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Total body irradiation with a sweeping 60Cobalt beam
1Division of Medical Physics, British Columbia Cancer Agency, Vancouver, Canada.
Purpose:
This article describes the physical, technical, and dosimetric aspects of total body irradiation (TBI).
Methods And Materials:
The continuous head swivel motion of a standard 60Cobalt unit has been used to obtain a sweeping beam that encompasses the entire length of the patient in TBI. A perspex beam flattener designed to remove the inverse square fall-off in beam intensity along the sweep axis provides a 90% field length of 200 cm in air at a treatment source-to-skin distance of 160 cm. The anterior-posterior parallel pair setup permits accurate placement of customized lead compensators to limit the dose to lungs.
Results:
Measured beam profiles, dose buildup curves, and percentage depth dose for the technique are presented. With compensators in place, the variation in lung dose is shown to be within +/- 5% of the prescribed tumor dose.
Conclusions:
A sweeping beam TBI technique has been devised using a standard 60Cobalt unit. The technique allows the patient to be treated in supine and prone positions, which facilitates the accurate placement of lung compensators to limit the dose to lung tissue.