Confidential: not for distribution. Submitted to IOP Publishing for peer review 3 October 2007
Dosimetric evaluation of a Monte Carlo IMRT treatment planning system incorporating the MIMiC P Rassiah-Szegedi1,4, M Fuss2, D Sheikh-Bagheri3, M Szegedi4, S Stathakis1,4, J Lancaster4, N Papanikolaou1,4, and B Salter5 Cancer Therapy and Research Center, San Antonio, 78229 Texas University of Oregon Health Science Center, Portland, Oregon Allegheny Hospital, Pittsburg, PA University of Texas Health Science Center at San Antonio, 78229 Texas Huntsmen Cancer Center, Salt Lake City, Utah Email:
[email protected] Short title: Evaluation of MC MIMiC IMRT Classification: 52.65.Pp Monte Carlo methods 87.00.00 Biological and medical physics Key words: MIMiC, Monte Carlo, lung lesions, inhomogeneity correction, SBRT
Abstract: The high dose per fraction delivered to lung lesions in stereotactic body radiation therapy (SBRT) demands high dose calculation and delivery accuracy. The inhomogeneous density in the thoracic region along with the small fields used typically in IMRT treatments poses a challenge in the accuracy of dose calculation. In this study we dosimetrically evaluated a pre-release version of a Monte Carlo planning system (PEREGRINE 1.6b, NOMOS, Cranberry Township, PA), which incorporates the modeling of serial tomotherapy IMRT treatments with the binary Multileaf Intensity Modulating Collimator (MIMiC). The aim of this study is to show the validation process of PEREGRINE 1.6b since it was used as a benchmark to investigate the accuracy of doses calculated by a finite size pencil beam (FSPB) algorithm for lung lesions treated on the SBRT dose regime via serial tomotherapy in our previous study. Doses calculated by PEREGRINE were compared against measurements in homogenous and inhomogeneous materials carried out on a Varian 600C with a 6 MV photon beam. Phantom studies simulating various sized lesions were also carried out to explain some of the large dose discrepancies seen in the dose calculations with small lesions. Doses calculated by PEREGRINE agreed to within 2% in water and up to 3% for measurements in an inhomogeneous phantom containing lung, bone and unit density tissue.
Dosimetric evaluation of a clinical implementation of IMRT Monte Carlo treatment planning system with serial tomotherapy
Abstract: The high dose per fraction delivered to lung lesions in stereotactic body radiation therapy (SBRT) demands high dose calculation and delivery accuracy. The inhomogeneous density in the thoracic region along with the small fields used typically in IMRT treatments poses a challenge in the accuracy of dose calculation. In this study, we dosimetrically evaluated a pre-release version of a Monte Carlo planning system (PEREGRINE MC 1.6b, North American Scientific, Cranberry Township, PA), which incorporates the modeling of serial tomotherapy IMRT treatments with the binary Multileaf Intensity Modulating Collimator (MIMiC). The aim of this study is to show the commissioning process of PEREGRINE MC 1.6b since it was used as a benchmark to investigate the accuracy of doses calculated by a finite size pencil beam (FSPB) algorithm for lung lesions treated on the SBRT dose regime via serial tomotherapy in our previous study. Doses calculated by PEREGRINE MC were compared against measurements in homogenous and inhomogeneous materials carried out on a Varian 600C with a 6 MV beam. Several MIMiC source models were evaluated and the final model agreed to within 2% in water and up to 3% for measurements in an inhomogeneous phantom containing lung, bone and unit density tissue.
Key words: MIMiC, Monte Carlo, lung lesions, inhomogeneity correction, SBRT
1. Introduction Due to the high, hypo-fractionation scheme, Stereotactic Body Radiation Therapy (SBRT) of small pulmonary lesions, demands high confidence that the doses delivered to a volume or point correspond with doses prescribed by the treating physician. However, heterogeneities in tissue density, a hallmark of pulmonary target locations, have rendered accurate dose calculation a challenge. The use of small 3-dimensional conformal radiotherapy (3DCRT) or intensity-modulated radiation therapy (IMRT) beams, which lack electronic equilibrium further compounds this problem (Jones and Das, 2005). The finite size pencil beam (FSPB) model commonly employed by multiple radiation treatment planning systems, has been shown to accurately predict doses in unit density media (Ostapiak et al, 1997, Sohn et al 2003). However, when the finite size pencil beam model is used in conjunction with an inhomogeneity correction such as the effective path length (EPL) in small fields and in an inhomogeneous media, dose prediction is much less accurate. This is because the finite size pencil beam/ effective path length (FSPB/EPL) calculates dose in inhomogeneous environment such as the chest using 2 key steps: (1) initial calculations are based on the assumption of a homogenous water density equivalent medium, then (2) the dose is adjusted to the non-water medium based on a CT density data derived tissue heterogeneity correction only in the direction of the primary beam. This process, along with an assumption of charge particle equilibrium makes it almost impossible for such algorithms to achieve dose accuracy of 3% or less (AAPM Report 85 2004) in the inhomogeneous environment of the thorax, though in a homogeneous region they work very well.
In our previous work (Rassiah-Szegedi et al 2006) we reported the dosimetric discrepancies between FSPB/EPL and Monte Carlo (MC) calculation for pulmonary SBRT target volumes using serial tomotherapy. MC was used as a benchmark to evaluate the accuracy of doses calculated by the treatment planning system (TPS). The clinical relevance of our findings of
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lower actual delivered than computed and prescribed SBRT doses was based on a statement that the MC model used agreed with validations to within 2% in homogenous and 3% in inhomogeneous media. The purpose of this paper is to describe the validation of the MC calculation method employed in support of these earlier reported findings. Given that the accuracy of the underlying MC model, PEREGRINE MC, (Nomos Crop. Cranberry Township, PA) used, is well characterized (Schach 2000, Hartman 2001, Heath and Seuntjens 2004), we focused on the validation of the serial tomotherapy specific binary multi-leaf collimator (MIMiC) instantiation of the model. Serial tomotherapy as a technology has proven to be reliable and its dosimetry such as abutting fields is well researched and understood (Low DA et al 2001). Besides being less expensive with faster delivery and plan development time with no difference in plan quality compared to helical tomotherapy (Fuss M, Shi CY and Papanikolaou N 2006), the versatility of serial tomotherapy compared to helical tomotherapy allows for treatment device extensions for smaller beamlets such as the beak (Salter BJ 2001) and couch angulations (Salter BJ et al 2001). Hence serial tomotherapy is used as one of the modes to deliver IMRT treatments in our clinic, alongside other IMRT technologies including helical tomotherapy and static field MLC based IMRT
2. Materials and Methods The Monte Carlo code used in this study is a pre-release version of PEREGRINE MC® 1.6b, which is interfaced with Corvus 5.0 (Nomos Corp., Cranberry Township, PA) and integrates the MIMiC delivery device. Following the development of an optimized treatment plan using the OctTree dose-point allocation approach, the user is given the option to calculate a finalized dose distribution in a conventional manner (i.e.using the treatment planning system (TPS) inherent FSPB/EPL dose calculation algorithm) or PEREGRINE MC calculation. PEREGRINE MC, developed by Lawrence Livermore National Laboratory and licensed to NOMOS Corporation for distribution uses the BEAMnrc MC code to simulate the linear accelerator head, which extends from the top of the bremsstrahlung target to the bottom
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of the monitor chamber. The electron beam incident on the target is assumed monoenergetic with no divergence with an initial uniform radius of 0.1 cm (Hartmann et al, 2001). The output of the BEAMnrc used here is a phase space file containing 30 million particles with each particle described in 5 dimensions (E, X, Y, U, V) i.e. E is the particles energy, X and Y are positional coordinates, U and V directional coordinates (Rogers, 1995). The 40 cm x 40 cm phase space file is scored below the monitor chamber. The phase space file is then used to generate a set of correlated histograms (beam source model). The energies and trajectories of the photons in these histograms accurately simulate the underlying gigabyte-size phase space file to better than 1% (Schach AE, 1999). The beam source model reduces the run time of a calculation and eliminates the necessity of storing large files for every treatment unit created. The agreement between PEREGRINE MC (source model based) and EGSnrc (phase space file based) has been shown to be within 1% for a 10 x 10 cm field (Heath E et al 2004). During simulation, the beam source model is sampled to generate particles that are tracked through the treatment specific beam modifiers (collimator jaws, MIMiC, Multileaf collimator etc.) and patient. The electron total cutoff energy is 0.611 MeV in air and 2 MeV in metal. In the patient, cutoff energies of 0.521 MeV for electron and 0.01 MeV for photon were used. The Monte Carlo code used in PEREGRINE MC is described in detail by Hartmann (2001).
The beam source model, a description of the beam modifiers (material, density and dimensions), calibration factor that converts monitor units to dose and backscatter coefficients as a function of jaw opening, form the device file in PEREGRINE MC. In PEREGRINE MC 1.6b, the MIMiC is described as one of the beam modifiers in the device file. The MIMiC consists of 2 banks of 20 pneumatically driven vanes which make up a nominal maximum field size of 20 x 4 cm. Each vane is projected at isocenter at nominal 1 cm (width) and retracts by 1.6 or 0.8 cm. The 8 cm thick vanes are made of sintered tungsten with a density of 17.0 g cm3
. The top plane of the MIMiC is located 52 cm from the target in the Varian 600C.
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The 3D density grid was derived from the CT calibration curve, material specification table and patient or phantom CT image set. The dose is scored in overlapping spheres called dosels with a radius of 2 mm. During simulation, the user chooses a statistical quality factor (SQF), where SQF is the fractional error. The run is terminated when the standard deviation of the watch dosel is less than SQF x W (where W is the dose in the watch dosel). The watch dosel is determined after a fix number of runs as the dosel with the largest dose. When PEREGRINE MC completes the dose calculation, the dose scored in the dosel grid is mapped onto the CT grid on Corvus, which is approximately 0.9 mm x 0.9 mm. The dose distribution can then be viewed on the CT images at the Corvus workstation.
2.1. The commissioning procedure The commissioning procedure of the MIMiC model in PEREGRINE MC involves three steps 2.1.1.
Interpolating the incident monoenergetic electron energy on the bremsstrahlung target. In our treatment unit, this energy is 5.32 MeV, which was inferred from the 10 cm x 10 cm depth dose measurement taken from the 600C Varian 6 MV accelerator.
2.1.2.
Setting the dose per monitor unit (cGy/ MU) which is 1 cGy/MU for a 10 cm x 10 cm field, 100 cm source to surface distance, at the point of dose maximum in our clinic.
2.1.3.
Validating the calculations against measurements. The validation process includes evaluation of 2.1.3.1. Output factors Due to volume dose averaging and positional uncertainties introduced by ion chambers in very small fields, EDR films were used to characterize the change in output for field sizes ranging from 1 cm x 0.8 cm to 20 cm x 3.2 cm. All measurements, including film calibration were carried out at 5 cm depth in 30 cm x 30 cm plastic water slabs (CIRS, VA USA) with 10 cm of plastic water used to provide back scatter. All measurements were normalized to a 10 cm x 10 cm open field at 5 cm depth.
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2.1.3.2.
Percentage depth doses (PDD) and profiles for a range of field sizes. The Blue
phantom which is a 3 dimensional water phantom from Wellhofer Dosimetrie, Schwarzenbach, Germany, with a positioning accuracy of ± 0.05 cm, was used to measure depth doses for fields sizes depicted by the MIMiC using a CC13 thimble chamber (active volume of 0.13 cm3, an active length of 0.58 cm, inner diameter of 0.6 cm and 0.4 cm wall thickness), (Wellhofer Dosimetrie, Schwarzenbach, Germany). The depth doses were shifted upstream by 0.18 cm (0.6 x radius) (AAPM Report 67, 2004) to account for the effective point of measurement. Profiles at various depths (1.5 cm, 5 cm, 10 cm and 20 cm) and field sizes were measured with EDR films in the CIRS plastic water phantom.
2.1.3.3.
Vane leakage Leakage and transmission measurements were carried out in
water phantom using the Exradin A1 chamber with a collecting volume of 0.056 cm3 and an inner diameter of 0.4 cm at 300V bias. Measurements were taken with the ion chamber perpendicular to the beam central axis at 3 positions, 1) the junction of 4 opposing vanes (central axis), 2) between 2 adjacent vanes and 3) the middle of a vane with all the leaves closed to characterize the leakage and transmission of the MIMiC as shown in figure 1. Intervane gaps were not included in the modeling of the MIMiC; therefore the vane density in the model was empirically decreased to account for intervane leakage through the use of increased transmission for the vane. Accounting for leakage in this manner is not reasonable since the dose delivered in serial tomotherapy is delivered in continuous arcs, and the effect of leakage is thus blurred over the entire treatment volume.
An iterative method employing both calculation and measurement was used to ascertain the necessary reduction in vane density in the model. Plans were generated based on CT data acquired from a customized heterogeneous thoracic phantom (CIRS, VA, USA). The thoracic phantom contained 1, 2, 3 and 4 cm diameter cylindrical tumor nodules of unit density embedded into lung equivalent material with various inserts for TLDs, MOSFETs
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and BANG GEL as shown in figure 2. MOSFETs (Thomson Nielsen) were placed in the phantom during the CT scan to ensure that the calculation algorithm had information on the higher density of the MOSFET (silicon oxide). The phantom was scanned with 3 mm slice thickness on the PQ 5000 (Philips Medical, MA, USA). Computations were carried out with various vane densities (ex. 17 gcm-3, 14.2 gcm-3, 14.0 gcm-3 and 13.8 gcm-3) to ascertain the best agreement between calculation and measurement. MOSFET measurements in regions exposed primarily to leakage dose were made and compared to calculation. The MOSFET 20 patient dose verification system (Thomson & Nielsen Electronics Ltd., Ottawa, ON, Canada) with dual-bias dual-MOSFETs (model TN-502RD) and active region of 0.04 mm2 was used in this study. The MOSFETs were calibrated prior to use and were used with vendor-defined bias setting appropriate to the dose level being measured. Used in this manner, the MOSFETs have an uncertainty of ±2% and isotropic response to within ±2.5% for 180° (Chuang CF 2002). Irradiations were carried out on a Varian 600CD equipped with MIMiC.
2.1.3.4. Intensity modulated treatment plans. Serial tomotherapeutic IMRT plans, were developed for all 4 sizes of the tumor nodules available in the thoracic phantom. TLDs (TLD-100, LiF: Mg, Ti, Bicron USA) were placed in the center of the tumor nodule, which was located centrally within the lung as shown in figure 2. TLDs were also placed in the low-density lung area, both in and out of the axial treatment plane. Irradiations were carried out on a Varian 600CD equipped with MIMiC. The TLDs chips with dimensions of 3.2 mm x 3.2 mm x 0.89 mm were calibrated individually and had an uncertainty of ± 3%. The 4 cm tumor nodule was also replaced by a cylindrical BANG gel canister for relative 3D dose distribution measurement. The irradiated gel was later scanned with the
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OCTOPUS (Hu and Maryanski 2004) laser CT scanner (MGS Research Inc., Madison, CT).
2.2. PEREGRINE MC calculations. PEREGRINE MC calculations for scenarios described in sections 2.1.3.1 and 2.1.3.2 were carried out in a homogenous CT phantom which has a uniform equivalent CT number of 0 (water), electron density of unity and dimensions of 50 cm x 50 cm x 30 cm.
Dose
distributions were calculated with Beam Utilities in Corvus 5.0 with a SQF of 0.5 with approximately 3 hours of calculation time required for such high precision dose calculation on an eight (8) station parallel processing array. Calculations for scenarios described in sections 2.1.3.3 and 2.13.4 which were plans generated on the thoracic phantom were calculated to attain a SQF of 1 with an approximate calculation time of 2 hours.
3. Results and discussions Output factors measured, as described above, and predicted by PEREGRINE MC for MIMiC field sizes ranging from a single open vane (1.0 cm x 0.8 cm) to all vanes open (20.0 cm x 3.2 cm) are presented in figure 3. Agreement to within 2 % is demonstrated. Three different source models were tested before the appropriate model was chosen. The initial source model had a 3 mm diameter parallel electron beam incident on the target. The profiles calculated with this model were found to overestimate the dose at the horns by 3%. Such discrepancies can be resolved by manipulating the parameters such as size or shape of the incident electron beam (Sheikh-Bagheri and Rogers, 2002). Whenever the phase space file is recomputed, a new set of histograms (source model) must be created and replaced in the device file. Therefore a second source model was developed using a much smaller electron beam of 1 mm diameter to isolate the reason for this discrepancy. This time, an underestimation was observed in the horn region. The off axis ratio at 80 % of the 20 x 4 cm field for both the above mentioned models was interpolated based on the measured profile to arrive at an expected 2.4
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mm ideal size for the incident electron beam. The field size of 20 x 4 cm was chosen because it is the largest field size with the MIMiC and any discrepancy in the horn region would be more obvious with a larger field. The profiles calculated with all three models were compared to measurement and are shown in figure 4. The change in the fitted beam energy for all threeelectron beam sizes was negligible (less than 2 keV) and was not observed in the depth dose curve. This is because the beam energy is inferred from the 10 cm x 10 cm measured PDD. The profiles and PDD calculated with this model agreed to within 2 % in the low gradient region and within 2 mm in the high gradient region as shown in figures 5 and 6.
The intervane leakage measured at 5 cm depth was 0.39% and the vane transmission was 0.26%. The agreement between leakage measurement via treatment plans and calculation was 0.1 ± 0.6 % for a vane density of 14.0 gcm-3 as shown in table 1. Hence a vane density of 14.0 gcm-3 was used for plan generation. Decreasing the vane density in PEREGRINE MC from 17.0 gcm-3 to 14.0 gcm-3 increased the transmission by 0.07%
PEREGRINE MC calculated doses in the center of the unit density target with diameter of 1– 4 cm were within 1% of TLD measurement as shown in table 2, where position #1, #2, #3 and #4 correspond to the center of target with 1 cm, 2 cm, 3 cm and 4 cm diameter. Regardless of lesion size PEREGRINE MC is capable of predicting dose to the center of the lesion accurately. Table 3 shows the agreement between calculation and TLD measurement in the low density region (lung) located in 4 arbitrary positions within the treatment plane. Figure 7 shows the relative 100% isodose line agreement of bang gel measurement and PEREGRINE MC normalized to the prescribed dose. The PEREGRINE MC predicted isodose line was within 2 mm of measurement at the 100% isodose level.
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4. Conclusion The agreement between measured and PEREGRINE MC 1.6b calculated depth doses and profiles in water with MIMiC for both the 1 cm and 2 cm modes was within 2%, which is comparable to other MC planning systems calculations (Deng et al 2003). Through a series of measurements and calculations, an optimal vane density of 14.0 gcm-3 was determined for this particular model. In an inhomogeneous environment the PEREGRINE MC 1.6b MIMiC model is accurate to better than 3.0 %. The doses reported for plans with PEREGRINE MC 1.6b are therefore subject to a maximum of 3.0 % inaccuracy when an SQF of 2 is used to calculate treatment plans. Compared with FSPB/EPL based dose calculations, which were documented to predict IMRT, lung dosimetry with variable accuracy, PEREGRINE 1.6b Monte Carlo calculations for serial tomotherapy can provide more accurate dose predictions. Thus, the clinical relevance of this work is related to the requirement to provide the radiation oncologist with accurate dose predictions that allow clinical radiation dose prescriptions based on realistic estimates of local tumor control and normal tissue dose exposure. While alternate IMRT planning and delivery techniques such as static field MLC based IMRT, helical tomotherapy and intensity-modulated arc therapy have become clinically available or will become available shortly, serial tomotherapy systems are still widely used and a significant number of patients are treated by those systems on a daily basis, many of which may benefit from more accurate dose delivery predictions.
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AAPM Report 67 1999 Protocol for clinical dosimetry of high-energy photon and electron beams Report of Task Group No. 51 of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) (Madison, WI: Medical Physics Publishing).
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Chuang CF, Verhey LJ and Xia P 2002 Investigation of the use of MOSFET for clinical IMRT dosimetric verification. Med Phys. 29 1109-15.
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Deng J, Ma CM, Hai J, and Nath R. 2003 Commissioning 6MV photon beams of a stereotactic radiosurgery system for Monte Carlo treatment planning. Med. Phys. 30 3124 -3134.
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Fuss M, Shi CY and Papanikolaou N 2006 Tomotherapeutic stereotactic body radiation therapy: Techniques and comparison between modalities. Acta Oncologica 45 953 -960
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Harmann Siantar C L, Walling R S, Daly T P, Faddegon B, Albright N, Bergstrom P, Bielejaw A F, Chuang C, Garrett D, House R K, Knapp D, Wieczorek D J and Verhey L J 2001 Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom. Med. Phys. 28 1322– 1337
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Heath E, Seuntjens J, and Sheikh-Bagheri D 2004 Dosimetric evaluation of the clinical implementation of the first commercial IMRT Monte Carlo treatment planning system at 6 MV. Med. Phys. 31 2771-2779.
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Jones A O and Das I J 2005 Comparison of inhomogeneity correction algorithms in small photon fields. Med Phys. 32 766-76.
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9. Low D A, Mutic S, James F, Dempsey J F, Markman J, Chao K S C and Purdy J A 2001 Abutment dosimetry for serial tomotherapy. Medical Dosimetry 26 79–82. 10. Ostapiak O Z, Zhu Y and Van Dyk J 1997 Refinements of the finite-size pencil beam model of three-dimensional photon dose calculation Med Phys. 24 743-50. 11. Rassiah-Szegedi P, Salter B J, Fuller C D, Blough M, Papanikolaou N, Fuss M 2006 Monte Carlo characterization of target doses in stereotactic body radiation therapy (SBRT). Acta Oncol. 45 989-94. 12. Rogers D W, Faggegon B A, Ding G X, Ma C M, We J, Mackie T R. 1995 BEAM: a Monte Carlo code to simulate radiotherapy treatment units. Med Phys. 22 503-24. 13. Salter B J, Hevezi J M, Sadeghi A, Fuss M, Herman T S 2001. An oblique arc capable patient positioning system for sequential tomotherapy. Med Phys. 28 2475-88. 14. Salter B J 2001 NOMOS Peacock IMRT utilizing the Beak post collimation device. Med Dosim. 26 37-45. 15. Saw C B, Ayyangar K M, Thompson R B, Zhen W, and Enke C A 2001 Commissioning of the peacock system for intensity modulated radiation therapy. Med Dosim. 26 37-45. 16. Sohn J W, Dempsey J F, Suh T S, Low D A. 2003 Analysis of various beamlet sizes for IMRT with 6 MV photons. Med Phys. 30 2432-9. 17. Schach von Witteneau A E. 2000 Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations. Med. Phys. 27 935-947. 18. Schach von Witteneau A E. 1999 Correlated histogram representation of Monte Carlo derived medical accelerator photon-output phase space. Med. Phys. 26 1196-1210. 19. Sheikh-Bagheri D and Rogers D W O 2002, Sensitivity of megavoltage photon beam Monte Carlo simulations to electron beam and other parameters. Med Phys. 29 391402.
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20. Xu Y, Wuu Cheng-Shie, Maryanski M J 2004 Performance of a commercial optical CT scanner and polymer gel dosimeters for 3-D dose verification. Med Phys. 31 3024-33.
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Table 1: Leakage and transmission evaluation. Values shown are % deviations in MC calculation with various vane densities from TLD measurement.
Vane density gcm-3 TLD position 1 2 3 4 5 6 7 8 9 10 11 12
17.0
14.2
14.0
13.8
-4.02 -5.00 -4.43 -2.10 -2.29 -4.34 -5.02 -3.96 -2.29 -4.34 -5.02 -3.96
-0.68 -1.00 0.23 0.57 0.27 -1.54 0.00 0.20 -1.25 0.02 -0.17 -0.60
-0.35 -0.67 0.57 0.90 0.93 -0.71 0.66 0.20 -0.36 0.46 0.27 -0.27
0.65 0.33 1.23 1.90 1.43 1.13 1.00 0.70 0.21 -0.50 -1.17 0.17
Average Std Dev
-3.90 1.08
-0.33 0.67
0.14 0.59
0.59 0.85
Table 2: Dose deviation at the center of the unit density target
Target diameter (cm)
TLD (Gy)
MC (Gy)
% Deviation * PEREGRINE MC
1.0 2.0 3.0 4.0
6.1 ± 0.2 6.5 ± 0.2 6.0 ± 0.2 6.3± 0.2
6.2 6.5 6.0 6.4
0.7 0.3 0.0 0.5
*Deviation per prescribed dose (6 Gy) of PEREGRINE MC calculated dose from measurement.
Table 3: Dose deviation in lung. Postion 1-4 indicates 4 arbirtary positions located within the treatment plane in the lung. Position
TLD (Gy)
MC (Gy)
1 2 3 4
0.66 ± 0.02 0.80 ± 0.02 0.79 ± 0.02 1.13 ± 0.03
0.52 0.85 0.94 1.01
% Deviation * PEREGRINE MC
-2.3 0.8 2.5 -2.0
*Deviation per prescribed dose (6 Gy) of PEREGRINE MC calculated dose from measurement.
Figure 1: Schematic diagram of the positions of the leakage and transmission measurements taken with the ion chamber orientated perpendicular to the axis at 3 positions, 1) the junction of 4 opposing vanes (central axis), 2) between 2 adjacent vanes and 3) the middle of a vane with all the leaves closed to characterize the of the MIMiC.
Figure 2 (a) CIRS thoracic phantom with 4 sizes of unit density tumor nodules embedded in lung equivalent material. (b) CT slice showing possible locations for MOSFETs (A), TLD (B) and Bang Gel (C) which replaces the tumor nodule.
Figure 3: Comparison of relative output factors versus equivalent field size for the Clinac 600C 6 MV beam with the MIMiC.
Figure 4: The agreement of calculated profiles for field size of 20 cm x 4 cm at 5 cm depth when the diameter of the electron beam incident on the target is varied in the MC simulation (3 mm dashed line, 2.4 mm smooth line and 1 mm dotted line) and measured values (+). The 2.4 mm diameter used in this study agreed with measurements within 2 %/ mm.
Figure 5: Ion chamber measured (dashed line) and PEREGRINE MC calculated (smooth line) relative depth dose agreed to within 1%/mm.
Figure 6: Comparison of profiles at 1.5 cm, 5.0 cm and 10 cm depths for MIMiC with all vanes open in 2 cm mode (6a). Comparison of cross-plane off axis profile at 1.5 cm depth for field sizes ranging from 1.0 cm x 0.82 cm to 3.4 cm x 20 cm (6b)
Figure 7: Illustration of the relative 100% isodose line agreement of bang gel measurement and PEREGRINE MC inside a 4 cm diameter bang gel canister using the CIRS phantom. PEREGRINE MC isodose line agrees to within 2 mm of measurement.
1
2
3
2 (a)
2(b)
A C B)
1.1
Measurement MC
Output factor
1
0.9
0.8
0.7
0.6 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0 2
Equivalent square / cm
4.5
5.0
5.5
6.0
1.1
1
Relative depth dose
0.9
0.8
0.7 3 mm Measurement
0.6 2.4 mm 1 mm
0.5 0
2
4
6 Off axis distance /cm
8
10
12
Measurement
1.0
MC
Relative Depth Dose
0.8
10 cm x 10
0.6 20 cm x 3.3
2 cm x 1.6
0.4
0.2
0.0 0
5
10
15 Depth / cm
20
25
30
Measurement MC
6 (a)
1.2
Measurement MC
5a
1.2 1
Relative dose
0.8
0.8
0.6 0.6 0.4 0.4
0.2
0.2 0
0
0 0
2
4
6
8
/ cm 4 Off axis distance 6 8 Off axis distance / cm
2
10
12 10
14 12
14
Measurement MC
6 (b)
1.2
1
0.8 Relative Dose
Relative dose
1
0.6
0.4
0.2
0 0
2
4
6 Off axis distance/ cm
8
10
12
4.0 cm
