Use of axiliary collimating devices in the treatment for breast cancer with 60Co teletherapy units. II. Dose to the skin.

Special auxiliary collimating devices are often used in radiation therapy for breast cancer in order to treat the chest wall with tangential 60Co fields. Some of these devices are manufactured with a permanently attached Lucite end plate which increases the skin dose due to electron contamination. The relative surface dose was measured on a 60Co irradiator utilizing the collimating device with and without the Lucite plate. A parallel plate extrapolation chamber was used. The surface dose was measured as a function of the distance from the end of the device, the position in the field, and the angle of incidence for typical chest wall fields. These results are used to compare the dose to the skin of the chest wall for several treatment regimes. The data indicate that if the Lucite end plate is permanently attached, the skin dose may be enhanced to the extent that the use of additional bolus may be neither necessary nor desirable.