DOSE DISTRIBUTION IN TOTAL BODY PHOTON IRRADIATION

Abstract

In the total body photon irradiation (TBI) of leukemic and lymphoma patients prior to bone marrow transplantation, bilateral opposing beams of the fully opened collimator produced by a 6 MV Clinac 2100C with perspex beam spoiler of 1 cm thick were used to irradiated the Randophantom at a large target surface distance (TSD). The measurements of the radiation doses at the surface and inside the phantom were performed using LiF Thermoluminescent dosemeters. The dose at the center of the abdomen at the umbilicus level was taken as a reference dose.
The first study without bolus and shieldings showed the high surface doses of 125 % and 128 % at the right and left lateral neck respectively. The rest of them were not more than 17% higher than the reference dose. The largest different doses from the reference for various organs inside the phantom were 33 % at the larynx and 29% at the cervical cord. The doses at the brain, right lung and left lung were 12 %, 16 % and 18 % higher than the reference dose respectively.
In the second study, the bolus was added to the neck region for uniformity of dose distribution and one half value thickness of shielding blocks were used for both lungs and brain. The surface doses at right and left lateral neck were reduced to 109 % and 104 % respectively. The doses at larynx and cervical cord were 5 % and 2 % higher than the reference dose. The doses variations within ± 10 % were obtained at the surface and inside the phantom while the dose in the shielding region were reduced to approximately 50%