Dosimetric considerations of very large cobalt-60 fields

Because of the growing trend toward the use of larger radiation fields, a special cobalt-60 unit has been constructed at the Princess Margaret Hospital which provides field sizes up to 160 x 50 cm at 90 cm from the source. Conventional percentage depth doses and tissue-air ratios have been published up to 35 x 35 cm fields. This report considers measurements that were performed to extend these data up to an equivalent field of 75 x 75 cm. Various problems associated with very large field dosimetry are considered. Effects of radiation backscattered from the floor are not negligible. The inverse-square law shows much larger deviations than at more conventional field sizes. Build-up characteristics were also found to be much worse than for smaller fields; however, appropriate electron filters have improved this. Usually, tissue-air ratio and percentage depth dose measurements are obtained under infinite phantom conditions. However, patients are not infinite phantoms. The difference between infinite and non-infinite phantoms has been found to be significant