Chibani Omar, C-M Ma Charlie
Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111.
Med Phys. 2014 May;41(5):051712. doi: 10.1118/1.4873318.
To present a new accelerated Monte Carlo code for CT-based dose calculations in high dose rate (HDR) brachytherapy. The new code (HDRMC) accounts for both tissue and nontissue heterogeneities (applicator and contrast medium).
HDRMC uses a fast ray-tracing technique and detailed physics algorithms to transport photons through a 3D mesh of voxels representing the patient anatomy with applicator and contrast medium included. A precalculated phase space file for the(192)Ir source is used as source term. HDRM is calibrated to calculated absolute dose for real plans. A postprocessing technique is used to include the exact density and composition of nontissue heterogeneities in the 3D phantom. Dwell positions and angular orientations of the source are reconstructed using data from the treatment planning system (TPS). Structure contours are also imported from the TPS to recalculate dose-volume histograms.
HDRMC was first benchmarked against the MCNP5 code for a single source in homogenous water and for a loaded gynecologic applicator in water. The accuracy of the voxel-based applicator model used in HDRMC was also verified by comparing 3D dose distributions and dose-volume parameters obtained using 1-mm(3) versus 2-mm(3) phantom resolutions. HDRMC can calculate the 3D dose distribution for a typical HDR cervix case with 2-mm resolution in 5 min on a single CPU. Examples of heterogeneity effects for two clinical cases (cervix and esophagus) were demonstrated using HDRMC. The neglect of tissue heterogeneity for the esophageal case leads to the overestimate of CTV D90, CTV D100, and spinal cord maximum dose by 3.2%, 3.9%, and 3.6%, respectively.
A fast Monte Carlo code for CT-based dose calculations which does not require a prebuilt applicator model is developed for those HDR brachytherapy treatments that use CT-compatible applicators. Tissue and nontissue heterogeneities should be taken into account in modern HDR brachytherapy planning.
介绍一种用于高剂量率（HDR）近距离放射治疗中基于CT的剂量计算的新型加速蒙特卡罗代码。新代码（HDRMC）兼顾了组织和非组织异质性（施源器和造影剂）。
HDRMC使用快速射线追踪技术和详细的物理算法，通过包含施源器和造影剂的代表患者解剖结构的三维体素网格来传输光子。使用预先计算的（192）Ir源相空间文件作为源项。HDRM针对实际计划的计算绝对剂量进行校准。采用一种后处理技术将非组织异质性的精确密度和组成纳入三维体模。利用来自治疗计划系统（TPS）的数据重建源的驻留位置和角度方向。结构轮廓也从TPS导入以重新计算剂量体积直方图。
首先将HDRMC与MCNP5代码在均匀水中的单个源以及水中加载的妇科施源器方面进行基准测试。通过比较使用1立方毫米与2立方毫米体模分辨率获得的三维剂量分布和剂量体积参数，也验证了HDRMC中使用的基于体素的施源器模型的准确性。HDRMC在单个CPU上5分钟内可以计算出典型HDR宫颈癌病例的2毫米分辨率三维剂量分布。使用HDRMC展示了两个临床病例（宫颈癌和食管癌）的异质性效应示例。食管癌病例中忽略组织异质性分别导致靶区适形度90%、靶区适形度100%和脊髓最大剂量高估3.2%、3.9%和3.6%。
针对那些使用与CT兼容施源器的HDR近距离放射治疗，开发了一种基于CT剂量计算的快速蒙特卡罗代码，该代码不需要预先构建施源器模型。在现代HDR近距离放射治疗计划中应考虑组织和非组织异质性。
