Does fractionation decrease the risk of breast cancer induced by low-LET radiation?

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.