Brenner D J
Center for Radiological Research, Columbia University, New York, New York 10032, USA.
Radiat Res. 1999 Feb;151(2):225-9.
Whether fractionation decreases the risk of breast cancer induced by low-LET radiation is a question of some importance. Analyses of the data for TB cohorts who were exposed to multiple fluoroscopies show an apparently similar breast cancer risk compared with those for the acutely exposed A-bomb survivors. However, the fluoroscopy cohorts were subjected to very much lower-energy photons (60-80 kVp) compared with the A-bomb survivors; the increased RBE associated with the low photon energies to which these fluoroscopy cohorts were exposed suggests that, in comparison to the risk estimates for the A-bomb survivors, the risk estimates from the X-ray fluoroscopy cohorts are increased because of the lower-energy X rays and decreased by a similar amount due to fractionation, resulting in an overall apparent equality of risk. Thus the results from the most powerful epidemiological data sets available for assessing breast cancer risks after fractionated exposure to low-LET radiation (the fluoroscopy cohorts) are quite consistent with a lower radiation risk for a fractionated exposure in comparison to an acute exposure. In general, for any cancer site, estimates of the dose-rate effectiveness factor (DDREF) generated by comparing the results for A-bomb survivors with those for the TB fluoroscopy cohorts should probably be roughly doubled from their apparent values because of the increased RBE of the fluoroscopy X rays.
分次照射是否会降低低线性能量传递（low-LET）辐射诱发乳腺癌的风险是一个颇为重要的问题。对接受多次荧光透视检查的结核病队列数据进行分析后发现，与急性受照的原子弹幸存者相比，其患乳腺癌的风险明显相似。然而，与原子弹幸存者相比，荧光透视检查队列所接受的光子能量要低得多（60 - 80 kVp）；这些荧光透视检查队列所接触的低光子能量相关的相对生物效应（RBE）增加表明，与原子弹幸存者的风险估计相比，X射线荧光透视检查队列的风险估计因较低能量的X射线而增加，又因分次照射而降低了类似的量，从而导致总体风险看似相等。因此，可用于评估分次低LET辐射后乳腺癌风险的最有力流行病学数据集（荧光透视检查队列）的结果与急性照射相比，分次照射的辐射风险较低这一观点相当一致。一般而言，对于任何癌症部位，由于荧光透视X射线的RBE增加，如果将原子弹幸存者的结果与结核病荧光透视检查队列的结果进行比较来生成剂量率有效性因子（DDREF）的估计值，那么其估计值可能应比表观值大致翻倍。
