Quality MediPhys LLC, 17 Jade Lane, Denville, New Jersey 07834, USA.
Med Phys. 2010 Jul;37(7):3611-20. doi: 10.1118/1.3439586.
In this study, the authors have quantified the two-dimensional (2D) perspective of skin dose increase using EBT film dosimetry in phantom in the presence of patient immobilization devices during conventional and IMRT treatments.
For 6 MV conventional photon field, the authors evaluated and quantified the 2D bolus effect on skin doses for six different common patient immobilization/support devices, including carbon fiber grid with Mylar sheet, Orfit carbon fiber base plate, balsa wood board, Styrofoam, perforated AquaPlast sheet, and alpha-cradle. For 6 and 15 MV IMRT fields, a stack of two film layers positioned above a solid phantom was exposed at the air interface or in the presence of a patient alpha-cradle. All the films were scanned and the pixel values were converted to doses based on an established calibration curve. The authors determined the 2D skin dose distributions, isodose curves, and cross-sectional profiles at the surface layers with or without the immobilization/support device. The authors also generated and compared the dose area histograms (DAHs) and dose area products from the 2D skin dose distributions.
In contrast with 20% relative dose [(RD) dose relative to dmax on central axis] at 0.0153 cm in the film layer for 6 MV 10 x 10 cm2 open field, the average RDs at the same depth in the film layer were 71%, 69%, 55%, and 57% for Orfit, balsa wood, Styrofoam, and alpha-cradle, respectively. At the same depth, the RDs were 54% under a strut and 26% between neighboring struts of a carbon fiber grid with Mylar sheet, and between 34% and 56% for stretched perforated AquaPlast sheet. In the presence of the alpha-cradle for the 6 MV (15 MV) IMRT fields, the hot spot doses at the effective measurement depths of 0.0153 and 0.0459 cm were 140% and 150%, (83% and 89%), respectively, of the isocenter dose. The enhancement factor was defined as the ratio of a given DAH parameter (minimum dose received in a given area) with and without the support device. For 6 MV conventional 10 x 10 cm2 field, the enhancement factor was the highest (3.4) for the Orfit carbon fiber plate. As for the IMRT field, the enhancement factors varied with the size of the area of interest and were as high as 3.8 (4.3) at the hot spot of 5 cm2 area in the top film layer (0.0153 cm) for 6 MV (15 MV) beams.
Significant 2D bolus effect on skin dose in the presence of patient support and immobilization devices was confirmed and quantified with EBT film dosimetry. Furthermore, the EBT film has potential application for in vivo monitoring of the 2D skin dose distributions during patient treatments.
本研究通过在常规和调强放疗治疗中使用 EBT 胶片剂量学对患者固定装置存在时的皮肤剂量进行二维（2D）透视评估和量化。
对于 6MV 常规光子场，作者评估和量化了六种常见患者固定/支撑设备对皮肤剂量的 2D 附加效应，包括带有聚酯薄膜的碳纤维网格、Orfit 碳纤维基板、轻木基板、泡沫聚苯乙烯、有孔 AquaPlast 片和 Alpha 摇篮。对于 6MV 和 15MV 调强放疗场，在固体体模上方堆叠两层胶片，在空气界面或存在患者 Alpha 摇篮的情况下进行曝光。所有胶片均进行扫描，并根据已建立的校准曲线将像素值转换为剂量。作者在有或没有固定/支撑设备的情况下确定了表面层的 2D 皮肤剂量分布、等剂量曲线和横截面轮廓。作者还生成并比较了来自 2D 皮肤剂量分布的剂量面积直方图（DAH）和剂量面积产品。
与中央轴上 0.0153cm 处的 6MV10×10cm2 开放场的 20%相对剂量（RD 剂量与 dmax 的比值）相比，在同一胶片层深度处，Orfit、轻木、泡沫聚苯乙烯和 Alpha 摇篮的平均 RD 分别为 71%、69%、55%和 57%。在同一深度，碳纤维网格带聚酯薄膜的支撑物下的 RD 为 54%，相邻支撑物之间的 RD 为 26%，而拉伸的有孔 AquaPlast 片的 RD 为 34%至 56%。在存在 6MV（15MV）调强放疗场的 Alpha 摇篮时，有效测量深度为 0.0153cm 和 0.0459cm 的热点剂量分别为等中心剂量的 140%和 150%（83%和 89%）。增强系数定义为有和没有支撑设备时给定 DAH 参数（给定区域内接收的最小剂量）的比值。对于 6MV 常规 10×10cm2 场，Orfit 碳纤维板的增强系数最高（3.4）。对于调强放疗场，增强系数随感兴趣区域的大小而变化，在顶部胶片层（0.0153cm）热点处高达 5cm2 区域的热点处为 3.8（4.3）（6MV（15MV）束）。
通过 EBT 胶片剂量学证实并量化了患者支撑和固定装置存在时对皮肤剂量的显著 2D 附加效应。此外，EBT 胶片具有在患者治疗期间用于 2D 皮肤剂量分布的体内监测的潜力。
