Olch Arthur J, Gerig Lee, Li Heng, Mihaylov Ivaylo, Morgan Andrew
Radiation Oncology Department, University of Southern California and Children's Hospital Los Angeles, Los Angeles, California 90027.
Department of Physics, Ottawa Hospital Regional Cancer Centre, Carleton University, Ottawa, Ontario K1S 5B6, Canada and Faculty of Medicine, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
Med Phys. 2014 Jun;41(6):061501. doi: 10.1118/1.4876299.
The dosimetric impact from devices external to the patient is a complex combination of increased skin dose, reduced tumor dose, and altered dose distribution. Although small monitor unit or dose corrections are routinely made for blocking trays, ion chamber correction factors, e.g., accounting for temperature and pressure, or tissue inhomogeneities, the dose perturbation of the treatment couch top or immobilization devices is often overlooked. These devices also increase skin dose, an effect which is also often ignored or underestimated. These concerns have grown recently due to the increased use of monolithic carbon fiber couch tops which are optimal for imaging for patient position verification but cause attenuation and increased skin dose compared to the "tennis racket" style couch top they often replace. Also, arc delivery techniques have replaced stationary gantry techniques which cause a greater fraction of the dose to be delivered from posterior angles. A host of immobilization devices are available and used to increase patient positioning reproducibility, and these also have attenuation and skin dose implications which are often ignored. This report of Task Group 176 serves to present a survey of published data that illustrates the magnitude of the dosimetric effects of a wide range of devices external to the patient. The report also provides methods for modeling couch tops in treatment planning systems so the physicist can accurately compute the dosimetric effects for indexed patient treatments. Both photon and proton beams are considered. A discussion on avoidance of high density structures during beam planning is also provided. An important aspect of this report are the recommendations the authors make to clinical physicists, treatment planning system vendors, and device vendors on how to make measurements of surface dose and attenuation and how to report these values. For the vendors, an appeal is made to work together to provide accurate couch top models in planning systems.
患者体外设备的剂量学影响是皮肤剂量增加、肿瘤剂量降低和剂量分布改变的复杂组合。尽管通常会对挡块托盘、电离室校正因子（例如考虑温度、压力或组织不均匀性）进行小的监测单元或剂量校正，但治疗床面或固定装置的剂量扰动常常被忽视。这些装置也会增加皮肤剂量，而这种影响也常常被忽略或低估。由于整体式碳纤维床面的使用增加，这些问题近来愈发受到关注。这种床面对于患者体位验证成像而言是最佳的，但与它们常常取代的“网球拍”式床面相比，会造成衰减并增加皮肤剂量。此外，弧形照射技术已取代了固定机架技术，后者会使更大比例的剂量从后角照射。有许多固定装置可用于提高患者体位的重复性，而这些装置也存在衰减和皮肤剂量方面的影响，却常常被忽视。任务组176的这份报告旨在展示对已发表数据的调查，这些数据说明了患者体外各种设备剂量学效应的程度。该报告还提供了在治疗计划系统中对床面进行建模的方法，以便物理学家能够准确计算索引患者治疗的剂量学效应。光子束和质子束均在考虑范围内。还提供了关于在射束规划期间避免高密度结构的讨论。本报告的一个重要方面是作者向临床物理学家、治疗计划系统供应商和设备供应商提出的关于如何测量表面剂量和衰减以及如何报告这些值的建议。对于供应商，呼吁他们共同努力，在规划系统中提供准确的床面模型。
