Decker W E, Erickson B, Albano K, Gillin M
Medical College of Wisconsin, Department of Radiation Oncology, Milwaukee 53226, USA.
Int J Radiat Oncol Biol Phys. 2001 Jun 1;50(2):561-7. doi: 10.1016/s0360-3016(01)01542-5.
Few dose specification guidelines exist when attempting to perform high-dose-rate (HDR) dosimetry. The purpose of this study was to model low-dose-rate (LDR) dosimetry, using parameters common in HDR dosimetry, to achieve the "pear-shape" dose distribution achieved with LDR tandem and ovoid applications.
Radiographs of Fletcher-Suit LDR applicators and Nucletron "Fletcher-like" HDR applicators were taken with the applicators in an idealized geometry. Traditional Fletcher loadings of 3M Cs-137 sources and the Theratronics Planning System were used for LDR dosimetry. HDR dosimetry was performed using the Nucletron Microselectron HDR UPS V11.22 with an Ir-192 source. Dose optimization points were initially located along a line 2 cm lateral to the tandem, beginning at the tandem tip at 0.5-cm intervals, ending at the sail, and optimized to 100% of the point A dose. A single dose optimization point was also placed laterally from the center of each ovoid equal to the radius of the ovoid (ovoid surface dose). For purposes of comparison, dose was also calculated for points A and B, and a point located 1 cm superior to the tandem tip in the plane of the tandem, (point F). Four- and 6-cm tandem lengths and 2.0-, 2.5-, and 3.0-cm ovoid diameters were used for this study. Based on initial findings, dose optimization schemes were developed to best approximate LDR dosimetry. Finally, radiographs were obtained of HDR applications in two patients. These radiographs were used to compare the optimization schemes with "nonoptimized" treatment plans.
Calculated doses for points A and B were similar for LDR, optimized HDR, and nonoptimized HDR. The optimization scheme that used tapered dose points at the tandem tip and optimized a single ovoid surface point on each ovoid to 170% of point A resulted in a good approximation of LDR dosimetry. Nonoptimized HDR resulted in higher doses at point F, the bladder, and at points lateral to the tandem tip than both the optimized plan or the LDR plan.
Optimized HDR allows specification of dose to points of interest, can approximate LDR dosimetry, and appears superior to nonoptimized HDR treatment planning, at least at the tandem tip. An optimization scheme is presented that approximates LDR dosimetry.
在尝试进行高剂量率（HDR）剂量测定时，几乎没有剂量规范指南。本研究的目的是利用HDR剂量测定中常见的参数对低剂量率（LDR）剂量测定进行建模，以实现LDR串联和卵形体应用所达到的“梨形”剂量分布。
在理想化几何结构下拍摄Fletcher-Suit LDR施源器和Nucletron“类Fletcher”HDR施源器的X线片。LDR剂量测定采用3M Cs-137源的传统Fletcher加载方式和Theratronics治疗计划系统。使用带有Ir-192源的Nucletron Microselectron HDR UPS V11.22进行HDR剂量测定。剂量优化点最初位于串联施源器外侧2 cm的一条线上，从串联施源器尖端开始，以0.5 cm的间隔，到施源器尾翼结束，并优化至A点剂量的100%。在每个卵形体中心外侧与卵形体半径相等的位置（卵形体表面剂量）也设置一个单一的剂量优化点。为了进行比较，还计算了A点、B点以及在串联施源器平面内位于串联施源器尖端上方1 cm处的点（F点）的剂量。本研究使用了4 cm和6 cm的串联施源器长度以及2.0 cm、2.5 cm和3.0 cm的卵形体直径。基于初步研究结果，制定了剂量优化方案以最佳地近似LDR剂量测定。最后，获取了两名患者HDR应用的X线片。这些X线片用于将优化方案与“未优化”的治疗计划进行比较。
LDR、优化后的HDR和未优化的HDR的A点和B点计算剂量相似。在串联施源器尖端采用渐变剂量点并将每个卵形体上的单个卵形体表面点优化至A点剂量的170%的优化方案，能很好地近似LDR剂量测定。未优化的HDR在F点、膀胱以及串联施源器尖端外侧的点处产生的剂量高于优化方案或LDR方案。
优化后的HDR能够对感兴趣的点进行剂量规范，可近似LDR剂量测定，并且至少在串联施源器尖端似乎优于未优化的HDR治疗计划。提出了一种近似LDR剂量测定的优化方案。
