Biological determinants of radiation-induced human breast cancer.

This is the second in a three part series on the hypothetical risk from X-ray mammography. It will review those aspects of breast anatomy, histology, physiology, and pathology which are pertinent to radiation carcinogenesis. Radiation-induced breast cancers are histologically identical to the naturally occurring type in that they arise from the ductal epithelium and consist of a similar proportion of infiltrating and intraductal lesions. As indicated from mortality/incidence ratios, their prognosis is no worse, and may even be better, than that of naturally occurring lesions. Their location within the breast may depend on the distribution of absorbed radiation. Possible explanations for the increased resistance to radiation effect in women over 30 years of age at time of exposure will be evaluated. these include regression of the glandular target tissue, hormonal changes, and parity. Examples of age-related sensitivity and hormonal dependence in other radiation-induced human and animal tumors will be discussed.