Does Fractionation Decrease the Risk of Breast Cancer Induced by Low-LET Radiation?

Abstract

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.

Effects of Radiation Exposure From Cardiac Imaging

“…Excess cancer risk was noted in each of these cohorts, and risk generally increased with attained age until about age 50 at which point it plateaued in several studies (Figure 7). The authors of a systematic review of most of this data (46) concluded that excess risk of breast cancer depends linearly on dose with a downturn at high doses, where cell death may occur, and that risk is similar between acute and fractionated high-dose-rate exposures, but lower with protracted low-dose rate exposures, although others have alternatively interpreted some of this data to conclude that fractionation also somewhat reduces risk (47). These studies were also generally compatible with the Life Span Study in terms of excess cancer rates (Figure 7) (40,46), although there was some variation in rates between studies, leading the authors of the systematic review to point out that “no simple unified summary model adequately describes the excess risks in all groups” (46).…”

Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know

“…In general, protracted exposures to x-or ␥-radiation are associated with lower risks than those of an acute exposure to the same total dose, both for cancer and other endpoints (10,11).…”

Radiation-associated circulatory disease mortality in a pooled analysis of 77,275 patients from the Massachusetts and Canadian tuberculosis fluoroscopy cohorts

“…The modification to excess relative risk/Gy with time after exposure for IHD and cerebrovascular disease (CeVD) in a US cohort of persons who received X-radiation treatment for peptic ulcer13 are consistent with those observed here. It may be significant that the type of radiation used in this study, moderate energy X-rays, is quite similar to the type of low energy, and largely unfiltered, fluoroscopy X-rays used here20, albeit for therapeutic rather than diagnostic purposes, and contrasts with the rather higher energy radiation to which nuclear workers and the LSS2122 were exposed (Supplementary Table S6); it is well known that higher-energy gamma rays are less biologically effective per unit dose than X-rays in relation to a number of experimental endpoints, in particular chromosome translocations, dicentrics, cell transformation, cell killing, specific locus mutations and various others23. Also, a typical chest fluoroscopy in the period 1930–1950, when most of the dose in the cohort was incurred, would last about 15 s and patients would receive 0.01–0.10 Gy, and thus should not be considered a low dose-rate exposure24; in this respect it is similar to the peptic ulcer study13 and to the LSS2122, and contrasts with the generally low dose-rate exposure in most of the other moderate/low dose cohorts listed in Supplementary Table S6.…”