Chromosome aberration yield following radiotherapy-biological dosimetry for non-uniform exposures

Chromosomal aberration analysis was carried out in peripheral blood lymphocytes of cancer patients following radiotherapy of lungs, cervix and spine. Whole blood cultures were set up by standard procedure and the samples were analysed for the presence of chromosomal aberrations. Radiotherapy in the pelvic region involving large doses (6 Gy) showed an over dispersed distribution of dicentrics. But when the doses were fractionated (3 fractions of 2 Gy) the distribution was found to be near Poisson. Spine irradiation covering almost all the lymphocytes pools indicated a Poisson distribution. The data shows that depending on the sites of exposure, the distribution of dicentrics in cells vary and hence there is a non-uniform distribution of lymphocytes in the body. From the yield of dicentrics the mean dose to the lymphocytes was calculated using in vitro dose response curve generated for cobalt-60 gamma rays. The average dose to the lymphocytes was found to be 1/6th of the partial body dose. On the basis of the non-Poisson distribution of aberrations, the fraction of lymphocytes irradiated mean dose to the fraction and part of the body exposed was calculated in a case of acute pelvic irradiation case of 6 Gy. The fraction of cells irradiated was calculated to be 4.4%, and the portion of the body exposed was approximately 24%. The dose to the irradiated fraction was found to be 5.4 Gy which is in agreement with the given dose of 6 Gy. (author)