Breast cancer is one of the most common cancers, but remains one of the most technically challenging to treat with radiation therapy. The shape and size diversity of the breast or chest wall, its superficial location, and its direct proximity to organs at risk (OARs) make this location particularly challenging. Conventional 3D conformal radiation therapy techniques (3D-CRT) can fail to provide appropriate target dose conformity and homogeneity while sparing OARs, in particular, for locally advanced breast cancers involving the internal mammary chain (IMC).