Markman J, Williamson J F, Dempsey J F, Low D A
Radiation Oncology Center, Mallinckrodt Institute of Radiology, St. Louis, Missouri 63110, USA.
Med Phys. 2001 Feb;28(2):147-55. doi: 10.1118/1.1339224.
Intracavitary vaginal applicators typically incorporate internal shielding to reduce dose to the bladder and rectum. While dose distributions about a single colpostat have been extensively measured and calculated, these studies neglect dosimetric perturbations arising from the contralateral colpostat or the intrauterine tandem. Dosimetric effects of inhomogeneities in brachytherapy is essential for both dose-based implant optimization as well as for a comparison with alternate modalities, such as intensity modulated radiation therapy. We have used Monte Carlo calculations to model dose distributions about both a Fletcher-Suit-Delclos (FSD) low dose-rate system and the microSelectron high dose-rate remote afterloading system. We have evaluated errors, relative to a Monte Carlo simulation based upon a complete applicator system, in superposition calculations based upon both precalculated single shielded applicator dose distributions as well as single unshielded source dose distributions. Errors were largely dominated by the primary photon attenuation, and were largest behind the shields and tandem. For the FSD applicators, applicator superposition showed differences ranging from a mean of 2.6% at high doses (>Manchester Point A dose) to 4.3% at low doses (<Manchester Point A dose) compared to the full geometry simulation. Source-only superposition yielded errors higher than 10% throughout the dose range. For the HDR applicator system, applicator superposition-induced errors ranging from 3.6%-6.3% at high and low doses, respectively. Source superposition caused errors of 5%-11%. These results indicate that precalculated applicator-based dose distributions can provide an excellent approximation of a full geometry Monte Carlo dose calculation for gynecological implants.
腔内阴道施源器通常内置屏蔽装置，以减少对膀胱和直肠的辐射剂量。虽然已对单个阴道柱状施源器周围的剂量分布进行了广泛测量和计算，但这些研究忽略了对侧阴道柱状施源器或宫腔管引起的剂量学扰动。近距离放射治疗中不均匀性的剂量学效应对于基于剂量的植入优化以及与其他放疗方式（如调强放射治疗）的比较都至关重要。我们使用蒙特卡罗计算方法对Fletcher-Suit-Delclos（FSD）低剂量率系统和微型Selectron高剂量率远程后装系统周围的剂量分布进行建模。我们评估了基于预先计算的单个屏蔽施源器剂量分布以及单个无屏蔽源剂量分布的叠加计算相对于基于完整施源器系统的蒙特卡罗模拟的误差。误差主要由初级光子衰减主导，在屏蔽和宫腔管后方最大。对于FSD施源器，与全几何模拟相比，施源器叠加显示高剂量（>曼彻斯特A点剂量）时平均差异为2.6%，低剂量（<曼彻斯特A点剂量）时为4.3%。仅源叠加在整个剂量范围内产生的误差高于10%。对于高剂量率施源器系统，施源器叠加在高、低剂量时分别引起3.6%-6.3%的误差。源叠加导致5%-11%的误差。这些结果表明，基于预先计算的施源器剂量分布可以为妇科植入物的全几何蒙特卡罗剂量计算提供极好的近似值。
