Department of Radiation Oncology, Universitätsmedizin Mannheim, Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany.
Int J Radiat Oncol Biol Phys. 2013 Mar 15;85(4):1127-33. doi: 10.1016/j.ijrobp.2012.08.005. Epub 2012 Sep 14.
Intraoperative radiation therapy (IORT) with low-energy x-rays is used to treat the tumor bed during breast-conserving surgery. The purpose was to determine the relative biologic effectiveness (RBE) of 50-kV x-rays for inactivation of cells irradiated in a tumor-bed phantom.
The RBE was determined for clonogenic inactivation of human tumor and normal cells (MCF7, human umbilical vein endothelial cells, normal skin fibroblasts), and hamster V79 cells. The 50-kV x-rays from the Intrabeam machine (Carl Zeiss Surgical) with a spherical 4-cm applicator were used. Cells were irradiated in a water-equivalent phantom at defined distances (8.1-22.9 mm) from the applicator surface. The 50-kV x-rays from a surface therapy machine (Dermopan, Siemens) were included for comparison; 6-MV x-rays were used as reference radiation.
At 8.1-mm depth in the phantom (dose rate 15.1 Gy/h), mean RBE values of 50-kV x-rays from Intrabeam were 1.26 to 1.42 for the 4 cell types at doses yielding surviving fractions in the range of 0.01 to 0.5. Confidence intervals were in the range of 1.2 and 1.5. Similar RBE values were found for 50-kV x-rays from Dermopan for V79 (1.30, CI 1.25-1.36, P=.74) and GS4 (1.42, CI 1.30-1.54, P=.67). No significant dependence of RBE on dose was found for Intrabeam, but RBE decreased at a larger distance (12.7 mm; 9.8 Gy/h).
An increased clinically relevant RBE was found for cell irradiation with Intrabeam at depths in the tumor bed targeted by IORT. The reduced RBE values at larger distances may be related to increased repair of sublethal damage during protracted irradiation or to hardening of the photon beam energy.
术中低能 X 射线放射治疗（IORT）用于保乳手术时治疗肿瘤床。本研究旨在确定在肿瘤床模型中照射细胞时，50kV X 射线的相对生物效应（RBE）。
本研究通过克隆性灭活实验来确定 RBE，所用细胞包括人肿瘤细胞（MCF7）、人脐静脉内皮细胞、正常皮肤成纤维细胞）和仓鼠 V79 细胞。使用 Intrabeam 机器（Carl Zeiss Surgical）的 50kV X 射线，其球形 4cm 施源器。细胞在水等效模型中于距施源器表面 8.1-22.9mm 处接受照射。为进行比较，还包括了来自表面治疗机（Dermopan，Siemens）的 6MV X 射线；6MV X 射线被用作参考辐射。
在模型中 8.1mm 深度（剂量率为 15.1Gy/h），对于剂量产生 0.01 至 0.5 存活分数范围内的 4 种细胞类型，Intrabeam 50kV X 射线的平均 RBE 值为 1.26 至 1.42。置信区间在 1.2 和 1.5 之间。对于 V79（1.30，置信区间 1.25-1.36，P=.74）和 GS4（1.42，置信区间 1.30-1.54，P=.67）细胞，Dermopan 50kV X 射线的 RBE 值也相似。对于 Intrabeam，RBE 与剂量无明显相关性，但在较大距离（12.7mm；9.8Gy/h）时 RBE 降低。
在 IORT 靶向肿瘤床的深度，Intrabeam 细胞照射的 RBE 显著增加。在较大距离时 RBE 值降低，可能与亚致死损伤修复增加有关，也可能与光子束能量硬化有关。
