Use of auxiliary collimating devices in the treatment for breast cancer with 60Co teletherapy units. I. Dosimetric considerations.

The dosimetry of tangential field treatment for breast cancer using an auxiliary collimating device on 60Co teletherapy machines is reviewed. The position of the device with respect to the source axis has negligible effect on either the dose rate or the dose distribution. Measurements with thermoluminescent dosimeters in an anatomic phantom verify the accuracy of the method of correction of isodose distributions for sloping skin. Possible systematic errors in dosimetry are identified.