Watanabe Y, Roy J N, Harrington P J, Anderson L L
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
Int J Radiat Oncol Biol Phys. 1998 Jul 15;41(5):1201-7. doi: 10.1016/s0360-3016(98)00170-9.
We have generated three-dimensional (3D) lookup tables for dosimetric analysis and optimization of high-dose rate (HDR) gynecological treatments using the Henschke applicator. The new dosimetry data have been compared with two-dimensional (2D) data currently in use. The 3D dosimetry tables have been implemented in an existing cervix treatment-planning system and have been evaluated through analysis of clinical cases.
A general Monte Carlo N-Particles (MCNP) transport code was used to compute absorbed dose distributions around the intrauterine tandem and tungsten-shielded ovoid separately. The dosimetry data are represented in the x-y coordinate system for the intrauterine tandem table. The 3D table for the ovoid contains a radial dose function and an anisotropy function, as formulated in the spherical coordinate system. Absorbed dose at a spatial point is calculated by applying bilinear interpolation for the anisotropy function and linear interpolation for the radial dose function. The geometry factor for a finite line source is used. 3D dose calculations and optimization were performed for 20 treatments of 10 patients. The absorbed dose to critical structures, bladder and rectum, was compared by applying both the 2D table currently in use and the new tables.
The new 2D table for the intrauterine tandem yields doses different by less than 10% from those with the current table. The 3D table for the shielded ovoids shows as large as a factor of 4 reduction of dose behind the shield compared with the present 2D table. This shielding effect leads to 21.6 +/- 9.3% and 20.0 +/- 6.6% dose reduction at rectum and bladder, respectively, for actual treatments.
Our analysis indicates a need for patient-specific 3D dosimetry to permit more accurate dosimetric evaluation of HDR cervix treatments using shielded applicators. We have also shown that a Monte Carlo simulation code enabled us to derive the lookup tables necessary for 3D planning.
我们生成了三维（3D）查找表，用于使用亨施克施源器对高剂量率（HDR）妇科治疗进行剂量分析和优化。已将新的剂量学数据与目前使用的二维（2D）数据进行了比较。3D剂量学表已在现有的子宫颈治疗计划系统中实施，并通过临床病例分析进行了评估。
使用通用的蒙特卡罗N粒子（MCNP）输运代码分别计算子宫内串联器和钨屏蔽卵形体周围的吸收剂量分布。子宫内串联器表的剂量学数据在x-y坐标系中表示。卵形体的3D表包含在球坐标系中制定的径向剂量函数和各向异性函数。通过对各向异性函数应用双线性插值和对径向剂量函数应用线性插值来计算空间点处的吸收剂量。使用有限线源的几何因子。对10名患者的20次治疗进行了3D剂量计算和优化。通过应用当前使用的2D表和新表比较了关键结构膀胱和直肠的吸收剂量。
新的子宫内串联器2D表产生的剂量与当前表产生的剂量相差不到10%。与当前的2D表相比，屏蔽卵形体的3D表显示屏蔽后面的剂量降低了4倍之多。这种屏蔽效应导致实际治疗中直肠和膀胱的剂量分别降低21.6±9.3%和20.0±6.6%。
我们的分析表明，需要针对患者的3D剂量学，以便对使用屏蔽施源器的HDR子宫颈治疗进行更准确的剂量学评估。我们还表明，蒙特卡罗模拟代码使我们能够导出3D规划所需的查找表。
