Three dimensional conformal radiation therapy or 3D-CRT uses computers and special imaging techniques to define the size, shape and size of the tumor. Imaging with computer-assisted tomography (CT or CAT scan), magnetic resonance imaging (MRI) and positron emission tomography (PET) scans are used to define the extent of the disease and the critical structures we need to avoid. Our doctors then precisely configure the radiation beams to the size and shape of the tumor.