Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film

Physica Medica xxx (2016) xxx–xxx

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Original paper

Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film Trent Aland a,c,d,⇑, Rebecca Moylan b, Tanya Kairn b,c, Jamie Trapp c a

Icon Integrated Cancer Care, 9 MeLennan Court, North Lakes, Qld 4509, Australia Genesis Cancer Care, Wesley Medical Centre, Suite 1, 40 Chasely St, Auchenflower, Qld 4066, Australia c School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, Qld 4000, Australia d Epworth Radiation Oncology, 32 Erin Street, Richmond, Vic 3121, Australia b

a r t i c l e

i n f o

Article history: Received 5 July 2016 Received in Revised form 20 September 2016 Accepted 26 October 2016 Available online xxxx Keywords: Gafchromic In vivo Dosimetry Imaging EBT3

a b s t r a c t In this work, the apparent treatment dose that kV planar or CBCT imaging contributes to Gafchromic EBT3 film used for in vivo dosimetry, was investigated. Gafchromic EBT3 film pieces were attached to a variety of phantoms and irradiated using the linear accelerator’s built-in kV imaging system, in both kV planar mode and CBCT mode. To evaluate the sensitivity of the film in the clinical scenario where dose contributions are received from both imaging and treatment, additional pieces of film were irradiated using base doses of 50 cGy and then irradiated using selected kV planar and CBCT techniques. For kV planar imaging, apparent treatment doses of up to 3.4 cGy per image pair were seen. For CBCT, apparent treatment doses ranged from 0.22 cGy to 3.78 cGy. These apparent doses were reproducible with and without the inclusion of the 50 cGy base dose. The contribution of apparent treatment dose from both planar kV as well as CBCT imaging can be detected, even in conjunction with an actual treatment dose. The magnitude of the apparent dose was found to be dependent on patient geometry, scanning protocol, and measurement location. It was found that the apparent treatment dose from the imaging could add up to 8% of additional uncertainty to the in vivo dosimetry result, if not taken into account. It is possible for this apparent treatment dose to be accounted for by subtraction of the experimentally determined apparent doses from in vivo measurements, as demonstrated in this work. Ó 2016 Published by Elsevier Ltd on behalf of Associazione Italiana di Fisica Medica.

1. Introduction In vivo dosimetry is essential for ensuring the overall quality for a patient receiving radiation therapy, and has been extensively investigated and recommended in the literature and by international bodies [1]. While in vivo dosimetry can be performed with a variety of detectors, the use of radiochromic film as an in vivo detector is desirable due to its near tissue equivalence [2,3], weak energy dependence [4–6], as well as its general robustness. Additionally, its thin two dimensional construction allows radiochromic film to provide an accurate measurement of skin dose [7– 11], as well as other in vivo applications [12,13]. More recently, Moylan et al. [14] investigated the use of small pieces of Gafchromic EBT film (generations 2 and 3) for the purposes of in vivo dosimetry. They found that pieces as small as 5 mm  5 mm could accurately be used for in vivo dosimetry of both photon and electron treatment beams. Before radiochromic films can be ⇑ Corresponding author at: Icon Integrated Cancer Care, 9 MeLennan Court, North Lakes, Qld 4509, Australia. E-mail address: [email protected] (T. Aland).

confidently used for in vivo measurements during clinical radiotherapy treatments, however, it is necessary to establish the effect on the film measurement caused by the dose delivered during the kilovoltage (kV) imaging procedures that are completed as part of many contemporary radiotherapy treatments. The application of Gafchromic EBT and EBT2 film (International Specialty Products, Wayne NJ, USA) in kV beams has been investigated in terms of energy dependence [15–18], in the determination of backscatter factors [19,20], and in an in vivo dosimetry application for intraoperative breast radiotherapy [21]. In all cases, these films were found to be sufficiently sensitive to kV beams to allow accurate kV dose measurements to be made. Gafchromic EBT3 film has been observed to be under respond in the kV range if the film has been calibrated in the MV range [6,22]. This under-response was shown to be up to 22% in the red channel for a 1 Gy dose delivery for a beam energy of 70 kVp [22]. Nobah et al. [23] measured skin doses on a RANDO phantom using Gafchromic EBT3 film and observed doses of up to 3.7 cGy when using a pelvis based kV cone beam computed tomography (CBCT) protocol on a Varian iX linac.

http://dx.doi.org/10.1016/j.ejmp.2016.10.020 1120-1797/Ó 2016 Published by Elsevier Ltd on behalf of Associazione Italiana di Fisica Medica.

Please cite this article in press as: Aland T et al. Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film. Phys. Med. (2016), http://dx.doi.org/10.1016/j.ejmp.2016.10.020

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T. Aland et al. / Physica Medica xxx (2016) xxx–xxx

To be used effectively as an in vivo dosimeter, the Gafchromic EBT3 film would ideally be placed onto the patient during setup and then removed at the completion of treatment. This would therefore mean that the dosimeters would be subject to any forms of verification imaging, which may include kV planar imaging or kV cone beam computed tomography (CBCT), depending on departmental protocols and imaging equipment available. Due to the detectable skin doses that have been reported for CBCT and kV planar imaging [23–28], especially for Gafchromic EBT3 films [23], and the known under response of Gafchromic EBT3 film to kilovoltage beams [6,22], it is apparent that this dose will contribute to in vivo dosimetry results. This may then lead to substantial inaccuracies if the imaging dose is ignored when Gafchromic EBT3 film is used for in vivo dosimetry, especially when the film is calibrated under an MV photon beam [6,22]. In this work we examined the apparent treatment dose contribution (ie. The kV imaging dose presented as an MV treatment dose) that kV planar and CBCT verification imaging has on in vivo dosimetry for Gafchromic EBT3 film, and also the apparent treatment dose contribution of the imaging dose when combined with a 50 cGy treatment dose. 2. Materials and methods A Varian trilogy linear accelerator (Varian Medical Systems, Palo Alto, CA) with on board imaging (OBI) (version 1.5) was used to irradiate pieces of GafChromic EBT3 film with dimensions of 25 mm  25 mm. The OBI system can acquire kV planar and CBCT images with predefined scan protocols (kVp, mA, and ms). The OBI hardware features the ability to perform CBCT using either a half gantry rotation (referred to as ‘full fan’) or a full gantry rotation (referred to as ‘half fan’). To determine the dose contribution that planar kV imaging added to the film, a simple solid water phantom of 20 cm thickness was positioned isocentrically (source-surface distance of 90 cm) on the couch top of the linear accelerator. The gantry was then oriented in the 90o position so that the kV source was positioned directly above the phantom and the kV detector was directly below the phantom. Film pieces, which were placed on the surface of the phantom in the centre of the beam, were then irradiated using six different planar scan protocols, details of which can be found in Table 1. Each film piece was irradiated several times in order to show a visible dose (shown in Table 1), all results were then normalised to indicate a total dose from two images. Two images were chosen as the normalisation point given that, clinically, two kV images are used for patient positioning. To determine the dose contribution that CBCT imaging added to the film, three different phantoms were placed at isocentre with pieces of film placed on the left, anterior, and right sides of the phantoms (refer to Fig. 1). The phantoms used were a head (model 605), thorax (model 002LFC), and pelvis (model 002PRA) phantom manufactured by CIRS (CIRS, Norfolk, VA). Several different scan protocols were then used to irradiate the film placed on the phantoms – both with and without 1.0 cm thick jelly bolus. The use of bolus was included given that many treatments include bolus

22°

RT

LT

180°

Fig. 1. Thorax/Pelvis phantom with film locations on right, anterior, and left (patient orientation). Blue arc is the CBCT head protocol (with full bow tie), red arc is the CBCT thorax/pelvis protocol (with half bow tie). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and in vivo measurement locations are likely to be underneath the bolus. For the bolus measurements, the bolus was placed over all film pieces. The scan protocols are summarised in Table 2. Half value layer (HVL) values for 100, 110, and 125 kV were 3.6, 3.9, and 4.6 mm of aluminium respectively, measured with no filtration. Where the full bow tie filter has been used, the gantry start and stop angles were 22° and 178° respectively. Where the half bow tie filter has been used, the gantry start and stop angles were 178° and 182° respectively. This setup is illustrated in Fig. 1. All gantry angles were defined according to the IEC 61217 scale. The film was calibrated by delivering a series of known doses using the MV beam, as would usually be done as part of any in vivo dose measurement. This method is not expected to provide accurate measurements of kV imaging dose [6,22], for which the film should be calibrated using an appropriate kV beam quality [29]. The phrase ‘‘apparent treatment dose” is used in this study to indicate that the goal of this work was not to provide accurate measurements of imaging doses, but rather to evaluate the effect of kV imaging on the accuracy of the MV treatment dose measurement (termed ‘‘treatment dose” in this work). Where a combination of the two has been used, this will be referred to simply as ‘‘dose”. For each of the test cases, measurements were repeated three times to separate pieces of film in each location and the results averaged. No treatment dose was delivered to these films. A subset of the tests were then repeated using pieces of film that had been pre-irradiated with 50 cGy of treatment dose from a 6 MV photon beam. The purpose of this further testing was to mimic the actual clinical use of the film. The 50 cGy treatment dose is reduced below the usual 2–3 Gy prescription dose because treatment beams are delivered from multiple directions and not all will be directly incident on the film. The subset of tests can be found in Table 3. All films were scanned pre- and post-irradiation using an Epson V700 flatbed scanner (Epson Corporation, Suwa, Japan) in transmission and professional modes, with all image enhancements switched off. Scanning was performed using a resolution of 75 DPI and the images were saved as 48 bit tagged image file format

Table 1 List of kV planar imaging protocols investigated. No filtration was added. kV Protocol

Times irradiated

kV

mA

ms

Field size (cm)

HVL (mm Al)

Pelvis-Lat-Lg Pelvis-AP-Med Head-AP Abdo-Lat Thorax-AP Pelvis-Lat-Med

4 4 8 4 8 4

120 75 100 85 75 105

200

630 50 40 200 25 400

15  15

4.3 2.7 3.6 3.1 2.7 3.8

Please cite this article in press as: Aland T et al. Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film. Phys. Med. (2016), http://dx.doi.org/10.1016/j.ejmp.2016.10.020

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T. Aland et al. / Physica Medica xxx (2016) xxx–xxx Table 2 List of CBCT protocols investigated for each phantom. Listed mA and ms values are per CBCT projection. CBCT Protocol

kV

mA

ms

Projections

Field size

Filtration

Phantom

High quality head Standard dose head Low dose thorax Pelvis

100 100 110 125

80 20 20 80

25 20 20 13

360 360 655 655

25  25  18 cm

Full bow tie

Head

45  45  16 cm

Half bow tie

Thorax/Pelvis

Table 3 List of CBCT/kV planar protocols measured using a 50 cGy base treatment dose. Protocol

Phantom/Film Location

Bolus

CBCT: High quality head CBCT: High quality head CBCT: Low dose thorax CBCT: Pelvis kV Planar: Pelvis-Lat-Lg

Head – Right Head – Right Thorax – Mid Pelvis – Mid Solid water

YES NO NO NO NO

(TIFF) images. All scanning precautions as outlined by Aland et al. [30] and more recently reviewed by Devic et al. [31] were adhered to. Films were all scanned in the centre of the scan bed in order to avoid the necessary inclusion of any lateral offset corrections [32]. The resulting scanned images were then imported into ImageJ software (National Institute of Health, Bethesda, USA) where data was extracted from the red colour channel and converted to optical density and dose using previously described methods [30]. Conversion from optical density to treatment dose was performed using a film-specific calibration curve measured with 6 MV photons in a 10  10 cm field. Calibration treatment doses ranged from 4.3 cGy up to 215 cGy, being the local procedure for calibrating film for dosimetric use. Although it has been shown that using a MV based film calibration will result in an underestimate of kV dose [6,22], verifying the actual kV dose is not the intent of this work. Uncertainty analysis was also performed and was assessed using the same methodology as that used by Aland et al. [30] for a ‘clinically likely’ scenario. This methodology essentially breaks down the uncertainty into components (calibration curve fitting, post irradiation development, film uniformity, readout reproducibility, water equivalence, film scanning orientation, and readout noise) and combines them in quadrature to give a total uncertainty, expressed as a percentage relative to a fixed dose. For this work, uncertainty is expressed relative to the dose of interest and not a fixed dose. This was chosen given that two very different sets of data have been analysed. 3. Results Results of the uncertainty analysis [30] for the film used in this application were determined to be 10.6% for the films irradiated with imaging only. The key contributors to this were the film response fit (3.5%) and the measurement reproducibility (9.7%). Where a base dose was used, the uncertainty reduced to 4.2% which was largely due to the decrease in uncertainty of film reproducibility at this dose level. The results for the various kV planar imaging protocols that were investigated are presented in Table 4. This data has also been presented graphically in Fig. 2 to highlight the relationship between apparent treatment dose and filament current multiplied by exposure time (termed mAs). Measured apparent treatment doses from the various CBCT protocols are presented in Tables 5–7, being for the head, thorax, and pelvis phantoms, respectively. Although it is unlikely that a thorax protocol would be used for a pelvis CBCT or a pelvis protocol used for a thorax CBCT, these results have been included to aid in the

Table 4 Measured apparent treatment doses from various kV planar imaging protocols (see Table 1 for protocol details). Reported doses are for two images. kV Protocol

Dose (cGy per 2 images)

Pelvis-Lat-Lg Pelvis-AP-Med Head-AP Abdo-Lat Thorax-AP Pelvis-Lat-Med

3.4 0.0 0.0 0.2 0.0 1.2

Fig. 2. Plot of measured apparent treatment dose from kV planar imaging pairs for the various exposure settings (mAs) of each kV planar imaging protocol. For all techniques, mA was set to 200.

Table 5 Measured apparent treatment doses from various CBCT protocols using the head phantom. Reported doses are for a single CBCT. CBCT Protocol

Film Location

Bolus

Dose (cGy per CBCT)

Standard dose head

Left Ant Right Left Ant Right Left Ant Right Left Ant Right

NO

0.22 0.22 0.33 0.26 0.23 0.40 1.45 0.96 1.69 1.58 0.99 2.21

Standard dose head

High quality head

High quality head

YES

NO

YES

discussion. Measured apparent treatment doses ranged from 0.22 cGy to 2.21 cGy for the head phantom, 0.82 cGy to 1.32 cGy for the thorax phantom (thorax protocols only), and 2.61 cGy to 3.78 cGy for the pelvis phantom (pelvis protocols only). The subset of measurement results that were repeated with a 50 cGy base treatment dose are shown in Table 8 for both CBCT protocols as well as kV planar imaging. Expected doses are also shown and represent the sum of 50 cGy treatment dose plus the respective measured apparent treatment doses from Tables 4–7.

Please cite this article in press as: Aland T et al. Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film. Phys. Med. (2016), http://dx.doi.org/10.1016/j.ejmp.2016.10.020

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T. Aland et al. / Physica Medica xxx (2016) xxx–xxx

Table 6 Measured apparent treatment doses from various CBCT protocols using the thorax phantom. Reported doses are for a single CBCT. CBCT Protocol

Film Location

Bolus

Dose (cGy per CBCT)

Low dose thorax

Left Ant Right Left Ant Right Left Ant Right Left Ant Right

NO

1.12 1.31 0.82 1.10 1.32 1.00 2.04 3.13 2.93 2.77 3.88 3.27

Low dose thorax

Pelvis

Pelvis

YES

NO

YES

Table 7 Measured apparent treatment doses from various CBCT protocols using the pelvis phantom. Reported doses are for a single CBCT. CBCT Protocol

Film Location

Bolus

Dose (cGy per CBCT)

Low dose thorax

Left Ant Right Left Ant Right Left Ant Right Left Ant Right

NO

1.30 1.49 1.26 1.18 1.81 1.47 2.61 3.20 2.79 2.71 3.78 2.85

Low dose thorax

Pelvis

Pelvis

YES

NO

YES

4. Discussion Apparent treatment doses of up to 3.4 cGy were measured for the kV planar image pairs, this being for the Pelvis-Lat-Lg kV protocol. The Pelvis-Lat-Lg kV protocol represents a worst case scenario and would not typically be used clinically, due to its high kV and mAs values, however has been included for completeness. For the remaining protocols tested, the measured apparent treatment doses were 1.2 cGy or lower. With further verification measurements, Fig. 2 could be used as a guide to indicate whether or not a particular mAs value would lead to a measurable amount of apparent treatment dose on a piece of GafChromic EBT3 film used in an in vivo dosimetry application. For example, from the results presented here, where the mAs is less than 10, an apparent treatment dose was not measurable for any kV imaging protocols, and therefore the imaging would not have contributed to any in vivo dosimetry results. For the CBCT results, all combinations of phantom and imaging protocols resulted in detectable levels of apparent treatment dose as measured with the Gafchromic EBT3 film. This highlights the importance of taking this apparent treatment dose into account, or by excluding imaging from the in vivo dosimetry measurements.

For the head protocols, as expected, the high quality head protocol resulted in higher apparent treatment doses compared to the standard dose head protocols. On average, the high quality head protocol resulted in apparent treatment doses 5.3 greater than that from the standard dose head protocol (averaged across all measurements with and without bolus). This agrees well with the fact that the mAs used for the high quality head protocol is 5 times greater than that for the standard dose head. Similarly, for the thorax and pelvis protocols that were delivered to the same phantoms, the apparent treatment doses are 2.4 time greater for the pelvis protocol compared to the thorax protocol (averaged across all measurements on the same phantoms both with and without bolus). This also agrees well with the mAs ratio between the pelvis and thorax protocols, being 2.3. Based upon these results for the head, thorax, and pelvis protocols and on the well published direct relationship between dose and mAs, it may be possible to estimate the expected apparent treatment dose based on the mAs alone (for protocols with the same number of projections and trajectories). This would be useful where measurements have been performed in order to quantify the apparent treatment dose for a given protocol, and then the protocol settings are later adjusted. Across some results, measured apparent treatment doses were marginally greater when bolus was present. This observed increase in apparent treatment dose at depth, although not statistically significant, does agree with results from Giaddui et al. [25] and with dose profiles presented by Tomic et al. [26] and Nobah et al. [23], which all show an increase in dose at shallow depths compared with the surface. When the film was positioned in the anterior location on the pelvis / thorax phantoms, higher apparent treatment doses were measured. This is expected given that the phantoms are thinner in this dimension, and because this position is in line with the gantry start and stop angles and is therefore exposed to the greatest amount of radiation, as demonstrated by Wen et al. [28]. Some slight differences were noted between the left and right measurements for the thorax and pelvis protocols. Although this is not expected, it is not inconsistent with previously published results [25,26] which show no real pattern as to why one side of the phantom receives a higher dose than another. Table 8 presents a range of clinically likely scenarios that would be encountered in an in vivo dosimetry application, with the exception of the Pelvis-Lat-Lg kV protocol. The measured doses agree well with the expected doses and it can be seen that where an expected dose of 50 cGy is to be measured, the addition of a CBCT will add, on average, an additional 4% to the result. Taking into account the wider results reported in this work, for a pelvis protocol, this additional apparent treatment dose could be anywhere up to 8%. Furthermore, where the expected treatment dose is less than the nominal 50 cGy use here, the contribution of CBCT dose to the in vivo result will be greater and, where possible, should be accounted for. From this work, as demonstrated by the agreement between measured and expected doses in Table 8, it should be possible to,

Table 8 Measured doses for various CBCT and kV imaging protocols where a 50 cGy base treatment dose was delivered. Pelvis-Lat-Lg represents the kV planar imaging protocol and doses presented for this protocol are for two images. CBCT/kV with 50 cGy base dose Protocol

Phantom/Film Location

Bolus

Dose measured (cGy)

Dose expected (cGy)

High quality head High quality head Low dose thorax Pelvis Pelvis-Lat-Lg

Head – Right Head – Right Thorax – Ant Pelvis – Ant Solid water

YES NO NO NO NO

52.2 51.9 50.8 52.4 54

52.2 51.7 51.3 53.2 53.4

Please cite this article in press as: Aland T et al. Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film. Phys. Med. (2016), http://dx.doi.org/10.1016/j.ejmp.2016.10.020

T. Aland et al. / Physica Medica xxx (2016) xxx–xxx

at least, partially account for the contribution that verification imaging will have on in vivo dosimetry results. This could be achieved by experiment with real patient data, or via approximation using data such as that provided in this study. In doing so, this should lead to a reduction in the overall measurement uncertainty, however further investigations are required, including the impact of patient size, imaging parameters such as field of view and scan length, as well as dosimeter placement. 5. Conclusions When Gafchromic EBT3 film is used as an in vivo dosimeter, the additional dose from verification imaging can become clinically significant, adding anywhere up to 8% to a 50 cGy treatment dose. It has been found that the contribution of dose from planar kV imaging as well as CBCT imaging is measurable on Gafchromic EBT3 film and the measured apparent treatment dose is dependent not only patient geometry, but scanning protocol (predominantly mAs and CBCT type), measurement location, and inclusion of bolus. For accurate in vivo dosimetry measurements, this apparent treatment dose should be taken into account by either accompanying in vivo dosimetry measurements with separate phantom measurements to estimate the apparent treatment dose from kV planar/CBCT imaging, by accumulating and referring to a local library of imaging doses for commonly used protocols, by estimating the dose based on the data provided herein, or by removing the imaging from the in vivo dosimetry measurement altogether. References [1] Kutcher GJ, Coia L, Gillan M, Hanson WF, Leibel S, Morton RJ, et al. Comprehensive QA for radiation oncology: Report of AAPM Radiation Therapy Committee Task Group 40. Med Phys 1994;21:581–618. [2] Arjomandy B, Tailor R, Zhao L, Devic S. EBT2 film as a depth-dose measurement tool for radiotherapy beams over a wide range of energies and modalities. Med Phys 2012;39:912–21. [3] International Specialty Products, ‘‘Gafchromic EBT2 self-developing film for radiotherapy dosimetry film”; 2010 (available URL: ). [4] Lindsay P, Rink A, Ruschin M, Jaffray D. Investigation of energy dependence of EBT and EBT-2 Gafchromic film. Med Phys 2010;37:571–6. [5] Butson MJ, Yu PKN, Cheung T, Alnawaf H. Energy response of the new EBT2 radiochromic film to X-ray radiation. Radiat Meas 2010;45:836–9. [6] Massillon G, Chiu-Tsao S, Domingo-Munoz I, Chan M. Energy dependence of the new Gafchromic EBT3 film: dose response curves for 50 kV, 6 and 15 MV Xray beams. Int J Med Phys Clin Eng Radiat Oncol 2012;1:60–5. [7] Nakano M, Hill RF, Whitaker M, Kim JH, Kuncic Z. A study of surface dosimetry for breast cancer radiotherapy treatments using Gafchromic EBT2 film. J Appl Clin Med Phys 2012;13:83–97. [8] Devic S, Seuntjens J, Abdel-Rahman W, Evans M, Olivares M, Podgorsak EB, et al. Accurate skin dose measurements using radiochromic film in clinical applications. Med Phys 2006;33:1116–24. [9] Quach KY, Morales J, Butson MJ, Rosenfeld AB, Metcalfe PE. Measurement of radiotherapy X-ray skin dose on a chest wall phantom. Med Phys 2000;27:1676–80.

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Please cite this article in press as: Aland T et al. Effect of verification imaging on in vivo dosimetry results using Gafchromic EBT3 film. Phys. Med. (2016), http://dx.doi.org/10.1016/j.ejmp.2016.10.020