Australas Phys Eng Sci Med (2014) 37:303–309 DOI 10.1007/s13246-014-0260-2
SCIENTIFIC PAPER
A comparison of surface doses for very small field size x-ray beams: Monte Carlo calculations and radiochromic film measurements J. E. Morales • R. Hill • S. B. Crowe T. Kairn • J. V. Trapp
•
Received: 18 October 2013 / Accepted: 4 March 2014 / Published online: 20 March 2014 Ó Australasian College of Physical Scientists and Engineers in Medicine 2014
Abstract Stereotactic radiosurgery treatments involve the delivery of very high doses for a small number of fractions. To date, there is limited data in terms of the skin dose for the very small field sizes used in these treatments. In this work, we determine relative surface doses for small size circular collimators as used in stereotactic radiosurgery treatments. Monte Carlo calculations were performed using the BEAMnrc code with a model of the Novalis Trilogy linear accelerator and the BrainLab circular collimators. The surface doses were calculated at the ICRP skin dose depth of 70 lm all using the 6 MV SRS x-ray beam. The calculated surface doses varied between 15 and 12 % with decreasing values as the field size increased from 4 to 30 mm. In comparison, surface doses were measured using Gafchromic EBT3 film positioned at the surface of a Virtual Water phantom. The absolute agreement between calculated and measured surface doses was better than 2.0 % which is well within the uncertainties of the Monte Carlo calculations and the film measurements. Based on these results, we have shown that the Gafchromic EBT3 film is suitable for surface dose estimates in very small size fields as used in SRS.
J. E. Morales (&) S. B. Crowe T. Kairn J. V. Trapp School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD, Australia e-mail: [email protected] J. E. Morales R. Hill Department of Radiation Oncology, Chris O’Brien Lifehouse, Sydney, NSW 2006, Australia T. Kairn Genesis Cancer Care (Queensland), The Wesley Medical Centre, Brisbane, QLD, Australia
Keywords Stereotactic radiosurgery SRS Surface dosimetry skin dose Monte Carlo calculations Radiochromic film EBT3
Introduction Stereotactic radiosurgery (SRS) involves the delivery of a high radiation dose, typically 15–20 Gy, using small size radiation beams for treatments of lesions within the brain [1]. These lesions are usually malignant brain metastases or benign arterovenous malformations and require high spatial and dosimetric accuracy due to the need for accurate delivery to the small lesion as well as minimising radiation dose to other tissues such as the brain stem, optic chiasm and other critical structures. SRS treatments usually involves a number of beams using arcs and/or conformal fields in order to achieve an optimum dose distribution and normal tissue doses. Skin toxicity can also be a dose-limiting factor for radiotherapy treatment planning, despite the skin-sparing effect of megavoltage x-ray dose buildup [2–4]. Skin dose is a particular concern in SRS treatments, due to the high single-fraction doses delivered, and there is a growing interest in the incidence of skin toxicity associated with stereotactic body radiotherapy (SBRT or SABR) [47]. The ICRP defines the skin depth to be 70 lm which corresponds to the basal cell layer thickness [5]. Surface doses for megavoltage x-ray beams have been measured using a number of different detectors including parallel-plate ionisation chambers, thermoluminescent dosimeters (TLDs), metal– organic semiconductor field-effect transistors (MOSFETs) and various types of radiochromic film [2, 3, 6–19]. While some parallel-plate ionisation chambers, such as the Attix chamber are well characterised for dose measurements in the
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buildup region, they are not suitable for in vivo dosimetry measurements [2, 3, 20]. In addition to measurements, surface doses for x-ray beams can also be determined by Monte Carlo calculations. Monte Carlo methods are regarded as the gold standard for accurate dose calculations of ionising radiation beams [21–23] and there have been a number of studies which have used Monte Carlo methods for determining surface doses for conventional field sizes [2, 4, 24–27]. To date, there is limited data for surface doses for the very small field sizes as typically used in the delivery of SRS treatments [4, 28]. Paskalev et al.[29] examined the dosimetry of SRS beams, including surface doses measured with radiographic film, generated from a 10 MV x-ray beam and using 1.5 and 3 mm circular collimators [29] and found that the surface doses decreased slightly with increasing size of the collimator. In a comprehensive study of the dosimetry of the Brainlab m3 micro-MLC system, Ding et al. [28] used both experimental and Monte Carlo methods to characterise the dosimetry. As part of their results, the Monte Carlo depth dose data included surface doses for field sizes ranging from 6 9 6 to 100 9 100 mm2 with relative surface dose values ranging between 26 and 32 % of Dmax. The purpose of this study was to determine surface doses for a variety of very small size fields as defined by circular applicators ranging from 4 to 30 mm as used in SRS treatments and to establish the utility of radiochromic film for measuring such doses. Surface doses were calculated using Monte Carlo modelling of the linear accelerator using the BEAMnrc Monte Carlo code. In addition, surface doses were measured for these applicators using the Gafchromic EBT3 radiochromic film.
Materials and methods In this work, surface doses were determined for the 6 MV SRS x-ray beam as produced by a Novalis Trilogy linear accelerator (Varian Medical Systems, Palo Alto, USA) which has a thin flattening filter in order to produce a dose rate of 1,000 MU\min [30–32]. Beam collimation for the SRS x-ray beams was achieved by using the BrainLab circular applicators (BrainLab, Germany) with diameters of 4, 7.5, 10, 20 and 30 mm diameter as defined at the isocentre. The X and Y collimator jaws were set to 5 cm for all measurements and calculations. These jaw sizes were used during commissioning of this linear accelerator but it is noted that a recent notification from the manufacturer recommends different jaw settings for each circular collimator (Safety Notice 09-06-29.BAV.2). Commissioning Monte Carlo model The 6MV SRS x-ray beam was modeled using the BEAMnrc/EGSnrc Monte Carlo code (Version4, release
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2.3.2) [22, 33, 34]. Simulations were performed on supercomputing facilities at the Queensland University of Technology. The supercomputer at this location has 1924 64-bit Intel Xeon Cores. All of the specifications for the geometry and materials used within the linear accelerator model including the special SRS flattening filter were supplied by Varian Medical Systems. The adjustable incident electron beam parameters were optimized to an elliptical beam with Gaussian distributions of X = 0.4 mm and Y = 0.4 mm in FWHM and with no angular spread striking the tungsten target along the central axis. The optimization process involved the tuning of the initial electron energy to match the PDD curve for a 10 9 10 cm2 open field size. A reduced Chi squared method was applied to every PDD obtained by Monte Carlo. Energies were varied between 5.0 and 7.0 MeV in steps of 0.2 MeV. The reduced Chi squared results were plotted versus energy and a minimum was reached. This minimum value was taken to be the best value for Monte Carlo simulations. In our case the final value for the incident electron energy was 6.06 MeV. For both the BEAMnrc and DOSXYZnrc user codes, the electron cutoff energy (ECUT) and the photon cutoff energy were set to 0.521 MeV and 0.010 MeV respectively. PRESTA-I and PRESTA-II were turned on for the boundary crossing algorithm and electron-step algorithm respectively. The directional bremsstrahlung splitting variance reduction technique was also used with splitting factor of 1,000 [35, 36]. The initial testing of the BEAMnrc model was for an open field size of 10 9 10 cm2 and no circular applicator. A total of 4 9 107 histories were used. The initial model of the x-ray beam was verified by comparison of measured and calculated percentage depth dose data and cross-plane beam profiles as measured in a PTW MP3 water tank (PTW, Freiburg, Germany). The procedure for comparing data involved working out the absolute difference by subtracting the measured data from the Monte Carlo data at each measurement point. For the 10 9 10 cm2 open field size, depth doses and cross plane profiles were measured with a PTW Advanced Markus parallel-plate ionisation chamber (PTW, Freiburg, Germany) and PTW 60003 diamond detector (PTW, Freiburg, Germany). Surface Dose simulations for circular collimators After verifying the Monte Carlo model for the 10 9 10 cm2 open field size, phase space files were generated for the circular collimators with diameters of 4, 7.5, 10, 20 and 30 mm using the BEAMnrc code. Percentage depth doses were then calculated for each circular collimator using the DOSXYZnrc user-code (V4 r2-3-0) and compared with PDDs measured with a PTW 60012 diode
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(PTW, Freiburg, Germany) at a source to surface distance of 100 cm. Voxel sizes of 0.25 9 0.25 9 1.0 mm3 were used to score the dose for central-axis PDDs. A total of 8 9 109 incident particles were used to obtain statistical uncertainties of less than 2 %. For consistency, the same user-specified simulation parameters were used as in the phsp file calculation. To model electron transport as accurately as possible, a global ECUT of 0.521 MeV was specified which corresponds to an electron range of approximately 2.5 lm in water (NIST DATABASE at http:// physics.nist.gov/cgi-bin/Star/e_table.pl) [26]. The EXACT boundary crossing algorithm was turned on for surface dose calculations [37]. For the surface dose calculations with the circular collimators, additional calculations were carried out using the DOSXYZnrc user-code. The dose was scored in voxels with thicknesses of 10 lm for the first 0.1 mm depth, which includes the clinically relevant skin depth of 70 lm [38], as well as at depths of 1, 2 mm and dmax = 1.5 cm. All surface doses scored at or close to the surface of the phantom were normalized to Dmax.
the RIT software package V5.2 (Radiological Imaging Technology, Inc, USA). The RIT software needs a calibration curve which allows the conversion of the pixel value to absorbed dose. This was achieved by irradiating the film pieces with known doses of 0–3 Gy by using a 10 9 10 cm2 field size and the films were located at the depth of dmax. A median filter of 3 9 3 pixels was applied to all the scanned images. All films were read out approximately 24 h after their irradiation. For the surface dose measurements with the different collimators, the EBT3 film piece was placed at the surface of the Virtual Water phantom at an SSD of 100 cm. The uncertainty in the EBT3 film surface dose measurements was determined to be 2 % (1 Standard Deviation) using the ISO GUM methodology [42–44]. The factors contributing to uncertainties in surface dose measurements included: variation within the OD measurements for the pixels in the region of interest, variations due to film non-uniformity as well as uncertainties in the curve fit for the EBT3 film calibration curve.
Surface dose measurements using Gafchromic EBT3 film
Results and discussion
All measurements were performed with Gafchromic EBT3 radiochromic film (Ashland, Wayne, USA), batch number A10171102, which has a number of advantages over the earlier versions of this film including a polyester substrate which prevents the formation of Newton’s rings and the use of a symmetrical structure for the different layers in the manufacturing of the film [39, 40]. The EBT3 film has the active layer of 30 lm thickness which is located between the two polyester layers of 125 lm thickness. The EBT3 film sheet was cut into 2 9 2 cm2 pieces for both the dose calibration and surface dose measurements. The film piece was positioned at the surface and at a depth of Dmax between the Virtual Water blocks [41]. For each set of measurements a minimum of two EBT3 film pieces were used in this study, and the mean dose absorbed by these film pieces was used for analysis purposes. The process for preparing, reading out the films and analyzing the dose information was consistent with manufacturer’s recommendations. All films were read out on an Epson Expression 10000 XL (EPSON) flatbed scanner maintaining a fixed orientation of the film during the readout. The orientation chosen was landscape which keeps the short edge of the film parallel to the scan direction. The EPSON scan software package was used to scan films using the red channel only at a resolution of 150 DPI in transmission mode with all image adjustment features switched off. The dosimetric analysis was performed using
Commissioning Monte Carlo model The uncertainty in the Monte Carlo calculated depth doses was less than 1 % as determined within the DOSXYZnrc user code. Figures 1 and 2 show the comparison between the Monte Carlo calculated and measured central axis depth dose curves and cross-plane profiles respectively for the open 10 9 10 cm2 reference field. The agreement between the Monte Carlo calculated and the measured doses for the PDD shown in Fig. 1 was within 1 % for depths of 1.0–30.0 cm and within 2 % for depths within 0.5–0.99 cm, which is well within an acceptance criteria of 2 % [45, 46]. Measurements with the advanced Markus for depths between 0 and 0.5 cm are not shown as they were deemed unreliable due to the meniscus effect near water surface. Figure 2a shows the Monte Carlo calculated and measured profiles at three depths in water across the X jaws. The depths were 1.4, 10 and 20 cm. All profiles were normalized with their respective PDD value at each depth. Figure 2b shows the difference between calculated and measured data. A maximum absolute difference of 1.4 % was obtained at the penumbra regions. Surface dose simulations The comparison between the Monte Carlo calculated and measured central axis depth dose curves for the circular
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acceptance criteria of 2 % [45, 46]. The uncertainty in measurements was 1 %. Surface dose measurements using Gafchromic EBT3 film
Fig. 1 Percentage depth dose in water calculated for a 10 9 10 cm2 field. BEAMnrc/DOSXYZnrc versus measurement by an advanced markus ionisation chamber
Fig. 2 a Cross profiles at depth in water calculated by BEAMnrc/ DOSXYZnrc for a 10 9 10 cm2 field versus measurement with a diamond a depths of 1.4, 10 and 20, b absolute difference between calculation and measurement for each depth
collimators are shown in Fig. 3. For all the circular collimators the agreement between Monte Carlo calculated and measured percentage depth doses was within 1 % for depths between 0.5 and 30 cm. This is well within an
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A summary of the relative surface doses for the Monte Carlo calculations and the Gafchromic EBT3 measurements is presented in Table 1. The estimated uncertainty in the Gafchromic EBT3 film measurements was 2.0 %. For the Monte Carlo calculations, the surface dose calculations range from 15.0 % for the 4 mm applicator to 11.9 % for the 30 mm applicator. It is of interest to note that there is a decrease in the surface dose by just over 3 % from the smallest to the largest cone. However, this result is consistent with previous studies that reported decreases in the relative surface dose while using small field sizes and for increasing field sizes [28, 29]. The study by Ding et al. [28] found that the Monte Carlo calculated surface doses varied from 28 % for the 6 9 6 mm2 field, 26 % for the 12 9 12, 18 9 18 and 24 9 24 mm2 fields and then started increasing from a field size of 30 9 30 mm2. Similarly, the study by Paskalev et al. [29] found a reduction in the relative surface dose from the 1.5 and 3 mm diameter fields. While it is accepted that surface dose increases as a function of field size for megavoltage x-ray beams due to increasing scatter, most studies do not use very small field sizes of less than 3 cm for which lateral dose equilibrium is achieved [4]. The measured surface doses for the five circular collimators had relative surface values in the range between 13 and 15.5 %. These measured values were in agreement with the Monte Carlo calculated doses to within the 3 % measurement uncertainty. These results shows that the EBT3 film can be used for surface dose estimation in SRS beams and would have application in either benchmark relative dosimetry measurements or for in vivo surface dose measurements. It is important to know the skin dose as accurately as possible because skin toxicity can be a problem in SBRT treatments [47]. It is important to note that there have been some differences reported between MOSFETs and radiochromic film for breast radiotherapy treatments [48]. Furthermore, treatment planning systems may not adequately deal with skin dose calculation making it harder to know the exact skin dose for some treatments like early-stage non-small-cell lung cancer [47]. Therefore having an independent method to verify the skin dose is definitely important in order to achieve optimal skin sparing. Our model is one step forward in having such tool to evaluate
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Fig. 3 Percentage depth doses calculated by BEAMnrc/DOSXYZnrc and measured with a diode for: a 4 mm circular collimator, b 10 mm circular collimator, c 20 mm circular collimator and d 30 mm circular collimator
Table 1 Relative surface doses for the BrainLab SRS circular collimators determined by Monte Carlo calculations and Gafchromic EBT3 measurements Monte Carlo surface dose (% of Dmax)
EBT3 film surface dose (% of Dmax)
4
15.0
14.5
7.5
12.8
15.5
10
12.3
15.0
20
12.3
13.0
30
11.9
14.0
Circular collimator diameter (mm)
skin toxicity in treatments where conical collimators are used in SRS and SBRT treatments.
Conclusions This study has determined the surface doses for a 6MV SRS x-ray beam for very small field sizes from circular collimators with diameters ranging from 4 to 30 mm. The Monte Carlo method showed that the surface doses were: 15.0 % for 4 mm, 12.8 % for 7.5 mm, 12.3 % for 10 mm, 12.3 % for 20 mm and 11.9 % for 30 mm. The EBT3 film measurements produced the following surface doses:
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14.5 % for 4 mm, 15.5 % for 7.5 mm, 15.0 % for 10 mm, 13.0 % for 20 mm and 14.0 % for 30 mm. The uncertainty for EBT3 film measurements was 2 % (1 Standard Deviation). This work has shown that both methods gave consistent surface dose values and indicates that Gafchromic EBT3 film can be used for surface dose measurements in radiotherapy departments where Monte Carlo simulations are not available for stereotactic radiosurgery beams. Acknowledgments Computational resources and services used in this work were provided by the High Performance Computing and Research Support Unit, Queensland University of Technology, Brisbane, Australia. Also, we’d like to acknowledge that Dr S. B. Crowe’s contribution to this work was supported by the Australian Research Council through Linkage Grant No. LP110100401.
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Australas Phys Eng Sci Med (2014) 37:303–309 17. Roberson PL, Moran JM, Kulasekere R (2008) Radiographic film dosimetry for IMRT fields in the near-surface buildup region. J Appl Clin Med Phys 9(4):87–97 18. Xiang HF et al (2007) Build-up and surface dose measurements on phantoms using micro-MOSFET in 6 and 10 MV x-ray beams and comparisons with Monte Carlo calculations. Med Phys 34(4):1266–1273 19. Sors A et al (2013) An optimized calibration method for surface measurements with MOSFETs in shaped-beam radiosurgery. Physica Medica 20. Nakano M et al (2012) A study of surface dosimetry for breast cancer radiotherapy treatments using Gafchromic EBT2 film. J Appl Clin Med Phys 13(3):83–97 21. Rogers DWO (2006) Fifty years of Monte Carlo simulations for medical physics. Phys Med Biol 51(13) 22. Rogers DWO et al (1995) BEAM: a Monte Carlo code to simulate radiotherapy treatment units. Med Phys 22(5):503–524 23. Verhaegen F, Seuntjens J (2003) Monte Carlo modelling of external radiotherapy photon beams. Phys Med Biol 48(21) 24. Abdel-Rahman W et al (2005) Validation of Monte Carlo calculated surface doses for megavoltage photon beams. Med Phys 32(1):286–298 25. Kim JH, Hill R, Kuncic Z (2012) Practical considerations for reporting surface dose in external beam radiotherapy: a 6 MV X-ray beam study. Australasian Phys Eng Sci Med 35(3):271–282 26. Kim JH, Hill R, Kuncic Z (2012) An evaluation of calculation parameters in the EGSnrc/BEAMnrc Monte Carlo codes and their effect on surface dose calculation. Phys Med Biol 57(14):N267–N278 27. Apipunyasopon L, Srisatit S, Phaisangittisakul N (2013) An investigation of the depth dose in the build-up region, and surface dose for a 6 MV therapeutic photon beam: Monte Carlo simulation and measurements. J Radiat Res 54(2):374–382 28. Ding GX, Duggan DM, Coffey CW (2006) Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods. Phys Med Biol 51(10):2549–2566 29. Paskalev KA et al (2003) Physical aspects of dynamic stereotactic radiosurgery with very small photon beams (1.5 and 3 mm in diameter). Med Phys 30(2):111–118 30. Chang Z et al (2008) Dosimetric characteristics of novalis Tx system with high definition multileaf collimator. Med Phys 35(10):4460–4463 31. Dhabaan A et al (2010) Dosimetric performance of the new highdefinition multileaf collimator for intracranial stereotactic radiosurgery. J Appl Clin Med Phys 11(3):197–211 32. Yin F–F et al (2002) Dosimetric characteristics of Novalis shaped beam surgery unit. Med Phys 29(8):1729–1738 33. Kawrakow I (2000) Accurate condensed history Monte Carlo simulation of electron transport I EGSnrc, the new EGS4 version. Med Phys 27(3):485–498 34. Rogers DWO, Walters BRB, Kawrakow I (2005) BEAMnrc users manual ionizing radiation standards. National Research Council of Canada, Ottawa 35. Kawrakow I, Rogers DWO, Walters BRB (2004) Large efficiency improvements in BEAMnrc using directional bremsstrahlung splitting. Med Phys 31:2883–2898 36. Kawrakow I, Walters BRB (2006) Efficient photon beam dose calculations using DOSXYZnrc with BEAMnrc. Med Phys 33(8):3046–3056 37. Walters BRB, Kawrakow I (2007) Technical note: overprediction of dose with default PRESTA-I boundary crossing in DOSXYZnrc and BEAMnrc. Med Phys 34(2):647–650 38. ICRP59 (1991) The biological basis for dose limitation in the skin. Ann 22(2) 39. Casanova BV et al (2013) Dosimetric characterization and use of GAFCHROMIC EBT3 film for IMRT dose verification. J Appl Clin Med phys/American Coll Med Phys 14(2):4111
Australas Phys Eng Sci Med (2014) 37:303–309 40. Reinhardt S et al (2012) Comparison of Gafchromic EBT2 and EBT3 films for clinical photon and proton beams. Med Phys 39:5257 41. Hill R, Kuncic Z, Baldock C (2010) The water equivalence of solid phantoms for low energy photon beams. Med Phys 37(8):4355–4363 42. ISO Guide to the expression of uncertainties in measurement (1995). International Organisation for Standardization, Geneva 43. McEwen MR, Kawrakow I, Ross CK (2008) The effective point of measurement of ionization chambers and the build-up anomaly in MV X-ray beams. Med Phys 35(3):950–958 44. Hill R et al (2009) An evaluation of ionization chambers for the relative dosimetry of kilovoltage X-ray beams. Med Phys 36(9):3971–3981
309 45. Low DA et al (1998) A techinique for the quantitative evaluation of dose distributions. Med Phys 25(5):656–661 46. Ding GX (2002) Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of Monte Carlo simulations for a Varian 2100EX accelerator. Phys Med Biol 47:1025–1046 47. Hoppe BS et al (2008) Acute skin toxicity following stereotactic body radiation therapy for stage I non-small-cell lung cancer: Who is at risk?. Int J Rad Oncol Bio Phys 72:1283–1286 48. Kelly A et al (2011) Surface dosimetry for breast radiotherapy in the presence of immobilization cast material. Phys Med Biol 56(4):1001–1013
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SCIENTIFIC PAPER
A comparison of surface doses for very small field size x-ray beams: Monte Carlo calculations and radiochromic film measurements J. E. Morales • R. Hill • S. B. Crowe T. Kairn • J. V. Trapp
•
Received: 18 October 2013 / Accepted: 4 March 2014 / Published online: 20 March 2014 Ó Australasian College of Physical Scientists and Engineers in Medicine 2014
Abstract Stereotactic radiosurgery treatments involve the delivery of very high doses for a small number of fractions. To date, there is limited data in terms of the skin dose for the very small field sizes used in these treatments. In this work, we determine relative surface doses for small size circular collimators as used in stereotactic radiosurgery treatments. Monte Carlo calculations were performed using the BEAMnrc code with a model of the Novalis Trilogy linear accelerator and the BrainLab circular collimators. The surface doses were calculated at the ICRP skin dose depth of 70 lm all using the 6 MV SRS x-ray beam. The calculated surface doses varied between 15 and 12 % with decreasing values as the field size increased from 4 to 30 mm. In comparison, surface doses were measured using Gafchromic EBT3 film positioned at the surface of a Virtual Water phantom. The absolute agreement between calculated and measured surface doses was better than 2.0 % which is well within the uncertainties of the Monte Carlo calculations and the film measurements. Based on these results, we have shown that the Gafchromic EBT3 film is suitable for surface dose estimates in very small size fields as used in SRS.
J. E. Morales (&) S. B. Crowe T. Kairn J. V. Trapp School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD, Australia e-mail: [email protected] J. E. Morales R. Hill Department of Radiation Oncology, Chris O’Brien Lifehouse, Sydney, NSW 2006, Australia T. Kairn Genesis Cancer Care (Queensland), The Wesley Medical Centre, Brisbane, QLD, Australia
Keywords Stereotactic radiosurgery SRS Surface dosimetry skin dose Monte Carlo calculations Radiochromic film EBT3
Introduction Stereotactic radiosurgery (SRS) involves the delivery of a high radiation dose, typically 15–20 Gy, using small size radiation beams for treatments of lesions within the brain [1]. These lesions are usually malignant brain metastases or benign arterovenous malformations and require high spatial and dosimetric accuracy due to the need for accurate delivery to the small lesion as well as minimising radiation dose to other tissues such as the brain stem, optic chiasm and other critical structures. SRS treatments usually involves a number of beams using arcs and/or conformal fields in order to achieve an optimum dose distribution and normal tissue doses. Skin toxicity can also be a dose-limiting factor for radiotherapy treatment planning, despite the skin-sparing effect of megavoltage x-ray dose buildup [2–4]. Skin dose is a particular concern in SRS treatments, due to the high single-fraction doses delivered, and there is a growing interest in the incidence of skin toxicity associated with stereotactic body radiotherapy (SBRT or SABR) [47]. The ICRP defines the skin depth to be 70 lm which corresponds to the basal cell layer thickness [5]. Surface doses for megavoltage x-ray beams have been measured using a number of different detectors including parallel-plate ionisation chambers, thermoluminescent dosimeters (TLDs), metal– organic semiconductor field-effect transistors (MOSFETs) and various types of radiochromic film [2, 3, 6–19]. While some parallel-plate ionisation chambers, such as the Attix chamber are well characterised for dose measurements in the
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buildup region, they are not suitable for in vivo dosimetry measurements [2, 3, 20]. In addition to measurements, surface doses for x-ray beams can also be determined by Monte Carlo calculations. Monte Carlo methods are regarded as the gold standard for accurate dose calculations of ionising radiation beams [21–23] and there have been a number of studies which have used Monte Carlo methods for determining surface doses for conventional field sizes [2, 4, 24–27]. To date, there is limited data for surface doses for the very small field sizes as typically used in the delivery of SRS treatments [4, 28]. Paskalev et al.[29] examined the dosimetry of SRS beams, including surface doses measured with radiographic film, generated from a 10 MV x-ray beam and using 1.5 and 3 mm circular collimators [29] and found that the surface doses decreased slightly with increasing size of the collimator. In a comprehensive study of the dosimetry of the Brainlab m3 micro-MLC system, Ding et al. [28] used both experimental and Monte Carlo methods to characterise the dosimetry. As part of their results, the Monte Carlo depth dose data included surface doses for field sizes ranging from 6 9 6 to 100 9 100 mm2 with relative surface dose values ranging between 26 and 32 % of Dmax. The purpose of this study was to determine surface doses for a variety of very small size fields as defined by circular applicators ranging from 4 to 30 mm as used in SRS treatments and to establish the utility of radiochromic film for measuring such doses. Surface doses were calculated using Monte Carlo modelling of the linear accelerator using the BEAMnrc Monte Carlo code. In addition, surface doses were measured for these applicators using the Gafchromic EBT3 radiochromic film.
Materials and methods In this work, surface doses were determined for the 6 MV SRS x-ray beam as produced by a Novalis Trilogy linear accelerator (Varian Medical Systems, Palo Alto, USA) which has a thin flattening filter in order to produce a dose rate of 1,000 MU\min [30–32]. Beam collimation for the SRS x-ray beams was achieved by using the BrainLab circular applicators (BrainLab, Germany) with diameters of 4, 7.5, 10, 20 and 30 mm diameter as defined at the isocentre. The X and Y collimator jaws were set to 5 cm for all measurements and calculations. These jaw sizes were used during commissioning of this linear accelerator but it is noted that a recent notification from the manufacturer recommends different jaw settings for each circular collimator (Safety Notice 09-06-29.BAV.2). Commissioning Monte Carlo model The 6MV SRS x-ray beam was modeled using the BEAMnrc/EGSnrc Monte Carlo code (Version4, release
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2.3.2) [22, 33, 34]. Simulations were performed on supercomputing facilities at the Queensland University of Technology. The supercomputer at this location has 1924 64-bit Intel Xeon Cores. All of the specifications for the geometry and materials used within the linear accelerator model including the special SRS flattening filter were supplied by Varian Medical Systems. The adjustable incident electron beam parameters were optimized to an elliptical beam with Gaussian distributions of X = 0.4 mm and Y = 0.4 mm in FWHM and with no angular spread striking the tungsten target along the central axis. The optimization process involved the tuning of the initial electron energy to match the PDD curve for a 10 9 10 cm2 open field size. A reduced Chi squared method was applied to every PDD obtained by Monte Carlo. Energies were varied between 5.0 and 7.0 MeV in steps of 0.2 MeV. The reduced Chi squared results were plotted versus energy and a minimum was reached. This minimum value was taken to be the best value for Monte Carlo simulations. In our case the final value for the incident electron energy was 6.06 MeV. For both the BEAMnrc and DOSXYZnrc user codes, the electron cutoff energy (ECUT) and the photon cutoff energy were set to 0.521 MeV and 0.010 MeV respectively. PRESTA-I and PRESTA-II were turned on for the boundary crossing algorithm and electron-step algorithm respectively. The directional bremsstrahlung splitting variance reduction technique was also used with splitting factor of 1,000 [35, 36]. The initial testing of the BEAMnrc model was for an open field size of 10 9 10 cm2 and no circular applicator. A total of 4 9 107 histories were used. The initial model of the x-ray beam was verified by comparison of measured and calculated percentage depth dose data and cross-plane beam profiles as measured in a PTW MP3 water tank (PTW, Freiburg, Germany). The procedure for comparing data involved working out the absolute difference by subtracting the measured data from the Monte Carlo data at each measurement point. For the 10 9 10 cm2 open field size, depth doses and cross plane profiles were measured with a PTW Advanced Markus parallel-plate ionisation chamber (PTW, Freiburg, Germany) and PTW 60003 diamond detector (PTW, Freiburg, Germany). Surface Dose simulations for circular collimators After verifying the Monte Carlo model for the 10 9 10 cm2 open field size, phase space files were generated for the circular collimators with diameters of 4, 7.5, 10, 20 and 30 mm using the BEAMnrc code. Percentage depth doses were then calculated for each circular collimator using the DOSXYZnrc user-code (V4 r2-3-0) and compared with PDDs measured with a PTW 60012 diode
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(PTW, Freiburg, Germany) at a source to surface distance of 100 cm. Voxel sizes of 0.25 9 0.25 9 1.0 mm3 were used to score the dose for central-axis PDDs. A total of 8 9 109 incident particles were used to obtain statistical uncertainties of less than 2 %. For consistency, the same user-specified simulation parameters were used as in the phsp file calculation. To model electron transport as accurately as possible, a global ECUT of 0.521 MeV was specified which corresponds to an electron range of approximately 2.5 lm in water (NIST DATABASE at http:// physics.nist.gov/cgi-bin/Star/e_table.pl) [26]. The EXACT boundary crossing algorithm was turned on for surface dose calculations [37]. For the surface dose calculations with the circular collimators, additional calculations were carried out using the DOSXYZnrc user-code. The dose was scored in voxels with thicknesses of 10 lm for the first 0.1 mm depth, which includes the clinically relevant skin depth of 70 lm [38], as well as at depths of 1, 2 mm and dmax = 1.5 cm. All surface doses scored at or close to the surface of the phantom were normalized to Dmax.
the RIT software package V5.2 (Radiological Imaging Technology, Inc, USA). The RIT software needs a calibration curve which allows the conversion of the pixel value to absorbed dose. This was achieved by irradiating the film pieces with known doses of 0–3 Gy by using a 10 9 10 cm2 field size and the films were located at the depth of dmax. A median filter of 3 9 3 pixels was applied to all the scanned images. All films were read out approximately 24 h after their irradiation. For the surface dose measurements with the different collimators, the EBT3 film piece was placed at the surface of the Virtual Water phantom at an SSD of 100 cm. The uncertainty in the EBT3 film surface dose measurements was determined to be 2 % (1 Standard Deviation) using the ISO GUM methodology [42–44]. The factors contributing to uncertainties in surface dose measurements included: variation within the OD measurements for the pixels in the region of interest, variations due to film non-uniformity as well as uncertainties in the curve fit for the EBT3 film calibration curve.
Surface dose measurements using Gafchromic EBT3 film
Results and discussion
All measurements were performed with Gafchromic EBT3 radiochromic film (Ashland, Wayne, USA), batch number A10171102, which has a number of advantages over the earlier versions of this film including a polyester substrate which prevents the formation of Newton’s rings and the use of a symmetrical structure for the different layers in the manufacturing of the film [39, 40]. The EBT3 film has the active layer of 30 lm thickness which is located between the two polyester layers of 125 lm thickness. The EBT3 film sheet was cut into 2 9 2 cm2 pieces for both the dose calibration and surface dose measurements. The film piece was positioned at the surface and at a depth of Dmax between the Virtual Water blocks [41]. For each set of measurements a minimum of two EBT3 film pieces were used in this study, and the mean dose absorbed by these film pieces was used for analysis purposes. The process for preparing, reading out the films and analyzing the dose information was consistent with manufacturer’s recommendations. All films were read out on an Epson Expression 10000 XL (EPSON) flatbed scanner maintaining a fixed orientation of the film during the readout. The orientation chosen was landscape which keeps the short edge of the film parallel to the scan direction. The EPSON scan software package was used to scan films using the red channel only at a resolution of 150 DPI in transmission mode with all image adjustment features switched off. The dosimetric analysis was performed using
Commissioning Monte Carlo model The uncertainty in the Monte Carlo calculated depth doses was less than 1 % as determined within the DOSXYZnrc user code. Figures 1 and 2 show the comparison between the Monte Carlo calculated and measured central axis depth dose curves and cross-plane profiles respectively for the open 10 9 10 cm2 reference field. The agreement between the Monte Carlo calculated and the measured doses for the PDD shown in Fig. 1 was within 1 % for depths of 1.0–30.0 cm and within 2 % for depths within 0.5–0.99 cm, which is well within an acceptance criteria of 2 % [45, 46]. Measurements with the advanced Markus for depths between 0 and 0.5 cm are not shown as they were deemed unreliable due to the meniscus effect near water surface. Figure 2a shows the Monte Carlo calculated and measured profiles at three depths in water across the X jaws. The depths were 1.4, 10 and 20 cm. All profiles were normalized with their respective PDD value at each depth. Figure 2b shows the difference between calculated and measured data. A maximum absolute difference of 1.4 % was obtained at the penumbra regions. Surface dose simulations The comparison between the Monte Carlo calculated and measured central axis depth dose curves for the circular
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acceptance criteria of 2 % [45, 46]. The uncertainty in measurements was 1 %. Surface dose measurements using Gafchromic EBT3 film
Fig. 1 Percentage depth dose in water calculated for a 10 9 10 cm2 field. BEAMnrc/DOSXYZnrc versus measurement by an advanced markus ionisation chamber
Fig. 2 a Cross profiles at depth in water calculated by BEAMnrc/ DOSXYZnrc for a 10 9 10 cm2 field versus measurement with a diamond a depths of 1.4, 10 and 20, b absolute difference between calculation and measurement for each depth
collimators are shown in Fig. 3. For all the circular collimators the agreement between Monte Carlo calculated and measured percentage depth doses was within 1 % for depths between 0.5 and 30 cm. This is well within an
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A summary of the relative surface doses for the Monte Carlo calculations and the Gafchromic EBT3 measurements is presented in Table 1. The estimated uncertainty in the Gafchromic EBT3 film measurements was 2.0 %. For the Monte Carlo calculations, the surface dose calculations range from 15.0 % for the 4 mm applicator to 11.9 % for the 30 mm applicator. It is of interest to note that there is a decrease in the surface dose by just over 3 % from the smallest to the largest cone. However, this result is consistent with previous studies that reported decreases in the relative surface dose while using small field sizes and for increasing field sizes [28, 29]. The study by Ding et al. [28] found that the Monte Carlo calculated surface doses varied from 28 % for the 6 9 6 mm2 field, 26 % for the 12 9 12, 18 9 18 and 24 9 24 mm2 fields and then started increasing from a field size of 30 9 30 mm2. Similarly, the study by Paskalev et al. [29] found a reduction in the relative surface dose from the 1.5 and 3 mm diameter fields. While it is accepted that surface dose increases as a function of field size for megavoltage x-ray beams due to increasing scatter, most studies do not use very small field sizes of less than 3 cm for which lateral dose equilibrium is achieved [4]. The measured surface doses for the five circular collimators had relative surface values in the range between 13 and 15.5 %. These measured values were in agreement with the Monte Carlo calculated doses to within the 3 % measurement uncertainty. These results shows that the EBT3 film can be used for surface dose estimation in SRS beams and would have application in either benchmark relative dosimetry measurements or for in vivo surface dose measurements. It is important to know the skin dose as accurately as possible because skin toxicity can be a problem in SBRT treatments [47]. It is important to note that there have been some differences reported between MOSFETs and radiochromic film for breast radiotherapy treatments [48]. Furthermore, treatment planning systems may not adequately deal with skin dose calculation making it harder to know the exact skin dose for some treatments like early-stage non-small-cell lung cancer [47]. Therefore having an independent method to verify the skin dose is definitely important in order to achieve optimal skin sparing. Our model is one step forward in having such tool to evaluate
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Fig. 3 Percentage depth doses calculated by BEAMnrc/DOSXYZnrc and measured with a diode for: a 4 mm circular collimator, b 10 mm circular collimator, c 20 mm circular collimator and d 30 mm circular collimator
Table 1 Relative surface doses for the BrainLab SRS circular collimators determined by Monte Carlo calculations and Gafchromic EBT3 measurements Monte Carlo surface dose (% of Dmax)
EBT3 film surface dose (% of Dmax)
4
15.0
14.5
7.5
12.8
15.5
10
12.3
15.0
20
12.3
13.0
30
11.9
14.0
Circular collimator diameter (mm)
skin toxicity in treatments where conical collimators are used in SRS and SBRT treatments.
Conclusions This study has determined the surface doses for a 6MV SRS x-ray beam for very small field sizes from circular collimators with diameters ranging from 4 to 30 mm. The Monte Carlo method showed that the surface doses were: 15.0 % for 4 mm, 12.8 % for 7.5 mm, 12.3 % for 10 mm, 12.3 % for 20 mm and 11.9 % for 30 mm. The EBT3 film measurements produced the following surface doses:
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14.5 % for 4 mm, 15.5 % for 7.5 mm, 15.0 % for 10 mm, 13.0 % for 20 mm and 14.0 % for 30 mm. The uncertainty for EBT3 film measurements was 2 % (1 Standard Deviation). This work has shown that both methods gave consistent surface dose values and indicates that Gafchromic EBT3 film can be used for surface dose measurements in radiotherapy departments where Monte Carlo simulations are not available for stereotactic radiosurgery beams. Acknowledgments Computational resources and services used in this work were provided by the High Performance Computing and Research Support Unit, Queensland University of Technology, Brisbane, Australia. Also, we’d like to acknowledge that Dr S. B. Crowe’s contribution to this work was supported by the Australian Research Council through Linkage Grant No. LP110100401.
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