Applicator for optimum cobalt-60 primary breast treatments.

Overview

A breast applicator has been designed to optimize 3 field breast treatments for 60Co. The device has a six half value layer beam splitting block constructed in two sections. The larger permanently mounted section is sufficient for treating 90% of the patients. Slots are available for mounting cerroband blocks, and any of five brass half field wedges. A magnetically attached front and back-pointer assembly readily breaks away in the event of a collision between pointer and patient. With this design the breast applicator with wedges and blocks has in-field surface doses reduced to that of an open field without accessory devices. The 50-90% dose decrement of the radiation penumbra for the half field block is comparable to that for the field edge of a typical 6 MV X ray unit, although the 50-10% decrement is larger. The average out-of-field dose at the surface is 8% and is 5% at the depth of dose maximum. The combination for this applicator of sharp penumbra and low out-of-field dose leads to reduced lung and opposite breast doses. The latter was confirmed with TLD measurements on 10 patients, and yielded an average opposite breast dose of 230 cGy for a 4600 cGy prescription. Thus, half-field blocking devices do not preclude, as has been stated in the literature, acceptable opposite breast doses. In addition, proper design of these devices can significantly improve the radiation characteristics for primary breast treatments.