Radiation Oncology, Moffitt Cancer Center, Tampa, FL 33612, USA.
J Appl Clin Med Phys. 2012 Sep 6;13(5):3800. doi: 10.1120/jacmp.v13i5.3800.
Optimization of permanent seed implant brachytherapy plans for treatment of prostate cancer should be based on biological effective dose (BED) distributions, since dose does not accurately represent biological effects between different types of sources. Currently, biological optimization for these plans is not feasible due to the amount of time necessary to calculate the BED distribution. This study provides a fast calculation method, based on the total dose, to calculate the BED distribution. Distributions of various numbers of hybrid seeds were used to calculate total dose distributions, as well as BED distributions. Hybrid seeds are a mixture of different isotopes (in this study (125)I and (103)Pd). Three ratios of hybrid seeds were investigated: 25/75, 50/50, and 75/25. The total dose and BED value from each voxel were coupled together to produce graphs of total dose vs. BED. Equations were then derived from these graphs. The study investigated four types of tissue: bladder, rectum, prostate, and other normal tissue. Equations were derived from the total dose - BED correspondence. Accuracy of conversion from total dose to BED was within 2 Gy; however, accuracy of conversion was found to be better for high total dose regions as compared to lower dose regions. The method introduced in this paper allows one to perform fast conversion of total dose to BED for brachytherapy using hybrid seeds, which makes the BED-based plan optimization practical. The method defined here can be extended to other ratios, as well as other tissues that are affected by permanent seed implant brachytherapy (i.e., breast).
优化前列腺癌永久性种子植入近距离放疗计划应基于生物有效剂量（BED）分布，因为剂量不能准确代表不同类型源之间的生物学效应。目前，由于计算 BED 分布所需的时间量，这些计划的生物学优化是不可行的。本研究提供了一种快速计算方法，基于总剂量来计算 BED 分布。使用各种数量的混合种子分布来计算总剂量分布以及 BED 分布。混合种子是不同同位素的混合物（在本研究中为（125）I 和（103）Pd）。研究了三种混合种子的比例：25/75、50/50 和 75/25。每个体素的总剂量和 BED 值被耦合在一起，生成总剂量与 BED 的关系图。然后从这些图中推导出方程。该研究调查了四种组织：膀胱、直肠、前列腺和其他正常组织。从总剂量-BED 对应关系中推导出方程。从总剂量转换为 BED 的转换精度在 2 Gy 以内；然而，与低剂量区域相比，发现高总剂量区域的转换精度更好。本文介绍的方法允许使用混合种子快速将总剂量转换为 BED 进行近距离放疗，从而使基于 BED 的计划优化成为可能。此处定义的方法可以扩展到其他比例以及受永久性种子植入近距离放疗影响的其他组织（例如，乳房）。
