The dosimetry of cobalt-60 γ-ray total body irradiation before bone marrow transplantation

The dosimetric considerations of using conventional cobalt-60 unit total body irradiation (TBI) are presented. By extending the source-to-midplane distance (SMD) to 346 cm, a 92 x 98 cm2 rectangular field with diagonal dimension 134 cm was obtained. The results from the phantom measurements showed: (1) the effective field corresponding to an average-size patient is 25 x 25 cm2, and a method for estimating the effective field of human body is given; (2) the midplane doses are consistently higher than those of surfaces, but the dose ratio of midplane to surface decreases as the body thickness increases, and a significant negative correlation is existed between the dose ratio and thickness, thus a linear regression line is fitted; (3) the anterior-posterior (AP) or AP + bilateral irradiation will yield a more uniform dose distribution in the whole body than the bilateral irradiation; (4) the dose uniformity can apparently be improved by the tissue compensation, for which the technique is described