XRT of large intact breasts using mixed energy beams and/or a beam spoiler

Breasts with separations over 24 cm are often treated with external beam with energies of 10 MV and higher, in order to reduce the magnitude of the hot spots. The disadvantage of the higher energy beams is the reduced dose to superficial breast tissue in the buildup region. The authors evaluated 2 methods for avoiding this underdosage: 1) a beam spoiler with 15 MV X-rays and 2) combining 6 MV and 15 MV X-rays. For the beam spoiler, a 1.7 cm thick slab of PMMA positioned at 35 cm from isocenter, the authors measured the dose distribution in the build-up region using film in a polystyrene phantom and an ion chamber in a water tank. For the mixed beam, the authors calculated the dose distributions in the buildup region for different proportions of 6 MV and 15 MV photons. Both methods are practical for radiation therapy of large intact breasts