Cobalt-60 therapy unit for large field irradiation

A cobalt-60 therapy unit designed and constructed by The Ontario Cancer Institute is described. This unit is capable of treating large fields up to 50 × 160 cm2 at a conventional source to surface distance of 90 cm by using large collimator openings. Three selectable filters were incorporated. They all act as electron filters to improve the build-up characteristics of these beams. Two of the three filters also act as beam flatteners. The longitudinal collimators are individually driven so that it is possible to produce a vertical beam edge by closing one of the collimators completely. This condition is very useful for matching adjacent fields. The output of this unit, at its operating distance, ranges from 100–200 rad/min depending on which of the filters is chosen, which makes the treatment time shorter than most known facilities for very large field irradiation. This unit has been in routine use since 1977.