Tumors usually have an irregular shape. Three-dimensional conformal radiation therapy, or 3D-CRT, uses sophisticated computers and computer-assisted tomography scans (CT or CAT scans) and/or magnetic resonance imaging scans (MR or MRI) to create detailed, three-dimensional representations of the tumor and surrounding organs. Your radiation oncologist can then shape the radiation beams exactly to the size of the shape of your tumor. The tools used to shape radiation beams are multi-leaf collimators or blocks. Because the radiation beams are very precisely directed, nearby normal tissue receives less radiation exposure.