Monte Carlo dosimetry for 125I and 103Pd eye plaque brachytherapy with various seed models.
ABSTRACT Dose distributions are calculated for various models of 125I and 103Pd seeds in the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). The sensitivity to seed model of dose distributions and dose distributions relative to TG-43 are investigated.
Monte Carlo simulations are carried out with the EGSnrc user-code BrachyDose. Brachytherapy seeds and eye plaques are fully modeled. Simulations of one seed in the central slot of a 20 mm Modulay (gold alloy) plaque backing with and without the Silastic (silicone polymer) insert and of a 16 mm fully loaded Modulay/Silastic plaque are performed. Dose distributions are compared to those calculated under TG-43 assumptions, i.e., ignoring the effects of the plaque backing and insert and interseed attenuation. Three-dimensional dose distributions for different 125I and 103Pd seed models are compared via depth-dose curves, isodose contours, and tabulation of doses at points of interest in the eye. Results are compared to those of our recent BrachyDose study for COMS plaques containing model 6711 (125I) or 200 (103Pd) seeds [R. M. Thomson et al., Med. Phys. 35, 5530-5543 (2008)].
Along the central axis of a plaque containing one seed, variations of less than 1% are seen in the effect of the Modulay backing alone for different seed models; for the Modulay/Silastic combination, variations are 2%. For a 16 mm plaque fully loaded with 125I (103Pd) seeds, dose decreases relative to TG-43 doses are 11%-12% (19%-20%) and 14%-15% (20%) at distances of 0.5 and 1 cm from the inner sclera along the plaque's central axis, respectively. For the same prescription dose, doses at points of interest vary by up to 8% with seed model. Doses to critical normal structures are lower for all 103Pd seed models than for 125I with the possible exception of the sclera adjacent to the plaque; scleral doses vary with seed model and are not always higher for 103Pd than for 125I.
Dose decreases relative to doses calculated under TG-43 assumptions vary slightly with seed model (for each radionuclide). Dose distributions are sensitive to seed model; however, variations are generally no larger than the magnitudes of other systematic uncertainties in eye plaque therapy.
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ABSTRACT: This study explores the influence of source photon energy on eye plaque brachytherapy dose distributions for a 16mm COMS plaque filled with (103)Pd, (125)I, or (131)Cs sources or monoenergetic photon emissions ranging from 12keV to 100keV. Dose distributions were similarly created for all permutations of three common brachytherapy seed designs. Within this range, sources with average energy ≤22keV may reduce dose to the opposite eye wall by more than a factor of 2 while maintaining tolerable proximal sclera doses when prescribing to depths of 9mm or less. Current commercially-available brachytherapy sources can exhibit up to 15% relative dosimetric sensitivity to seed design at regions within the eye.Applied radiation and isotopes: including data, instrumentation and methods for use in agriculture, industry and medicine 05/2013; 79C:62-66. DOI:10.1016/j.apradiso.2013.04.034 · 1.06 Impact Factor
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ABSTRACT: Purpose: The purpose of this study was to measure the dose distributions for different Radiation Oncology Physics and Engineering Services, Australia (ROPES) type eye plaques loaded with I-125 (model 6711) seeds using GafChromic(®) EBT3 films, in order to verify the dose distributions in the Plaque Simulator™ (PS) ophthalmic 3D treatment planning system. The brachytherapy module of RADCALC(®) was used to independently check the dose distributions calculated by PS. Correction factors were derived from the measured data to be used in PS to account for the effect of the stainless steel ROPES plaque backing on the 3D dose distribution.Methods: Using GafChromic(®) EBT3 films inserted in a specially designed Solid Water™ eye ball phantom, dose distributions were measured three-dimensionally both along and perpendicular to I-125 (model 6711) loaded ROPES eye plaque's central axis (CAX) with 2 mm depth increments. Each measurement was performed in full scatter conditions both with and without the stainless steel plaque backing attached to the eye plaque, to assess its effect on the dose distributions. Results were compared to the dose distributions calculated by Plaque Simulator™ and checked independently with RADCALC(®).Results: The EBT3 film measurements without the stainless steel backing were found to agree with PS and RADCALC(®) to within 2% and 4%, respectively, on the plaque CAX. Also, RADCALC(®) was found to agree with PS to within 2%. The CAX depth doses measured using EBT3 film with the stainless steel backing were observed to result in a 4% decrease relative to when the backing was not present. Within experimental uncertainty, the 4% decrease was found to be constant with depth and independent of plaque size. Using a constant dose correction factor of T = 0.96 in PS, where the calculated dose for the full water scattering medium is reduced by 4% in every voxel in the dose grid, the effect of the plaque backing was accurately modeled in the planning system. Off-axis profiles were also modeled in PS by taking into account the three-dimensional model of the plaque backing.Conclusions: The doses calculated by PS and RADCALC(®) for uniformly loaded ROPES plaques in full and uniform scattering conditions were validated by the EBT3 film measurements. The stainless steel plaque backing was observed to decrease the measured dose by 4%. Through the introduction of a scalar correction factor (0.96) in PS, the dose homogeneity effect of the stainless steel plaque backing was found to agree with the measured EBT3 film measurements.Medical Physics 12/2013; 40(12):121709. DOI:10.1118/1.4828786 · 3.01 Impact Factor
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ABSTRACT: PURPOSE: Quantify the dosimetric adequacy of the 2003 American Brachytherapy Society report tumor margin recommendations for Collaborative Ocular Melanoma Study (COMS) eye plaque size selection for radiation coverage and clinical plaque placement uncertainties. METHODS AND MATERIALS: Plaque heterogeneity-corrected dose distributions were generated for the range of available COMS plaque diameters (φplaque) and radionuclides. These dose distributions were used to determine the radiation dose distribution diameter (φ℞) at the eye surface for each plaque as a function of central axis prescription depth (d℞) to assess adequacy of a 2-3-mm margin for various gross tumor volume (GTV) basal diameters (φGTV). Four sets of ellipsoidal tumors (φGTV=5, 8, 11, and 14mm) with a range of apical heights (dGTV=2-8mm) were contoured in a reference CT environment. Plaque placement uncertainties were quantified as circumferential displacements (Δ) at the outer scleral surface. Tumor dose-volume histograms were generated and compared for all Δ with D90 and D95 used to evaluate tumor margin adequacy. RESULTS: For equivalent φplaque and prescription depths, φ℞ values were typically 0.4-0.8mm less for 103Pd than for 125I or 131Cs. Δ≤3mm resulted in D90 and D95 values as low as 68% and 64% of the prescription dose, respectively. 103Pd plaque dose distributions were more sensitive than 125I or 131Cs to placement uncertainties. CONCLUSIONS: The American Brachytherapy Society-recommended tumor margin may be inadequate for prescription dose coverage given COMS plaque radiation characteristics and placement uncertainties. Better coverage is achieved assuming a GTV-to-planning target volume total basal expansion of 3mm or greater and/or prescribing beyond the tumor apex.Brachytherapy 03/2013; DOI:10.1016/j.brachy.2012.09.006 · 1.99 Impact Factor