Tissue compensation with lead for 60Co therapy.

Abstract Compensation for the effect of missing tissue, using lead filters, was investigated experimentally on a 60Co unit. Curves of lead filter thickness required to compensate for various tissue deficits at different depths were obtained, and from these estimates were made of the errors in tumour dose when compensating at a depth other than the tumour depth. As a means of minimising these errors, two depths (5 and 10 cm) were deduced as being representative of shallow and deep tumours, and the variation of filter thickness with field size investigated at these depths. The errors produced by changes in both SSD and in the use of equivalent squares for rectangular fields were also estimated and found to be negligible within the ranges examined. A semi-empirical method, using central axis depth dose data, was also developed and the resulting compensation curves compared with those obtained experimentally. The effectiveness of compensation was then checked experimentally for a typical surface shape.