Multileaf collimation in the treatment of the intact breast

PURPOSE: To compare multileaf-collimator beam shaping and conventional metal-alloy blocking in irradiation of the intact breast after breast- conserving surgery. MATERIALS AND METHODS: Transmission dose was measured in a phantom, by using both types of blocking with 6- and 15-MV photon beams. Buildup data were collected, using a 6-MV photon beam, comparing open fields to various beam modifiers. RESULTS: Contralateral breast transmission was reduced with the multileaf collimator from 4% to 1%. With the jaws and multileaf collimator, the primary beam component of 0.5% was eliminated. Buildup data for the multileaf collimator most closely resembled the surface dose when a blocking tray is not used and were slightly lower than those for the conventional mounted blocks. This relates to the use of the lower of the two sets of wedges on the multileaf collimator, which is closer to the patient and thus enhances the surface dose relative to the dose with an open field and no wedge. CONCLUSION: Multileaf-collimator blocking for primary breast treatment is similar to conventional blocking, and the transition from one technique to another should be uneventful. The transmission dose to the contralateral breast is decreased with the multileaf collimator.