Grigorov Grigor N, Chow James Cl
Grigor N Grigorov, Medical Physics Department, Grand River Regional Cancer Center, Kitchener, ON N2G 1G3, Canada.
World J Radiol. 2016 Jan 28;8(1):73-81. doi: 10.4329/wjr.v8.i1.73.
To study the leakage-penumbra (LP) effect with a proposed correction method for the step-and-shoot intensity modulated radiation therapy (IMRT).
Leakage-penumbra dose profiles from 10 randomly selected prostate IMRT plans were studied. The IMRT plans were delivered by a Varian 21 EX linear accelerator equipped with a 120-leaf multileaf collimator (MLC). For each treatment plan created by the Pinnacle(3) treatment planning system, a 3-dimensional LP dose distribution generated by 5 coplanar photon beams, starting from 0(o) with equal separation of 72(o), was investigated. For each photon beam used in the step-and-shoot IMRT plans, the first beam segment was set to have the largest area in the MLC leaf-sequencing, and was equal to the planning target volume (PTV). The overshoot effect (OSE) and the segment positional errors were measured using a solid water phantom with Kodak (TL and X-OMAT V) radiographic films. Film dosimetric analysis and calibration were carried out using a film scanner (Vidar VXR-16). The LP dose profiles were determined by eliminating the OSE and segment positional errors with specific individual irradiations.
A non-uniformly distributed leaf LP dose ranging from 3% to 5% of the beam dose was measured in clinical IMRT beams. An overdose at the gap between neighboring segments, represented as dose peaks of up to 10% of the total BP, was measured. The LP effect increased the dose to the PTV and surrounding critical tissues. In addition, the effect depends on the number of beams and segments for each beam. Segment positional error was less than the maximum tolerance of 1 mm under a dose rate of 600 monitor units per minute in the treatment plans. The OSE varying with the dose rate was observed in all photon beams, and the effect increased from 1 to 1.3 Gy per treatment of the rectal intersection. As the dosimetric impacts from the LP effect and OSE may increase the rectal post-radiation effects, a correction of LP was proposed and demonstrated for the central beam profile for one of the planned beams.
We concluded that the measured dosimetric impact of the LP dose inaccuracy from photon beam segment in step-and-shoot IMRT can be corrected.
采用一种针对步进式调强放射治疗（IMRT）的拟议校正方法研究漏射半影（LP）效应。
研究了从10个随机选择的前列腺IMRT计划中获取的漏射半影剂量分布。IMRT计划由配备120叶多叶准直器（MLC）的瓦里安21EX直线加速器执行。对于由Pinnacle(3)治疗计划系统创建的每个治疗计划，研究了由5个共面光子束生成的三维LP剂量分布，起始角度为0°，等间距为72°。对于步进式IMRT计划中使用的每个光子束，在MLC叶片排序中，第一个束段设置为具有最大面积，且等于计划靶体积（PTV）。使用带有柯达（TL和X-OMAT V）射线照相胶片的固体水模体测量过冲效应（OSE）和束段位置误差。使用胶片扫描仪（Vidar VXR-16）进行胶片剂量分析和校准。通过特定的个体照射消除OSE和束段位置误差来确定LP剂量分布。
在临床IMRT射束中测量到叶LP剂量呈非均匀分布，范围为射束剂量的3%至5%。在相邻束段之间的间隙处测量到过量剂量，表现为高达总BP的10%的剂量峰值。LP效应增加了PTV和周围关键组织的剂量。此外，该效应取决于束的数量和每个束的段数。在治疗计划中，在每分钟600监测单位的剂量率下，束段位置误差小于1mm的最大容差。在所有光子束中均观察到OSE随剂量率变化，直肠交叉点每次治疗的效应从1增加到1.3Gy。由于LP效应和OSE的剂量学影响可能会增加直肠放疗后效应，因此针对其中一个计划射束的中心束分布提出并演示了LP校正。
我们得出结论，步进式IMRT中光子束段LP剂量不准确的测量剂量学影响可以得到校正。
