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

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Abstract
Since radiotherapy is commonly part of treatment for breast cancer and since the untreated breast is bound to receive some dose (depending on the technique used), the observations discussed would lead to the prediction that subsequent frequency of breast cancer would be raised in the untreated breast and this increase in frequency would be more conspicuous the younger the age at which the treatment was given. Both predictions appear to be supported by observations. At 10-15 yr after treatment of breast cancer the frequency of cancer in the untreated breast was 4-fold greater than that expected from national statistics. The risk of occurrence of a 2nd primary breast cancer in the other breast was dependent on the age at diagnosis of a 1st breast cancer, in 1 survey being 84-fold higher for those aged 20-24 yr and 4-fold higher for those aged 40-44 yr at the time of the 1st diagnosis. There is as yet no evidence to tell how much of these observed differences may be attributed to biological factors and how much, if any, to radiotherapy. Known biological factors, such as cystic mastitis and the length of the time interval between menarche and the 1st pregnancy, seem responsible for only a 3- to 4-fold difference in risk of breast cancer. Now that the high radiation sensitivity of the human breast is established, the effect of radiation dose on the untreated breast must be evaluated before accepting quantitative conclusions about genetic or other influences derived from epidemiological reports on the occurrence of a 2nd cancer in individuals who have already had a 1st breast cancer.