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Verification of Monte Carlo calculations around a Fletcher Suit Delclos ovoid with normoxic polymer gel dosimetry
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2004 J. Phys.: Conf. Ser. 3 217 (http://iopscience.iop.org/1742-6596/3/1/031) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 206.214.93.182 This content was downloaded on 03/07/2017 at 15:44 Please note that terms and conditions apply.
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Institute of Physics Publishing doi:10.1088/1742-6596/3/1/031
Journal of Physics: Conference Series 3 (2004) 217–220 Third International Conference on Radiotherapy Gel Dosimetry
Verification of Monte Carlo calculations around a Fletcher Suit Delclos ovoid with normoxic polymer gel dosimetry K Gifford1, J Horton1, T Steger2, M Heard1, E Jackson2 and G Ibbott1 Deparment of Radiation Physics, The University of Texas M.D. Anderson Cancer Center, Houston, TX USA 2 Department of Imaging Physics, The University of Texas M.D. Anderson Cancer Center, Houston, TX USA
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[email protected] 1. Introduction Most modern brachytherapy treatment planning systems do not take into account the effect of the ovoid shields on the dose distributions. However, Meertens and Van Der Laarse have measured the effect of including the ovoid shields on the dose distribution [1] and have written a shielding correction algorithm [2] that has been incorporated into a commercial treatment planning system. Mohan [3] measured the perturbation effect of including the ovoid shields with an ion chamber in a water phantom and wrote a computer program to demonstrate the effect of the shields [4]. Williamson used Monte Carlo to calculate the perturbation of the dose distribution from the ovoid shields [5]. The goal of this work is to calculate the effect of including the anterior and posterior ovoid shields on the dose distribution around a Fletcher Suit Delclos (FSD) ovoid (Nucletron Trading BV, Leersum, Netherlands) and verify these calculations with normoxic polymer gel dosimetry. To date, no Monte Carlo results verified with dosimetry have been published for this ovoid. 2. Methods and materials The Monte Carlo code MCNPX (Monte Carlo N-Particle) version 2.5.c [6] was used to perform the simulations in this study. MCNPX is a general purpose Monte Carlo code for transporting neutrons, photons, electrons, as well as other particles in various geometries. In fact, MCNPX includes many convenient features such as a powerful geometry modeling tool and various tallies related to energy deposition, particle current and particle flux. Photon and electron transport can be carried out from 1 keV to 1000 MeV. Photon transport includes the photoelectric effect, coherent scattering, Compton scattering, and pair production. Characteristic x-ray production accompanying the photoelectric effect can also be simulated. Greater detail concerning the code can be found elsewhere [6]. Four active pellets and four inactive pellets were simulated. The calculations were performed in a sphere of water of radius 15 cm surrounded by a sphere of air of 100 cm radius. A mesh of 5×0.2×5 cm3 with grid resolution of 0.2×0.2×0.2 cm3 was overlaid on the geometry to tally photon energy deposition. The mesh was positioned so that it cut the ovoid in half length-wise and passed through both shields. A sufficient number of particles were run so that the maximum relative error was ±1% overall. Photon water kerma rates were converted to total dose delivered over 24 hours. A 5% solution of methacrylic acid gels were prepared in accordance with the recipe of Fong [7]. © 2004 IOP Publishing Ltd
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This formulation is known as MAGIC (Methacrylic and Ascorbic acid in Gelatin initiated by Copper). Five cylindrical glass vials of 2.5 cm diameter and 5.5 cm length were filled to serve as calibration gels. The ovoid gel phantom and gel calibration vials were wrapped in aluminum foil and stored at room temperature (22°C). The gels were allowed to stabilize for 48 hours after preparation. The right ovoid was situated in the gel phantom. A pillowcase was wrapped around the ovoid gel phantom to prevent any stray light from fogging the gel. The pellets were loaded into the ovoid. Each pellet had an air kerma strength of 33.013 U (427.725 MBq). This irradiation took place for 24 hours and yielded a maximum dose of approximately 20 Gy on the left lateral ovoid wall (-1.5,0). The dose response of the gel was assessed on a 60Co unit. Each vial was placed screw-cap down in a 30 × 30 × 30 cm3 water phantom. Water was filled so that the surface coincided with the top of each vial. A 10×10 cm2 field was centered on the middle of each vial at 80 cm SSD. Dose levels ranged from 2.5 Gy to 25 Gy. Forty eight hours elapsed between irradiation and readout. Comparisons between the Monte Carlo calculations and the gel measurements were performed on a relative dose basis. The Monte Carlo calculations and gel measurements were each normalized to their maximum dose and then compared. In addition to this, the gel and Monte Carlo data were analyzed with a binary agreement map program. If the Monte Carlo data met the ±3% or ±3mm criterion, that pixel was colored white. If the Monte Carlo data did not meet the ±3% or ±3mm criterion, that pixel was colored black. Regions less than 0.15 were deemed too noisy for comparison and colored gray. The gels were imaged with a 1.5 Tesla GE Signa MR scanner. Paraffin was heated and molded into the rectangular recess that housed the ovoid. This served to reduce any susceptibility artifacts that might arise due to dramatic changes in signal intensity. An in-house developed multi-planar multiplespin echo sequence with an echo train length of 12 was employed to image the gel phantom. The crusher scaling factors from Poon [8] were used to remove stimulated echoes. Each image was 512 × 512 pixels. A 26 cm field of view (FOV) was used yielding voxels of 0.05 × 0.05 × 0.3 cm3. The number of excitations (NEX) of 1 with phase FOV of 1 and repetition time (TR) of 5000 ms were used yielding an acquisition time of 1 hour 20 minutes. Echo times ranged from 23 ms to 276 ms. The acquisition was interleaved into two scans to reduce cross-talk between neighboring slices. The calibration vials and the gel phantom were placed on a rigid surface and positioned inside a head coil for the scan. R2 values were determined by a least squares fit to a mono-exponential in an IDL (Research Systems Inc., Boulder, CO) program. Individual points on the R2 to dose curve were determined by centering an ROI on each of the vials in the image and performing a median filter with a 5 point window to reduce noise. The planar gel data were filtered with a median filter with a 3 point window. 3. Results Figure 1 is a plot of the R2 versus dose curve for the normoxic gel ovoid irradiation. A second order polynomial fit was satisfactory. These data compare well with those of Fong [7] and De Deene [9]. Figure 2a is a plot of the normoxic gel ovoid irradiation versus the MCNPX 2.5.c simulation. The gel matches well laterally with the Monte Carlo calculations. Also, it predicts the perturbation of the dose distribution from the anterior and posterior shields. In fact, the gel data match the Monte Carlo data to within ±3% or ±3mm on 98% of points. Figure 2b is the agreement map for gel and MCNPX comparison in Figure 2a. Gel measurements indicated that the Monte Carlo calculations were within ±3% or ±3mm for 98% of the points tested.
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Figure 1. Dose response of the normoxic gel.
Figure 2. Comparison between gel measurements and Monte Carlo computations. 4. Conclusions The MAGIC gel measurements accurately predicted the perturbation of the ovoid shields on the dose distribution around an FSD ovoid. These data indicate that MCNPX can accurately calculate dose in the presence of the ovoid shields and other internal structures of the ovoid, such as the pellets.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Meertens H and Van der Laarse R 1985 Screens in ovoids of a cervix applicator Radiother. Oncol. 3 69–80 Van der Laarse R and Meertens H 1984 An algorithm for ovoid shielding of a cervix applicator Proc. 8th Int. Conf. on the use of computers in radiation therapy (Toronto, Canada) ed Cunningham J R, Ragan D and Van Dyk S pp 364–9 Mohan R, Ding I, Martel M, Anderson L and Nori D 1985 Measurements of radiation dose distributions for shielded cervical applicators Int. J. Radiat. Oncol. Biol. Phys. 11 861–8 Mohan R, Ding I, Toraskar J, Chui C, Anderson L and Nori D 1985 Computation of radiation dose distributions for shielded cervical applicators Int. J. Radiat. Oncol. Biol. Phys. 11 823–30 Williamson J 1990 Dose calculations about shielded gynecological colpostats Int. J. Radiat. Oncol. Biol. Phys. 19 167–78 Hendricks J 2003 MCNPX version 2.5.c LA-UR-03-2002,(Los Alamos, New Mexico, LANL) Fong P, Keil D, Does M and Gore J C 2001 Polymer gels for magnetic resonance imaging of radiation dose distributions at normal room atmosphere Phys. Med. Biol. 46 3105–13 Poon C and Henkelman R 1992 Practical T2 quantization for clinical applications J. Magn. Reson. Imag. 2 541–53 De Deene Y, Hurley C, Venning A, Vergote K, Mather M, Healy B and Baldock C 2002 A basic study of some normoxic polymer gel dosimeters Phys. Med. Biol. 47 3441–63
