A modified three-field technique for breast treatment

Abstract

Radiation therapy of breast cancer commonly uses three high-energy photon fields. The supraclavicular field and the two tangential fields are often matched on the skin. An analysis of the dose distribution in the matchline region shows a dose of up to 200% of the prescribed dose over the width of 8 mm at 3 cm depth. This is a result of the beam divergence and the complex geometry. A modification of the treatment technique has reduced the dose to less than 140% of the prescribed dose over approximately 4 mm. The modified technique uses a shielding block which covers the caudad half of the supraclavicular field and makes the caudad edge vertical. The cephalad edges of the two tangential fields are defined by a shielding block, which hangs vertically from the treatment head regardless of gantry and collimator position. This block obstructs a small portion of the radiation field and defines a vertical field edge. These three vertical field edges are brought into coincidence by adjustment of couch turntable angle and longitudinal couch position. The nearly perfect geometric match in three dimensions achieved in this manner further improves the cosmetic results after radiation treatment of breast cancer.