Afsharpour Hossein, D'Amours Michel, Coté Benoit, Carrier Jean-François, Verhaegen Frank, Beaulieu Luc
Département de Radio-Oncologie et Centre de Recherche en Cancérologie de l'Université Laval, Centre Hospitalier Universitaire de Québec, Québec, Canada.
Med Phys. 2008 Aug;35(8):3671-81. doi: 10.1118/1.2955754.
Standard algorithms for postimplant analysis of transperineal interstitial permanent prostate brachytherapy (TIPPB) are based on AAPM Task Group 43 formalism (TG-43), which makes use of a world entirely made of water. This entails an assignment of the prostate, surrounding organs at risk, as well as all brachytherapy seeds present in a permanent prostate implant to water. Brachytherapy seeds are generally made from high atomic number materials. Because of the simultaneous presence of many brachytherapy seeds in a TIPPB, there is a shielding effect causing an attenuation of energy of the emitted photons generally called the "interseed attenuation" (ISA). This study investigates the impact of seed designs and compositions on the interseed attenuation. For this purpose, six brachytherapy seeds covering a wide variety of seed design and composition were modeled with the GEANT4 Monte Carlo (MC) toolkit. MC has allowed calculation of the contribution of each major component (encapsulation and internal components) of a given seed model to ISA separately. The impact of ISA on real clinical implant configurations was also explored. Two clinical postimplant geometries with different brachytherapy seeds were studied with MC simulations. The change in the clinical parameter D90 was observed. This study shows that Nucletron SelectSeed (similar to the Oncura model 6711), ProstaSeed, and Best Medical model 2335 are the most attenuating designs with 4.8%, 3.9%, and 4.6% of D90 reduction, respectively. The least attenuating seed is a 103Pd seed encapsulated in a polymer shell, the IBt OptiSeed with 1.5%. Finally, based on this systematic study, a new seed design is proposed that is predicted to be the most waterlike brachytherapy seed and thus TG-43 compatible.
经会阴间质永久性前列腺近距离放射治疗（TIPPB）植入后分析的标准算法基于美国医学物理师协会任务组43形式主义（TG - 43），该形式主义使用的是一个完全由水构成的世界。这就需要将前列腺、周围的危险器官以及永久性前列腺植入物中存在的所有近距离放射治疗种子都视为水。近距离放射治疗种子通常由高原子序数材料制成。由于在TIPPB中同时存在许多近距离放射治疗种子，会产生一种屏蔽效应，导致发射光子的能量衰减，这种效应通常称为“种子间衰减”（ISA）。本研究调查了种子设计和成分对种子间衰减的影响。为此，使用GEANT4蒙特卡罗（MC）工具包对六种涵盖多种种子设计和成分的近距离放射治疗种子进行了建模。MC能够分别计算给定种子模型的每个主要成分（封装和内部成分）对ISA的贡献。还探讨了ISA对实际临床植入配置的影响。通过MC模拟研究了两种使用不同近距离放射治疗种子的临床植入后几何形状。观察到临床参数D90的变化。本研究表明，Nucletron SelectSeed（类似于Oncura 6711型号）、ProstaSeed和Best Medical 2335型号是衰减最大的设计，D90分别降低4.8%、3.9%和4.6%。衰减最小的种子是封装在聚合物壳中的103Pd种子，即IBt OptiSeed，其D90降低1.5%。最后，基于这项系统研究，提出了一种新的种子设计，预计它是最接近水的近距离放射治疗种子，因此与TG - 43兼容。
