Physical treatment planning of total-body irradiation: patient translation and beam-zone method

In the treatment of acute leukemias, high doses of chemotherapy and total-body irradiation (TBI) are used prior to bone marrow transplantation. However, the doses needed equal or exceed the tolerance of vital organs at risk. Thus precision and reliability of treatment are required. A new concept of TBI is introduced, combining a patient translation whole-body irradiation technique with the beam-zone method for individual treatment planning. The patient rests first in the prone, then in the supine position on a specially constructed flat couch and is moved horizontally with a preset constant velocity, completely through a vertical 60Co beam, twice for PA/AP TBI. This technique enlarges the useful beam size, improves the photon fluence uniformity, and reduces the depth-dose inhomogeneity. In translation irradiation, the product of velocity and integrated dose is the significant measurement. This velocity-dose product can be described sufficiently as a function of the local thickness and the local effective field size. Using the beam-zone method, calculation and checking of the dose at reference points in homogeneous, inhomogeneous, or shielded regions is simplified and reduces to taking dose measurements at the center of equivalent square phantoms. Since April 1983, about 20 leukemia patients received TBI by this individually planned translation-irradiation technique. A dose homogeneity within +/- 7% is achieved in the entire body.