Sanchez-Nieto B, Sanchez-Doblado F, Arrans R, Terron J A, Errazquin L
Dpto. Fisiología Médica y Biofísica, Universidad de Sevilla, Spain.
Med Dosim. 1993 Fall;18(3):107-11. doi: 10.1016/s0958-3947(06)80003-8.
The accuracy requirements in target dose delivery is, according to ICRU, +/- 5%. This is so not only in standard radiotherapy but also in total body irradiation (TBI). Physical dosimetry plays an important role in achieving this recommended level. The semi-infinite phantoms, customarily used for dosimetry purposes, give scatter conditions different to those of the finite thickness of the patient. So dose calculated in patient's points close to beam exit surface may be overestimated. It is then necessary to quantify the backscatter factor in order to decrease the uncertainty in this dose calculation. The backward scatter has been well studied at standard distances. The present work intends to evaluate the backscatter phenomenon under our particular TBI treatment conditions. As a consequence of this study, a semi-empirical expression has been derived to calculate (within 0.3% uncertainty) the backscatter factor. This factor depends lineally on the depth and exponentially on the underlying tissue. Differences found in the qualitative behavior with respect to standard distances are due to scatter in the bunker wall close to the measurement point.
根据国际辐射单位与测量委员会（ICRU）的规定，目标剂量输送的精度要求为±5%。不仅在标准放射治疗中如此，在全身照射（TBI）中也是如此。物理剂量测定在达到这一推荐水平方面起着重要作用。通常用于剂量测定目的的半无限体模，其散射条件与患者有限厚度的散射条件不同。因此，在靠近射束出射表面的患者点处计算的剂量可能会被高估。为了降低该剂量计算中的不确定性，有必要对反向散射因子进行量化。在标准距离下，反向散射已得到充分研究。本研究旨在评估我们特定的全身照射治疗条件下的反向散射现象。作为这项研究的结果，已经推导出一个半经验表达式来计算（不确定性在0.3%以内）反向散射因子。该因子与深度呈线性关系，与下层组织呈指数关系。相对于标准距离，在定性行为上发现的差异是由于靠近测量点的掩体壁中的散射所致。
