Medical Physics, Radiobiology and Radiation Protection Group, IPO Porto Research Center (CI-IPOP), Portuguese Oncology Institute of Porto (IPO Porto), Porto, Portugal.
Medical Physics Department, Portuguese Oncology Institute of Porto (IPO Porto), Porto, Portugal.
Health Phys. 2019 Nov;117(5):489-503. doi: 10.1097/HP.0000000000001079.
Out-of-field scattered and transmitted extrafocal radiation may induce secondary cancer in long-term survivors of external radiotherapy. Pediatric patients have higher life expectancy and tend to receive higher secondary radiation damage due to geometric and biological factors. The goal of this study is to characterize the location and the magnitude of extrafocal dose regions in the case of three-dimensional conformal radiotherapy and volumetric arc therapy, to apply this information to clinical treatment cases, and to provide mitigation strategies. Extrafocal dose has been investigated in a Varian TrueBeam linac equipped with a high-definition 120 multileaf collimator using different physical and virtual phantoms, dose calculation (including Monte Carlo techniques), and dose measurement methods. All Monte Carlo calculations showed excellent agreement with measurements. Treatment planning system calculations failed to provide reliable results out of the treatment field. Both Monte Carlo calculations and dose measurements showed regions with higher dose (extrafocal dose areas) when compared to the background. These areas start to be noticeable beyond 11 cm from the isocenter in the direction perpendicular to the multileaf collimator leaves' travel direction. Out-of-field extrafocal doses up to 160% of the mean dose transmitted through the closed multileaf collimator were registered. Two overlapping components were observed in the extrafocal distribution: the first is an almost elliptical blurred dose distribution, and the second is a well-defined rectangular dose distribution. Extra precautions should be taken into consideration when treating pediatric patients with a high-definition 120 multileaf collimator to avoid directing the extrafocal radiation into a radiosensitive organ during external beam therapy.
在三维适形放疗和容积弧形调强放疗中，离焦区散射和透射的额外焦点外辐射可能会导致长期接受外部放射治疗的患者发生继发性癌症。儿科患者的预期寿命较长，由于几何和生物学因素，他们更容易受到继发性辐射损伤。本研究的目的是描述三维适形放疗和容积弧形调强放疗中离焦剂量区域的位置和大小，将这些信息应用于临床治疗病例，并提供缓解策略。在配备高清 120 多叶准直器的瓦里安 TrueBeam 直线加速器中，使用不同的物理和虚拟体模、剂量计算（包括蒙特卡罗技术）和剂量测量方法，对离焦剂量进行了研究。所有蒙特卡罗计算都与测量结果吻合良好。治疗计划系统计算无法在治疗野外提供可靠的结果。与背景相比，蒙特卡罗计算和剂量测量都显示出具有更高剂量的区域（离焦剂量区域）。这些区域在垂直于多叶准直器叶片行进方向的离轴方向上从等中心开始，超过 11 厘米后开始变得明显。在穿过闭合多叶准直器的平均剂量的 160%范围内，记录到离焦区外的额外剂量。在离焦分布中观察到两个重叠的分量：第一个是几乎椭圆形的模糊剂量分布，第二个是定义明确的矩形剂量分布。在使用高清 120 多叶准直器治疗儿科患者时，应特别注意避免在外部束治疗过程中将离焦辐射引导到敏感器官。
