A TRANSLATING-BED TECHNIQUE FOR TOTAL-BODY IRRADIATION

Total-body irradiation (TBI) is a therapy modality that is being used with increasing frequency, in conjunction with chemotherapy, for patie nts undergoing bone-marrow transplantation. At the Ottawa Regional Can cer centre a technique has been developed for the delivery of TBI to p atients prior to bone-marrow transplantation. In this technique patien ts are treated on a mobile couch at approximately 195 cm SSD with a fi eld size of 66.5 cm wide by 57 cm long. A computer-controlled stepping motor drives the patient couch at a user-selectable speed. The total dose delivered to the patient is a function of couch velocity, field s ize and patient separation. Treatment times are of the order of 10 min for each of the anterior and posterior fields for a 400 cGy fraction. It has been found that the conventional central axis tissue maximum r atio (TMR) and percentage depth dose (PDD) functions are not appropria te for describing dose delivered during dynamic treatment. To this end we have developed dynamic TMR and PDD functions. Extensive measuremen ts have been performed in an anthropomorphic water phantom to determin e the dose distributions in three dimensions and the efficacy of polym ethyl methacrylate (PMMA) beam spoilers as a replacement for anterior and lateral bolus. It has been found that 2.4 cm PMMA spoilers do prov ide full skin dose and negate the requirement for lateral bolus. This Tsr procedure is simple, rapid and appears to be well tolerated by the patients. 55 patients have been treated since the introduction of thi s technique in 1991.