A practical approach to uniform total body photon irradiation

Abstract

Photon total body irradiation (TBI) has been applied to treat several systemic malignancies. However, TBI studies have been limited by nonuniform dosimetry. A 16 MV technique was initiated to improve uniformity of dose in a practical manner. For high dose TBI, missing tissue compensators are designed from lateral tissue separations, intra-lung separations, average CT numbers of lung regions, tissue phantom ratios, and off axis ratios. A few days before treatment, CT scans are obtained and TBI is simulated in the treatment room. In the treatment room, back projections of the patient's lateral silhouette, arm outline, and CT levels are traced on a compensator tray. Lead sheets are scribed through a schematic of the tray, cut, and fixed to their appropriate positions on the tray. Doses are verified with thermoluminescent dosemeters and ion chambers. Most measurements at the temple, chest wall, mid-thighs, and mid-knees have been within 10% of prescribed doses. About 4 hours are required for compensator fabrication and dose verification. This approach has been found practical, substantially improving dose uniformity relative to prior 60Co techniques applied at this institution.