Ludbrook Joanna J S, Greer Peter B, Blood Paul, D'yachkova Yulia, Coldman Andrew, Beckham Wayne A, Runkel Jim, Olivotto Ivo A
Radiation Therapy Program, British Columbia Cancer Agency, Vancouver Island Centre, Victoria, BC, Canada.
Med Dosim. 2005 Summer;30(2):76-84. doi: 10.1016/j.meddos.2005.03.003.
The purpose of this study was to develop an evidence-based off-line setup correction protocol for systematic errors in prostate radiation therapy. Daily orthogonal electronic portal images were acquired from 30 patients. Field displacements were measured in the medial-lateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions for each treatment fraction. The off-line protocol corrects the mean field displacement found from n consecutive images, starting at a particular fraction of treatment, with a fixed tolerance level. Simulations were performed with the measured data to determine (1) how many images (n) should be averaged to determine the systematic error; (2) on which treatment fraction should the protocol be initiated; and (3) what tolerance level should be applied to determine whether the patient position should be corrected. Uncorrected systematic errors in the ML, SI, and AP directions were (mean position +/- 1 standard deviation [SD]): -0.7 +/- 2.2 mm, -1.5 +/- 1.3 mm, and 1.4 +/- 2.6 mm, respectively. Random errors (1 SD and range) were 1.9 mm (1.3 - 3.3), 1.5 mm (0. - 4.1), and 1.8 mm (1.0-2.6), respectively. A correction based on a single image taken on the first fraction actually increased the systematic errors in the ML and SI directions compared with no correction. More accurate correction of systematic errors was achieved with increasing number of images averaged, with only small benefit after 5 images. With fewer images averaged, delaying the start of the protocol resulted in more accurate correction because of the influence of unrepresentative positions at early fractions. The number of corrections made on patients with small (< 2 mm) systematic errors was minimized for tolerance values of 2 mm and n > or = 5 images averaged. The optimal off-line setup correction protocol would be to shift the patient by the mean displacement of the first 5 portal images of a radical course of radiation therapy. A small tolerance level should be utilized with 2 mm giving good accuracy with minimal unnecessary shifts.
本研究的目的是制定一种基于证据的离线设置校正方案，用于纠正前列腺放射治疗中的系统误差。从30例患者获取每日正交电子门静脉图像。在每个治疗分次中，测量在内外侧（ML）、上下（SI）和前后（AP）方向上的射野位移。离线方案从特定治疗分次开始，以固定的容差水平校正从n个连续图像中发现的平均射野位移。利用测量数据进行模拟，以确定：（1）应平均多少幅图像（n）来确定系统误差；（2）该方案应从哪个治疗分次开始；以及（3）应采用何种容差水平来确定是否应校正患者体位。ML、SI和AP方向上未校正的系统误差分别为（平均位置±1标准差[SD]）：-0.7±2.2 mm、-1.5±1.3 mm和1.4±2.6 mm。随机误差（1 SD和范围）分别为1.9 mm（1.3 - 3.3）、1.5 mm（0. - 4.1）和1.8 mm（1.0 - 2.6）。与不校正相比，基于第一次分次拍摄的单幅图像进行校正实际上增加了ML和SI方向上的系统误差。随着平均图像数量的增加，系统误差得到更准确的校正，平均5幅图像后收益甚微。平均图像数量较少时，由于早期分次中位置不具代表性的影响，推迟方案开始时间可实现更准确的校正。对于平均n≥5幅图像且容差值为2 mm的情况，系统误差较小（<2 mm）的患者进行的校正次数最少。最佳的离线设置校正方案是将患者按照根治性放射治疗疗程的前5幅门静脉图像的平均位移进行移动。应采用较小的容差水平，2 mm可提供良好的准确性，且不必要的移动最少。
