Three-dimensional conformal radiation therapy (3D CRT) uses advanced computer technology to tailor the radiotherapy beam to the exact size and shape of a tumor, while minimizing incidental irradiation of surrounding normal tissues. Using 3D CRT in patients with breast cancer allows the delivery of higher doses of radiation to the breast than would be possible with conventional external beam radiation therapy, with the objective of improving local control and, ultimately, survival. Intensity-modulated radiation therapy (IMRT), the latest conformal radiation therapy development, combines two advanced concepts to deliver 3D CRT to tumors at the higher dosages with enhanced precision: (1) inverse treatment planning with optimization by computer; and (2) computer-controlled intensity-modulation of the radiation beam during treatment.