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The yield of dicentric chromosomes has been recorded in peripheral blood lymphocytes of patients undergoing telecobalt therapy for glioblastoma. Blood samples were taken by venipuncture, prior to the first radiotherapy session and 24 h after 10, 20 and 30 Gy to the tumor volume. On the basis of the maximum likelihood method, the yield of chromosome aberrations was best fitted by a linear quadratic dose-response relationship. According to this relationship, the dose inducing ten dicentrics at the target volume is 58 Gy, a value considerably higher than those found after radiotherapy for mammary carcinoma (15 Gy) or for pelvic tumors (5.62 Gy). Our results indicate that, in the case of fractionated exposures, confined to a small volume of the body, it is not possible to estimate the total dose administered and that the method only provides an estimate of the proportion of the lymphocytes irradiated.