Ottawa Carleton Institute for Physics, Carleton University Campus, Ottawa, Ontario K1S 5B6, Canada.
Med Phys. 2010 Jan;37(1):368-76. doi: 10.1118/1.3271104.
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.
计算 COMS 标准化斑块中各种 125I 和 103Pd 种子模型的剂量分布。研究剂量分布对种子模型的敏感性以及与 TG-43 的剂量分布关系。
使用 EGSnrc 用户代码 BrachyDose 进行蒙特卡罗模拟。种子和眼斑均进行了全面建模。在带有和不带有 Silastic（硅聚合物）插件的 20mm Modulay（金合金）斑片中央槽中模拟一个种子，以及在完全填充的 16mm Modulay/Silastic 斑片中模拟一个种子。将剂量分布与 TG-43 假设下计算的剂量分布进行比较，即忽略斑片背衬和插件以及种子间衰减的影响。通过深度剂量曲线、等剂量曲线和眼球内感兴趣点的剂量列表，比较不同 125I 和 103Pd 种子模型的三维剂量分布。结果与我们最近对包含模型 6711（125I）或 200（103Pd）种子的 COMS 斑片的 BrachyDose 研究进行了比较[R. M. Thomson 等人，医学物理 35,5530-5543（2008）]。
在包含一个种子的斑片中，单独使用 Modulay 背衬的影响在不同种子模型中变化小于 1%；对于 Modulay/Silastic 组合，变化为 2%。对于完全填充 125I（103Pd）种子的 16mm 斑片，相对于 TG-43 剂量，距内巩膜 0.5 和 1cm 处沿斑片中央轴的剂量分别减少 11%-12%（19%-20%）和 14%-15%（20%）。对于相同的处方剂量，感兴趣点的剂量因种子模型而异，最大可达 8%。对于所有 103Pd 种子模型，关键正常结构的剂量均低于 125I，除了紧邻斑片的巩膜外；巩膜剂量随种子模型而变化，并不总是 103Pd 高于 125I。
与在 TG-43 假设下计算的剂量相比，剂量减少幅度随种子模型（对于每种放射性核素）略有变化。剂量分布对种子模型敏感；然而，变化通常不会大于眼球斑片治疗中其他系统不确定性的幅度。
