Cytokinesis-blocked micronucleus (CBMN) assay has been extensively used to evaluate the radiation sensitivity of human individuals. Using the CBMN assay, Scott et al (1998, 1999)demonstrated that a fraction of radiosensitive individuals in breast cancer case population was larger than in normal individual population. However, Vral et al were very skeptical about the Scott et al\u27 s findings (2002). Under the approval from the ethical committee of NIRS, peripheral blood was obtained from 46 normal healthy females, 131 breast cancer patients, 32 cervical cancer patients and 7 female head and neck cancer patients. Radiosensitivity of T-lymphocytes was assessed by using a CBMN assay. The frequencies of MN per binucleated cell in healthy donors were 0.031 (+/-0.010) and 0.151 (+/-0.066) for cells treated before and after X-ray-irradiation (2Gy), respectively. Spontaneous MN frequencies in cancer patients were significantly higher than healthy donors (p<0.001). Radiation sensitivities of breast- and head and neck-cancer patients were significantly higher than normal individuals (p<0.001). Cervical cancer patients were more resistant to irradiation than healthy donors, though the number of cases for statistical analysis was small (p<0.001). We are considering that the HPV infection affected the radiosensitivity of cervical cancer cases. Because it is widely believed that one key mechanism which leads to spontaneous micronucleus formation involves an imbalance of chromosomal segregation and a chromosomal instability in patients\u27 lymphocytes might be greater than that in normal individuals\u27 lymphocytes. Recently, Kuschel et al (2002) demonstrated that ratios in two SNPs on XRCC3 were significantly different between cancer patients and healthy females. Then, we can suppose that the radiation-related genes with low penetrance may be involved in tumorigenesis of mammary- and head & neck-cells, and also, in patients\u27 radiation susceptibility.The 12th International Congress of Radiation Research(ICRR
