Intensity modulated radiation therapy (IMRT) is a radiation technique that takes advantage of recent advances in imaging and computer technology. Doctors at UCSF have used IMRT to treat cancer patients since 1997, giving us more experience with IMRT than most other places in the country. IMRT planning starts with target and normal tissue definition, then dose prescription, and then inverse planning. Using computer optimization, multiple small fields are designed to give a complex and conformal radiation dose distribution. Because of improved target volume coverage and reduced dose to critical structures, IMRT has been widely accepted as an excellent treatment modality for multiple tumor types. Clinical results suggest that use of IMRT improves cancer control rates and decreases toxicity rates in certain tumor types, such as head and neck cancer, prostate cancer, and some brain tumors. At UCSF, we have 6 treatment machines capable of delivering IMRT plans. We routinely use IMRT for treatment of prostate and head and neck cancer, and we frequently use IMRT to treat other tumors in challenging locations.