Gifford Kent A, Horton John L, Jackson Edward F, Steger Theodore R, Heard Malcolm P, Mourtada Firas, Lawyer Ann A, Ibbott Geoffrey S
Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center, Houston, Texas 77030.
Department of Imaging Physics, The University of Texas M.D. Anderson Cancer Center, Houston, Texas 77030.
Med Phys. 2005 Jul;32(7Part1):2288-2294. doi: 10.1118/1.1944247.
The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33μGym2h-1 Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within ±2% or 2 mm for 98% of points compared with RCF measurements and to within ±3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.
用于宫颈癌腔内近距离放射治疗（ICB）的弗莱彻-休伊特-德尔克洛（FSD）椭圆体含有屏蔽物，可减少对膀胱和直肠的剂量。许多治疗计划系统（TPS）在剂量计算中未纳入屏蔽物及其他椭圆体结构。相反，TPS通过将各个源的剂量贡献相加来计算剂量，而忽略诸如屏蔽物之类的椭圆体结构。本研究的目的是利用蒙特卡罗方法计算围绕Selectron FSD椭圆体的剂量分布，并将这些计算结果与放射变色胶片（RCF）和常氧聚合物凝胶剂量测定法进行比较。使用MCNPX 2.5.c对单个带有和不带有屏蔽物的Selectron FSD椭圆体进行蒙特卡罗计算。在与椭圆体长轴平行且横向偏移1.25 cm的平面上进行RCF测量。在聚甲基丙烯酸甲酯体模中进行MAGIC凝胶测量。用四个33μGym2h-1的Cs-137粒籽照射RCF和MAGIC凝胶24小时。结果表明，与RCF测量相比，MCNPX计算的98%的点的剂量在±2%或2 mm范围内，与MAGIC凝胶测量相比，98%的点的剂量在±3%或3 mm范围内。得出的结论是，MCNPX 2.5.c在存在椭圆体屏蔽物的情况下能够准确计算剂量，RCF和MAGIC凝胶能够证明椭圆体屏蔽物对剂量分布的影响，且椭圆体屏蔽物可使剂量降低多达50%。
