Department of Radiation Oncology, Prince of Wales Hospital, Randwick, NSW 2031, Australia.
Med Phys. 2013 Dec;40(12):121709. doi: 10.1118/1.4828786.
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.
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(®).
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.
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.
本研究旨在使用 GafChromic(®) EBT3 胶片测量装有 I-125（型号 6711）种子的不同 Radiation Oncology Physics and Engineering Services, Australia（ROPES）型眼贴的剂量分布，以验证 Plaque Simulator™（PS）眼科三维治疗计划系统中的剂量分布。RADCALC(®) 的近距离治疗模块用于独立检查 PS 计算的剂量分布。从测量数据中得出校正因子，用于 PS 中，以说明不锈钢 ROPES 贴剂背板对三维剂量分布的影响。
使用插入特制的 Solid Water（水凝胶）眼球模型的 GafChromic(®) EBT3 胶片，在装有 I-125（型号 6711）的 ROPES 眼贴的中央轴（CAX）上，以 2mm 深度增量进行三维剂量分布测量。每次测量均在完全散射条件下进行，同时附有和不附有不锈钢贴剂背板，以评估其对剂量分布的影响。结果与 Plaque Simulator™ 计算的剂量分布进行比较，并通过 RADCALC(®) 进行独立检查。
在没有不锈钢背板的情况下，EBT3 胶片的测量结果与 PS 和 RADCALC(®) 的偏差分别在 2%和 4%以内，均位于贴剂 CAX 上。此外，RADCALC(®) 与 PS 的偏差也在 2%以内。使用 EBT3 胶片测量有不锈钢背板时，在 CAX 深度剂量测量中，发现与没有背板时相比，剂量降低了 4%。在实验不确定度范围内，发现这种 4%的降低在深度上是恒定的，与贴剂尺寸无关。在 PS 中使用常数剂量校正因子 T = 0.96，即水散射介质中的计算剂量在剂量网格的每个体素中降低 4%，可以在规划系统中准确模拟贴剂背板的影响。通过考虑贴剂背板的三维模型，还在 PS 中模拟了离轴轮廓。
通过 EBT3 胶片测量，对完全散射和均匀散射条件下均匀装载的 ROPES 贴剂的 PS 和 RADCALC(®) 计算剂量进行了验证。观察到不锈钢贴剂背板使测量剂量降低了 4%。通过在 PS 中引入标量校正因子（0.96），发现不锈钢贴剂背板的剂量均匀性效应与测量的 EBT3 胶片测量结果一致。
