Is there a real risk of radiation-induced breast cancer for postmenopausalwomen?

The risk of radiation-induced breast cancer decreases with increasing age a t exposure. Thus, for calculating the individual risk for a patient undergo ing mammography, age-related risk coefficients need to be used. In this rep ort, the results of epidemiological studies on risks of radiation-induced b reast cancer are reviewed indicating; that the available data do nut show t he risk to be enhanced for women exposed at the age of 55 years or older. T his lack of evidence is reflected by the fact that the risk coefficients re commended by national and international advisory bodies differ by a factor of 10 or more for age at exposure of 50-60 years or older. A hypothesis is proposed indicating that the risk of radiation-induced breast cancer might decrease considerably at the time of menopause. The hypothesis is based on the following line of arguments: (1) evidence has accumulated from molecula r genetic studies indicating that the development of colorectal cancer requ ires a cascade of subsequent mutations consisting of at least seven genetic events. (2) For colorectal cancer, the annual rates of incidence and morta lity increase with age to the power of 5-6. Thus, the number of mutation st eps (minus 1) is approximately reflected by the power of age dependence. (3 ) For western populations, the incidence and mortality of breast cancer up to the age of about 50 years increase with age to the power of about 6, ind icating that a similar number of genetic events might be involved in develo pment of breast cancer as has been identified for colorectal cancer. (4) Fo r women aged 50 years or older, breast cancer occurs at an annual rate that is about proportional to age or age squared. This may mean that after meno pause, the processes in the multistep mutation cascade leading to breast ca ncer are slowed down by a factor of about 3 or more. (5) The constant relat ive risk model of radiation carcinogenesis implies for solid cancers that r adiation acts by inducing additional mutations in the earlier steps of the multistep cascade. It is suggested that the break-point in the age-specific annual rate of breast cancer incidence at menopause is associated with a c orresponding drop in radiation sensitivity with respect to induction of bre ast cancer.