This study aims at the estimation of skin doses during small field radiotherapy with 6 MV photons and analysis of beam spectra at skin surface. The EGSnrc Monte Carlo code was used for spectral analysis and dose scoring in a water phantom. Percent skin dose (PSD) was calculated at a depth of 70 µm (relative to 10 cm depth), and the effects of field size, collimation, source-to-surface distance, and tissue inhomogeneity (bone/air) below the skin were evaluated. Low-energy photons and contaminant electrons from the machine head or back-scattered from underlying tissue were found to be the major contributors to skin dose. As the field size was reduced, the beam hardened, while the photon and electron fluences at the skin decreased compared to those at the reference depth of 10 cm. This resulted in a PSD reduction for fields smaller than the reference field size. Multi leaf collimators increased the PSD (up to 4%) while variation in source-to-skin dose showed a negligible effect. A substantial increase in PSD has been observed (up to 6%) when high Z material like bone was placed below the skin. In contrast, air as underlying material decreased the skin dose. The skin dose varied considerably with various clinical and geometric parameters. It is concluded that, although the skin doses were low for small fields compared to those for the reference field, skin doses may become substantial when escalated target doses are delivered with multi leaf collimators. Moreover, the presence of high Z materials such as bones or metallic implants below the skin can result in significant enhancement of the skin dose.

本研究旨在估算 6 MV 光子小场放射治疗期间的皮肤剂量，并分析皮肤表面的光束光谱。EGSnrc Monte Carlo 代码用于水体模型中的光谱分析和剂量评分。在 70 µm 深度（相对于 10 cm 深度）计算皮肤剂量百分比 (PSD)，并且皮肤下的场大小、准直、源到表面距离和组织不均匀性（骨/空气）的影响为评估。来自机器头部或从底层组织反向散射的低能光子和污染电子被发现是皮肤剂量的主要贡献者。随着场尺寸的减小，光束变硬，而与参考深度 10 cm 处的相比，皮肤处的光子和电子能量密度降低。这导致小于参考场大小的场的 PSD 减小。多叶准直器增加了 PSD（高达 4%），而源到皮肤剂量的变化显示出可以忽略不计的影响。当高时观察到 PSD 显着增加（高达 6%）像骨头这样的Z材料被放置在皮肤下方。相比之下，空气作为底层材料降低了皮肤剂量。皮肤剂量随各种临床和几何参数变化很大。得出的结论是，虽然小视野的皮肤剂量与参考视野的皮肤剂量相比较低，但当使用多叶准直器传送递增的目标剂量时，皮肤剂量可能会变得很大。此外，皮肤下存在高Z材料（例如骨骼或金属植入物）会导致皮肤剂量显着增加。