Premion Cancer Care, Wesley Medical Centre, Suite 1, 40 Chasely St, Auchenflower Qld 4066, Australia and Science and Engineering Faculty, Queensland University of Technology, G.P.O. Box 2434, Brisbane Qld 4000, Australia.
Med Phys. 2013 Nov;40(11):111708. doi: 10.1118/1.4823776.
Intensity modulated radiotherapy (IMRT) treatments require more beam-on time and produce more linac head leakage to deliver similar doses to conventional, unmodulated, radiotherapy treatments. It is necessary to take this increased leakage into account when evaluating the results of radiation surveys around bunkers that are, or will be, used for IMRT. The recommended procedure of applying a monitor-unit based workload correction factor to secondary barrier survey measurements, to account for this increased leakage when evaluating radiation survey measurements around IMRT bunkers, can lead to potentially costly overestimation of the required barrier thickness. This study aims to provide initial guidance on the validity of reducing the value of the correction factor when applied to different radiation barriers (primary barriers, doors, maze walls, and other walls) by evaluating three different bunker designs.
Radiation survey measurements of primary, scattered, and leakage radiation were obtained at each of five survey points around each of three different radiotherapy bunkers and the contribution of leakage to the total measured radiation dose at each point was evaluated. Measurements at each survey point were made with the linac gantry set to 12 equidistant positions from 0° to 330°, to assess the effects of radiation beam direction on the results.
For all three bunker designs, less than 0.5% of dose measured at and alongside the primary barriers, less than 25% of the dose measured outside the bunker doors and up to 100% of the dose measured outside other secondary barriers was found to be caused by linac head leakage.
Results of this study suggest that IMRT workload corrections are unnecessary, for survey measurements made at and alongside primary barriers. Use of reduced IMRT workload correction factors is recommended when evaluating survey measurements around a bunker door, provided that a subset of the measurements used in this study are repeated for the bunker in question. Reduction of the correction factor for other secondary barrier survey measurements is not recommended unless the contribution from leakage is separately evaluated.
调强放射治疗（IMRT）需要更多的射束照射时间，并产生更多的直线加速器机头漏射线，以达到与传统的未调制放射治疗相似的剂量。在评估用于 IMRT 的掩体周围的辐射调查结果时，有必要考虑到这种增加的漏射线。建议的方法是应用基于监测单位的工作量校正因子来校正次级屏蔽测量值，以在评估 IMRT 掩体周围的辐射测量值时考虑到这种增加的漏射线，这可能会导致对所需屏蔽厚度的过高估计。本研究旨在通过评估三种不同的掩体设计，为减少应用于不同辐射屏蔽（初级屏蔽、门、迷宫墙和其他墙壁）的校正因子值的有效性提供初步指导。
在三个不同的放射治疗掩体的每一个掩体周围的五个测量点处，获得了初级、散射和漏射线的辐射测量值，并评估了在每个点处漏射线对总测量辐射剂量的贡献。在每个测量点处，直线加速器旋转架设置在 0°至 330°的 12 个等距位置处进行测量，以评估辐射束方向对结果的影响。
对于所有三种掩体设计，在初级屏蔽物的内外侧测量到的剂量中，不到 0.5%，在掩体门外部测量到的剂量中，不到 25%，在其他次级屏蔽物外部测量到的剂量中，高达 100%，被认为是由直线加速器机头漏射线引起的。
本研究结果表明，对于在初级屏蔽物的内外侧进行的测量，IMRT 工作量校正因子是不必要的。在评估掩体门周围的测量值时，建议使用减少的 IMRT 工作量校正因子，但前提是对于所研究的掩体，重复进行本研究中使用的测量值的一部分。除非单独评估漏射线的贡献，否则不建议减少其他次级屏蔽物的测量值的校正因子。
