[Technic and dosimetry in total body irradiation with 18 MeV photons]

Total body irradiation (TBI) is used in our Institution in the conditioning regimen for bone marrow transplantation. The fractionation pattern consists of two daily fractions of 1.65 Gy repeated for 4 days (total dose 13.20 Gy in 8 fractions). Lung dose is reduced by means of lead custom shaped shields directly strapped to the patient surface. TBI is delivered by a THERAC 20 linear accelerator with two opposing AP-PA photon beams with a maximum energy of 18 MeV. Treatment distance is 340 cm and the patient is treated in a semi-standing position. Dosimetry studies in a homogeneous phantom were performed in the treatment geometry and consisted in the determination of: tissue maximum ratios (TMR) at different depths, absorbed dose along the median axis and the diagonal of the field, variation of the absorbed dose in the prescription point with different volumes of scattering material, and transmission of shielding and compensating material. A semi-empiric formula for the calculation of absorbed dose in a point has been obtained. A subsequent study in a Rando phantom with termoluminescent dosimeters (TLD) has shown a +/- 5% agreement between calculated and measured values and a +/- 7% homogeneity.