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. The goal of using 3D CRT in patients with prostate cancer is to allow higher doses of radiation to be delivered to the prostate 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 development of conformal radiation therapy, combines two advanced concepts to deliver 3D CRT: (1) inverse treatment planning with optimization by computer; and (2) computer-controlled intensity modulation of the radiation beam during treatment.