statistical uncertainties of TEPC counts represent a second source of uncertainties. This uncertainty component can however be reduced by considering long counting periods. This is feasible when using regular radiation sources but can be difficult to achieve during the proton therapy irradiation where the high proton dose rate (and consequently the high dose rate of stray radiations) limits the irradiation time to few minutes. To compute this statistical uncertainty, a Poisson statistics was applied to the counts of each channel which provides an uncertainty estimate for low-LET and high-LET events separately at each position around the source/ anthropomorphic phantom. The spectra processing methodology, described in Section 2.3, to allow the comparison of pure propane and propane-TE microdosimetric spectra also introduces uncertainties. Namely, for the LET-SW5 TEPC the extrapolation of counts at the lowest lineal energies adds uncertainties on gamma radiation which can be calculated following Moro et al. (2003). Finally, beam delivery and detector positioning errors can add up to the overall measurement uncertainties. Beam delivery uncertainties are imperatively within 5% to guarantee therapeutic accuracy and the precision of the proton dose at the target volume. Meanwhile, positioning errors and misplacements of 1e2 cm/1e2 should have a limited effect with an uncertainty remaining within 1%.
Fig. 1. Schematic view of the experimental setup used for the irradiation of TEPCs in reference neutron radiation fields; similar conditions were used for 137Cs and/or 60Co sources.
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Fig. 2. Schematic representation of irradiation conditions and TEPCs' setup positions.
3. Results and discussion 3.1. TEPC intercomparison in standard radiation fields 3.1.1. Low-LET threshold check with gamma sources Measurements with 137Cs and/or 60Co sources at a nominal dose rate of 10 mSv h1 were first conducted to determine the low-LET threshold specifically for each TEPC. Fig. 3 shows that HAWK v. 1 and v.2 have a threshold of 0.35 keV mm1. The LET-SW5 TEPC has the highest threshold for low-LET particles at 1.7 keV mm1. This will result in a strong under-response of the LET-SW5 TEPCs for the low-LET radiation. Also due to this threshold, the LET-SW5 TEPC normalized spectrum (yd(y) values) is higher than the HAWK normalized values. From Fig. 3, little difference is visible on the microdosimetric spectra for 137Cs and 60Co although one would expect slight differences considering the physical properties of both sources (Moro and Chiriotti, 2015); this is most likely due to the large binning size, especially for HAWK systems. Finally, one can see on this figure that the electron edge of pure propane TEPCs (HAWKs) is almost equivalent to that of propane-TE (LET-SW5) although larger differences are expected. This can again be partially explained by the use of a logarithmic scale and the low resolution of the plots. One additional explanation could be that the spectra of Hawks are calibrated by the manufacturer to set the proton edge at 150 keV um1 instead of 161 keV um1 (ICRU 49).
SW5 TEPC was again found to have a higher threshold compared to HAWK versions 1, 2 and 3. In addition, while focusing on the proton edge position, one can see that prior to spectra processing, the proton edge is located at 178 keV mm1 for both HAWK versions 1 and 2, at 143 keV mm1 for HAWK version 3 and at 134 keV mm1 for LET-SW5 TEPC. Such values are far from the position indicated by the manufacturer, i.e. 150 keV mm1, for pure-propane TEPCs (Waker and Aslam, 2010), which indicates that the gas pressure or the gas gain of HAWKs v. 1 and v.2 may have changed and thus these TEPCs require a gas re-filling and subsequent calibration. In this case, these TEPCs tend to over-estimate the high-LET component and a correction factor of ~18% (178/150) should be used to correctly compute the dosimetric quantities. After spectra processing, a better agreement between all spectra was found even if some differences could be observed due to the different simulated site sizes. Indeed the data processing involved: (1) rescaling of C3H8 yp-edge position to 131 keV mm1 to allow equivalence of to C3H8-TE gases, (2) extrapolating LET-SW5 to lower y-values down to 0.01 keV mm1 and (3) interpolation of HAWK spectral data in the merging region (see section 2.3). Microdosimetric spectra measured with the four TEPCs agree with literature data (Dicello et al., 1972) where a 252Cf source was found to present a single peak at ~60 keV mm1 which is mainly due to proton recoils (up to about 150 keV mm1) produced by elastic scattering. The higher yvalues are due to heavy recoil ions produced by elastic (C, N and O) or inelastic (He) scattering. Similar performance is observed for the four TEPCs with the 241Am-Be source (cf. Fig. 5).
3.1.2. High-LET performance check with neutron sources Fig. 4 shows the 252Cf microdosimetric spectra measured with the four TEPC systems before and after the data processing methodology. When looking at the unprocessed data (Fig. 4 left), LET-
3.2. TEPC intercomparison in the mixed/complex field of proton therapy
Fig. 3. Microdosimetric spectra measured with HAWK v.1, v.2 and v.3 (10 bins/decade) and LET-SW5 (25 bins/decade) considering 137Cs and/or 60Co sources.
3.2.1. Microdosimetric spectra analysis Fig. 6 shows microdosimetric spectra measured at positions A (45 , 225 cm), B (90 , 225 cm), C (135 , 225 cm) and D (270 150 cm) around the anthropomorphic phantom irradiated with a scanning proton beam; these spectra are acquired with the LETSW5 TEPC (left) or HAWK v.3 (right) and plotted after processing. Fig. 6 hence proves that spectra acquired at positions A and D are similar for y < 100 keV mm1 showing a peak around 5 keV mm1 mainly due to low-LET recoil protons from fast neutrons together with a significant contribution to the absorbed dose of particles of high lineal energies, up to 1000 keV mm1. For position A, the contribution of INC neutrons to the dose is most pronounced and this is expected considering the low angle of this position with respect to the beam direction (Farah et al., 2015; Mares et al., 2016). Meanwhile, spectra measured at positions B and D show different components although both positions are symmetrical with respect to the isocenter but subject to different attenuation due to the
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Fig. 4. 252Cf microdosimetric spectra measured with HAWK v.1, v.2 and v.3 (10 bins/decade) and LET-Sw5 (25 bins/decade) considering unprocessed spectra (left) and processed spectra (right).
Fig. 5. 241Am-Be microdosimetric spectra measured with HAWK v.1, v.2 and v.3 (10 bins/decade) and LET-Sw5 (25 bins/decade) considering unprocessed spectra (left) and processed spectra (right).
Fig. 6. Microdosimetric spectra measured with LET-SW5 (25 bins per decade) (left) and HAWK v.3 (10 bins per decase) (right) at positions A (45 , 225 cm), B (90 , 225 cm), C (135 , 225 cm) and D (270 150 cm) around the phantom irradiated with a scanning proton beam. Spectra were processed as described in Section 2.3.
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phantom orientation (higher in B direction). Position B is characterized by two distinct peaks where the first peak is at 4 keV mm1 and can be mainly attributed to secondary gammas generated inside the phantom. Trompier et al. (2007) also suggested the presence of scattered protons. The second peak is around 90 keV mm1 and has a sharp proton edge indicating that the main contribution to the spectra is due to lower energy recoil protons generated following the interactions of neutrons of a few MeV in the gas; such neutrons are the result of the high energy neutron slowing-down inside the phantom. These peaks are less pronounced at position D further confirming this analysis. Additionally, one has to keep in mind that at high angles (positions B, C and D), most of the internal neutrons are of lower energies and they are also slowed-down by the phantom (Agosteo, 2009). Spectra measured at position C demonstrates comparable features to those of position B, with the main peak at 60 keV mm1, where a sharp proton edge is visible together with a low contribution of INC neutrons in the high LETpart of the spectra. It should also be noted that the lower contribution of the low-LET particles in position C when compared to position B which can be explained by the thermalization of neutrons and the creation of secondary gammas inside the phantom. Meanwhile, Fig. 7 intercompares the microdosimetric spectra measured with the four TEPC systems and all positions showing good agreement for the four systems. This agreement is strongly due to the spectra processing methodology introduced in Section
2.3 which improved the low-LET threshold (for LET-SW5 TEPC) and adjusted the position of the proton edge (with highest impact for HAWKs v.1 and v.2). Finally, unprocessed and processed yF and yD values are given in Tables 2a and 2b highlighting the impact of the introduced spectra processing methodology and showing that yF and yD are highest at position A. The higher dose-mean lineal energy, yD can be explained by the larger contribution of high energy neutrons in position A. Statistical uncertainties on yD and yF values remain below 5% at all positions and with all used TEPCs. Extrapolation uncertainties calculated following Moro et al. (2003) for the LET-SW5 TEPC were below 2% at all positions. Hence, the overall measurement uncertainty on yD and yF values is largely affected by the calibration component (10%) and its maximum value is about 11.7% for the LET-SW5 TEPC set at position A. A similar overall uncertainty value is obtained for HAWKs. 3.2.2. Dosimetric quantities analysis Starting from the measured microdosimetric spectra and using the in-house Matlab routine, the dosimetric quantities were derived including the dead time value, quality factor, HLow (due to events below 10 keV mm1) and HHigh (due to events above 10 keV mm1) obtained for each TEPC. These are given in Tables 3 and 4. Based on the minute by minute analysis of TEPC counts, it was seen that low-LET counts (conventionally associated to gamma particles) were up to one order of magnitude larger than high-LET
Fig. 7. Microdosimetric spectra measured with HAWK v.1, v.2, v.3 (10 bins per decade) and LET-SW5 (25 bins per decade) at positions A (45 , 225 cm), B (90 , 225 cm), C (135 , 225 cm) and D (270 150 cm) around the phantom irradiated with a scanning proton beam. Spectra were processed as described in Section 2.3.
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Table 2a yF and yD values for the unprocessed spectra at positions A-D. Unprocessed (keV mm1)
Position A
Position B
yF
HAWK v.1 HAWK v.2 HAWK v.3 LET-SW5
yD
5.4 4.6 5.6 8.6
± ± ± ±
0.6 0.5 0.6 1.0
66.3 71.1 61.3 51.0
± ± ± ±
7.5 8.0 6.9 6.1
Position C
Position D
yF
yD
yF
yD
yF
5.0 ± 0.5 e 5.2 ± 0.5 7.7 ± 0.9
42.1 ± 4.8 e 42.6 ± 4.8 38.7 ± 4.7
4.5 ± 0.5 e 5.1 ± 0.6 10.0 ± 1.1
55.2 ± 6.3 e 48.7 ± 5.5 41.5 ± 5.0
4.4 3.4 4.4 7.2
yD ± ± ± ±
0.5 0.4 0.5 0.8
41.6 46.8 41.8 36.9
± ± ± ±
4.7 5.3 4.8 4.3
± ± ± ±
4.1 4.6 4.1 3.8
Table 2b yF and yD values for the processed spectra at positions A-D. Processed (keV mm1) HAWK v.1 HAWK v.2 HAWK v.3 LET-SW5
Position A
Position B
yF
yD
4.6 4.0 4.7 5.7
± ± ± ±
0.5 0.4 0.5 0.6
59.4 61.6 53.7 51.6
± ± ± ±
6.7 6.9 6.0 5.6
Position C
Position D
yF
yD
yF
yD
yF
3.7 ± 0.4 e 3.9 ± 0.4 4.2 ± 0.5
36.4 ± 4.1 e 37.6 ± 4.3 35.4 ± 4.1
3.7 ± 0.4 e 4.3 ± 0.5 4.7 ± 0.5
53.9 ± 6.2 e 42.6 ± 4.8 38.1 ± 4.4
3.7 2.9 3.7 4.2
yD ± ± ± ±
0.4 0.3 0.4 0.4
36.4 41.1 36.3 34.2
Table 3 Dosimetric data measured with the four TEPCs at positions A (45 , 225 cm) and B (90 , 225 cm) around the anthropomorphic phantom together with H*(10) values (mSv Gy1) measured with the FH-40G counter (gamma) and the Wendi-II rem-meter (neutron). A (45 , 2.25 m)
Position H*(10) values (mSv Gy
1
)
B (90 , 2.25 m)
Gamma ¼ 0.11; Neutron ¼ 1.83
TEPC
Quality factor
HAWK v.1 HAWK v.2 HAWK v.3 LET-SW5
7.5 7.6 7.8 8.0
± 0.83 ± 0.88 ± 0.85 ± 0.87
HLow (mSv Gy1) 0.11 0.13 0.08 0.08
± 0.012 ± 0.015 ± 0.009 ± 0.010
Gamma ¼ 0.03; Neutron ¼ 0.55 HHigh (mSv Gy1) 1.78 2.17 2.05 1.59
± ± ± ±
0.20 0.25 0.24 0.18
Quality factor
HLow (mSv Gy1)
HHigh (mSv Gy1)
7.6 ± 0.85 e 7.7 ± 0.84 7.9 ± 0.82
0.03 ± 0.003 e 0.02 ± 0.002 0.02 ± 0.002
0.52 ± 0.06 e 0.50 ± 0.06 0.42 ± 0.05
Table 4 Dosimetric data measured with the four TEPCs at positions C (135 , 225 cm) and D (270 , 150 cm) around the anthropomorphic phantom together with H*(10) values (mSv Gy1) measured with the FH-40G counter (gamma) and the Wendi-II rem-meter (neutron). C (135 , 2.25 m)
Position H*(10) values (mSv Gy
1
)
D (270 , 1.50 m)
Gamma ¼ 0.04; Neutron ¼ 0.79
Gamma ¼ 0.12; Neutron ¼ 1.69
TEPC
Quality factor
HLow (mSv Gy1)
HHigh (mSv Gy1)
Quality factor
HAWK v.1 HAWK v.2 HAWK v.3 LET-SW5
8.4 ± 0.96 e 8.9 ± 0.97 9.7 ± 0.92
0.04 ± 0.005 e 0.03 ± 0.003 0.02 ± 0.003
0.70 ± 0.08 e 0.62 ± 0.07 0.60 ± 0.07
6.3 6.1 6.3 6.6
counts (conventionally associated to neutron particles). Thus lowLET particles induced highest dead time values of 14% at position A, 4% at position B, 6% at position C and up to 20% at position D. For neutron radiation, the calculated dead time values did not exceed 2% at all measurement positions. Hence, in these specific experimental conditions, no distortions in the lineal energy spectrum due to large pile-up of pulses are expected (Aslam et al., 2011). Additionally, as the dose equivalent is strongly affected by the radiation quality factor, the rather high dead time on low-LET counts is expected to have little impact on the total H value (HLow þ HHigh) which is dominated by HHigh term. In fact, neutron dose equivalent rate values ranged from ~0.6 mSv h1 (at position B) to ~2.3 mSv h1 (at position A) while gamma dose equivalent rate values ranged from ~30 mSv h1 (at position B) to ~150 mSv h1 (at position D). Maximum statistical uncertainties on HLow values were within 10% for LET-SW5 TEPC set at position C while such uncertainties remained within 3% for the other systems and positions; 3% is also the maximum statistical uncertainty on HHigh for all TEPCs. For the
± ± ± ±
0.69 0.75 0.70 0.69
HLow (mSv Gy1) 0.15 0.18 0.12 0.11
± ± ± ±
0.017 0.020 0.014 0.013
HHigh (mSv Gy1) 1.64 1.95 1.86 1.50
± ± ± ±
0.19 0.22 0.21 0.17
LET-SW5 TEPC, the maximum overall measurement uncertainty on HLow values is hence in the order of 15% when adding statistical (10%), alpha calibration (10%), extrapolation (2%), proton beam delivery (5%) and detector positioning (1%) uncertainties. Meanwhile for HAWKs, the maximum overall HLow or HHigh uncertainty falls within 12%. Meanwhile, similar radiation quality factors are obtained with the four TEPC systems at all measurement positions (cf. Tables 3 and 4). Measured Q values range from 5.91 to 9.66 depending on the measurement position with the highest value registered at position C. Statistical uncertainties on Q factor were below 1% at all positions and regardless of the used TEPC. It should also be noted that the largest Q factor at position C does not correspond to the highest yD values which are found at position A (cf. Tables 2a and 2b). This can be explained by the fact that Q factors are a function of lineal energy and have a peak at ~100 keV mm1 (ICRP 60, 1991) while yD values continually increase with lineal energy. In addition, Tables 3 and 4 show a generally good agreement
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among the four TEPCs on H values at all positions. In addition, the variation of H values reflects the impact of the source-detector distance and the angular position with respect to beam axis; in agreement with literature neutron spectrometry findings (Farah et al., 2015; Mares et al., 2016). Indeed, the increased fluence of INCs in the forward direction and their higher energy range justify the largest H and H*(10) values at position A compared to those of positions B and C located at the same distance to isocenter. Moreover, position D involves larger stray radiation compared to position B, although both positions are perpendicular to the beam axis. This is due to the smaller isocenter-detector distance as well as to the reduced phantom attenuation (see phantom orientation) in D direction. The average ratio of H values measured at position D to those at position B was of 5.5 ± 0.5 for photon radiation (HLow) and of 3.5 ± 0.3 for neutron radiation (HHigh). Finally, Tables 3 and 4 show a reasonably good agreement between H values measured with TEPCs and H*(10) values measured with the FH-40G & Wendi-II counters. The difference for gamma radiation can be explained by the low-LET thresholds of TEPCs with the strongest disagreement (factor 2) for the LET-SW5 system. Meanwhile, the energy dependence of the dose equivalent response and measurement uncertainties can explain the deviation on the neutron doses (high-LET events) which is highest for the smallest angles with respect to beam axis (positions A and B). Indeed, while TEPCs are known to show small variations in the dose equivalent response (within ±10%) with neutron energies (Trompier et al., 2007), larger variability (up to ±50%) is observed for the Wendi-II rem-counter both for thermal and fast neutrons (Olsher et al., 2000).
proton therapy's stray neutrons (Alexeev et al., 1998; Trompier et al., 2007). However, this energy dependence has not been determined for the LET-SW5 TEPC. Additionally, this uncertainty cannot be fully acknowledged in the absence of a neutron metrology and dosimetry facility with mono-energetic beams of the right energy range (up to 200 MeV) and with appropriate reference instruments. Thus, the overall TEPC measurement uncertainty on H values is difficult to compute and should fall within 20%; a much lower value compared to other neutron rem-counters (Olsher et al., 2000; Farah et al., 2015) which further highlights the advantage of TEPCs. 3.3.3. Merging low and high-gain channels For HAWK systems, the merging of high and low gain channels remains a real challenge in the absence of clear recommendations from the manufacturer on the optimal method to perform this operation. A simple combination of the data, taking only into account the difference in bin width, leads to the observation of undesired peaks at the merging region. This effect was most pronounced for the proton therapy field probably because of lower counting statistics in this area compared to the performed experiments in reference fields. In fact, this problem is often solved by increasing the bin size which leads to a resolution loss. Hence, a decision was made here to interpolate the data between 18 and 45 keV mm1 for HAWK v.2 and v.3 and between 14 and 45 keV mm1 for HAWK v.1 to maintain a high resolution for all the other values in the microdosimetric spectrum. However, this solution is also not fully satisfactory especially since, in this interpolated region, data are not measured but rather interpolated which increases measurement uncertainties.
3.3. Study limitations and open questions 3.3.1. Calibration check and frequency When using TEPCs, special care should be taken during the calibration process. For HAWK systems, the proton edge position set by the manufacturer (150 keV mm1) does not correspond to the theoretical value of a 0.2 mg cm2 propane based TEPC (expected at 161 keV mm1). This shift on the microdosimetric spectra induces a systematic error on the HHigh values which is not expected with this micro-dosimetric approach. Additionally, the proton edge position can witness a shift from its theoretical value due to changes in gas pressure following strong/intense exposure. Although this drift can be difficult to quantify, it could lead to large errors on the dose estimate and induce therefore high measurement uncertainties (Dietze et al., 1988). Chiriotti et al. (2016) introduced an indirect method to monitor the site size of sealed TEPCs. This method requires performing the calibration with gamma and neutron radiation sources under the same settings and having high statistics. These conditions were not available in the current experiments where neutron and gamma source measurements were performed at different facilities, setups and statistics. Finally, the frequency at which TEPC systems should be calibrated is not defined and the common methodology of using correction coefficients which account for the shift in the proton edge position (verified with the inner alpha source for HAWKs) is questionable. 3.3.2. Measurement uncertainties Measurement uncertainties represent one of the most challenging issues in neutron dosimetry. The analysis done here show a maximum uncertainty of 15% on HLow for LET-SW5 TEPC and 12% for HAWKs. One should however note that this uncertainty estimate does not take into account the energy response of TEPCs which can be a major source of uncertainties. In fact, previous studies have shown the dose equivalent response of HAWK TEPCs to remain quasi flat (within 10e15%) at the entire energy range of
3.3.4. Extrapolation of low-LET events and binning resolution As previously mentioned, low-LET particles with y values below <1.7 keV mm1 were not measured by the LET-SW5 TEPC due its high threshold. An extrapolation with a positive slope was hence used to determine the lowest y values following the method by Varma (1983) since it resulted in the best agreement between all systems. Other methods for extrapolation were described in the literature such as using a zero or a negative slope. Nevertheless, the uncertainty due to the extrapolation of the low-LET events was estimated following the method by Moro et al. (2003) and was found to remain within 2% at all positions around the anthropomorphic phantom. As this extrapolation was only applied to correct for low-LET events when using the LET-SW5 TEPC, it remains of limited impact on the total H value and associated overall uncertainty (dominated by calibration uncertainties and counting statistics). Additionally in this work, the binning resolution was set to 10 bins per decade for HAWKs and 25 bins per decade for LET-SW5 TEPC. Larger binning could not be used to avoid distortions of the microdosimetric spectra. As a result of this low resolution, it is difficult to compute from the plots the position of the observed peaks or the proton edge. For HAWKs data, the proton edge region only involves 3 points so the uncertainty of the fitting can be quite high and was estimated to be around 30%, following Moro et al. (2015). This added new challenges to the data analysis and additional uncertainties to the derived dosimetric quantities. In fact, due to this limited resolution, the “theoretical” proton edge position (150 keV mm1 as given by the manufacturer) was used to rescale the proton TEPC to proton-equivalent TEPC. 3.3.5. Dead time and pile-up corrections In this work, a minute-by-minute management of dead time was considered while differently accounting for low/high-LET events. This lead to the calculation of gamma and neutronspecific dead time correction coefficients which were used to
J. Farah et al. / Radiation Measurements 96 (2017) 42e52
correct the number of counts, plot the microdosimetric spectra and deduce equivalent dose values. This minute-by-minute analysis is however complex and time consuming and proved to be of little impact in these experimental conditions where the dead time coefficient of high-LET particles (having the strongest impact on dose) remained below 2%. This minute by minute analysis proved nonetheless that the dead time coefficient provided by the HAWK output routine does not distinguish between low-LET and high-LET counts and tend to over-estimate the dead time coefficient for high-LET particles. Similarly, few counts were registered in the last channel of the TEPC. These are the result of pulse pile-ups which should be redistributed to the microdosimetric spectra. The distribution of these counts was not performed in this study as no proper methodology is yet available to perform this correction. The most appropriate correction method should also consider the different behavior of low and high-LET events when redistributing these counts. Yet again, in these particular experiments, the total number of counts registered in the last channel was very limited (<10 counts at positions B and D) which renders this correction negligible. 4. Conclusion This paper test the performance of four commercial TEPCs in reference irradiation conditions (with 137Cs, 60Co, and 252Cf sources) and in the complex stray radiation field of scanning proton therapy. The manuscript involves a strong effort on the data processing methodology to make full use of TEPCs. First, microdosimetric measurements with 137Cs and/or 60Co sources show variable performance of the systems due to the detector-specific threshold for low-LET radiation. Meanwhile, measurements with 252Cf and 241Am-Be neutron sources show similar behavior of the four TEPCs in the presence of high-LET radiation. For HAWK systems, the proton edge position set by the manufacturer remains controversial since it does not correspond to the theoretical value of a 0.2 mg cm2 propane based TEPC. Additionally, a shift in the position of the proton edge was registered for HAWKs v.1 and 2 indicating a change in the gas pressure; such detectors hence need re-calibration, a correction of 20% of neutron doses (which are over-estimated) or to be refilled again. In addition, microdosimetric spectra measured at four positions around the phantom in the proton therapy environment provide valuable information on the radiation quality as a function of measurement position. The largest proportion of high-LET neutron radiation was indeed measured at position A (45 from the beam axis, 225 cm from isocenter) while low-LET photons and thermal neutrons were mainly dominant at position B (90 from the beam axis, 225 cm from isocenter). The spectra processing methodology introduced in this work to account for differences among the four TEPCs in gas filling and gain, acquisition channels, etc. resulted in good adjustment of the data and a generally good overlapping of microdosimetric spectra both in reference conditions and in proton therapy's stray radiation environment. The processed spectra show good agreement both on dosimetric quantities (H and Q values) and microdosimetric quantities (yF and yD ). The extrapolation of low-LET events for the LET-SW5 TEPC only added a 2% uncertainty to the overall measurement uncertainty which fell below 15% and was clearly dominated by calibration (10%) and counting statistics uncertainties (up to 10%). For HAWKs, the uncertainty on H values remained below 12% at all positions around the phantom. Finally, H values measured with TEPCs proved to agree reasonably well with H*(10) records obtained with other instruments (FH40G and Wendi-II) considering measurement uncertainties and the
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