Matchline dosimetry of a three field technique for breast treatment using cobalt or 6 MV X rays

The matchline dose distribution between the tangential and supraclavicular fields used for treatment of breast cancer was investigated using 60Co and 6 MV X rays. Techniques which allow minimum, moderate, and maximum overlap between the fields as well as a geometric alignment technique were studied. For a given technique, comparable matchline widths and doses were obtained with either machine. Average matchline doses were slightly greater with 6 MV X rays, however, more tissue was irradiated to higher dose levels with 60Co. Among the techniques which do not geometrically align the three fields, doses as high as 150% or as low as 70% were observed depending on the amount of overlap between the fields. Matching the tangents 3.5 cm medial and lateral to the supraclavicular field center yielded average maximum and minimum matchline doses within ten percent of those with geometric alignment techniques. However, these distributions will vary with patient size and matchline length. Geometric alignment techniques offer the advantages of matchline dose uniformity and reproducibility over the patient population with maximum matchline doses of only 110-115%.