Total body irradiation using a temporarily modified Theratron 780

For Total Body Irradiation (TBI) purposes the secondary collimating system of a standard Cobalt-60 unit (an AECL Theratron 780) is removed and a flattening filter added to provide a 1800 mm diameter field at a source-midline distance of 1360 mm. A test dose is given to estimate the patient's dose distribution. Mid-line doses are estimated from thermoluminescent dosemeter (TLD) measurements of exit and entrance doses. These are compared with doses independently determined from tissue-air-ratio (TAR) values using a specifically designed spreadsheet. Generally boost doses to the head and lower leg are required to ensure a midline dose distribution within ± 4% of the prescribed dose. Comparisons between the two methods (Test dose and TAR) using an anthropomorphic phantom and subsequent in vivo results have demonstrated that neither method is superior to the other. Using both gives greater confidence in the planning procedure