The sensitivity of the human breast to cancer induction by ionizing radiation

It is often difficult to accept that cancer induction by small doses of radiation is more than a theoretical possibility but actually happens. Indeed it is only quite recently that confirmatory evidence of considerable weight has become available to support the conclusion that the breast may be more sensitive to cancer induction by ionizing radiation than any other human tissue. Two groups of female patients have been followed up for ten to 44 years (mean follow-up each exceeding 25 years) (in Nova Scotia by Myrden and Hiltz, 1969; BEIR, 1972, and in Massachusetts by Boice and Monson, 1977). In the first series breast cancer incidence was five times, in the other twice, what would be expected in controls (tuberculous patients treated without pneumothorax, or regional population statistics). Other series may fail to show an increase but this is to be expected given the numbers of subjects, the magnitude of the breast doses and/or the use of mortality instead of incidence as the index of effect. In the two surveys mentioned there was an approximately linear relationship between excess breast cancer incidence and number of fluoroscopies. This is as would be predicted from the simple hypothesis that cancer frequency caused by multiple small doses of radiation would be proportional to total accumulated dose.