LAUSR

Introduction Breast cancer is one of the most common cancers in women throughout the world. It is well known that ionising radiation is a potent carcinogen of breast, and its effect is modified by both age at its exposure and the reproductive history. The atomic bomb survivor study suggests that radiation-related risk of breast cancer of women is influenced by the ages of first menarche, first birth and menopause. With respect to radiation carcinogenesis, quality of radiation, such as density of ionisation and energy of radiation, is a critical determinant of the risk of radiogenic cancer. Experimental studies have shown that densely-ionising neutrons exhibit an increased risk of cancer compared to sparsely-ionising gamma and X rays. Little data are available, however, about the effect of parity (i.e. history of pregnancy) on carcinogenesis induced by neutrons. Material and methods Virgin female Sprague-Dawley rats at 35 weeks after birth (early middle age) were whole-body irradiated with gamma rays (Cs-137, 2 and 4 Gy) or fast neutrons of a mean energy of 2 MeV (0.05, 0.2 and 0.5 Gy). Parous rats, which gave birth and breast fed twice prior to 26 weeks of age, were also irradiated at 35 weeks. Rats were observed until 100 weeks and the incidence for mammary carcinoma, as compared to the non-irradiated group, was determined based on palpation records and pathological examination. Results and discussions Susceptibility to radiation in induction of mammary cancer is clearly dependent upon age at exposure. Gamma-irradiation significantly induced mammary cancer in virgin rats at 15 weeks old (mature young adult) with a similar extent to that at 3 weeks (before puberty) in our previous study, whereas the susceptibility was lost at 35 weeks. Neutron exposure at 35 weeks, however, still increased the risk of mammary carcinoma, suggesting that the susceptible age window for breast cancer induction was wider for neutrons than gamma rays. Importantly, parity at the time of exposure completely inhibited an increase in mammary cancer risk irrespective of radiation type. Pathological examination to identify if parity influences a specific subtype of mammary carcinomas is now undertaken. The above evidence sheds light on the precise estimation/prediction of breast cancer risk of nulliparous and parous women, for instance, in case of diagnostic and therapeutic radiation exposure. Conclusion This study suggests that parity significantly suppresses not only gamma-ray– but also neutron-induced mammary cancer.