Tumors are not regular — they come in different shapes and sizes. Three-dimensional conformal radiation therapy, or 3D-CRT, uses computers and special imaging techniques to show the size, shape and location of the tumor. Computer assisted tomography (CT or CAT scans), magnetic resonance imaging (MR or MRI scans) and/or positron emission tomography (PET scans) are used to create detailed, three-dimensional representations of the tumor and surrounding organs. Your radiation oncologist can then precisely tailor the radiation beams to the size and shape of your tumor with multileaf collimators or custom fabricated field shaping blocks. Because the radiation beams are very precisely directed, nearby normal tissue receives less radiation and is able to heal quickly.