Modification of a standard cobalt-60 unit for total body irradiation at 150 cm SSD

A cobalt-60 teletherapy unit has been modified to permit total body irradiation (TBI) with a vertical beam in a conventional treatment room. This technique has been implemented at low cost using a few easily made accessories. Removal of the adjustable collimator assembly provides a field 2.3 meters in diameter at 150 cm SSD. A copper flattening filter has been constructed to improve beam uniformity and remove electron contamination. Machine set up time for TBI requires less than 15 minutes and does not affect the routine clinical use of the unit. A dose rate of 32 cGy per minute (midplane) is attainable in a 20 cm thick patient. The dosimetry and technical aspects are presented in this paper.