Use of electron filters to improve the buildup characteristics of large fields from cobalt-60 beams.

It has been shown that for Cobalt-60 units capable of producing large fields (up to 40 X 40 cm2)at 80 cm, the dose distribution in the buildup region is quite different from that observed with small fields. First, the maximum dose occurs at 1-2 mm instead of 5 mm; and second, the maximum does in some extreme cases is 15%-20% higher than the dose at 5 mm. Thus, the dose is decreased by more than 15%-20% by 3 mm of tissue, suggesting severe electron contamination. In this report, it is shown that this contamination can be best removed by a filter of thickness 0.4 g/cm2 of medium atomic number, placed some 25 cm from the source inside the collimator. For existing units, positioning the filter inside the collimator is not practical. Acceptable results, however, can be obtained by placing the filter just below the collimator.