Design and characteristics of a facility for total-body and large-field irradiation.

A facility for total-body X ray irradiation has been built using two 4 MV linear accelerators, one supported from the ceiling and one placed in a floor pit. The maximum distance between the sources is 410 cm. The patient lies supine on a light, movable support with a stretched-canvas top, halfway between the sources where the field size is 80 X 220 cm2. A special flattening filter maintains the doserate variation in air within +/- 3% over the central 70 X 200 cm2 of the field, but some quality variations within the beam are noticeable. The doserate at 205 cm distance from the sources is variable between 0.05 and 0.8 Gy/min (half from each source). To permit treatment of large fields at higher doserates, the accelerators can be moved vertically to place the sources at 120 cm or 160 cm from the patient's midplane. For this purpose, independently movable collimators are provided and the flattening filter is designed to provide two options, one for the large total-body field and the other with less filtration covering a smaller solid angle. At 120 cm distance, each beam can provide a doserate of up to 1.1 Gy/min.