Intensity-modulated radiation therapy (IMRT) is a type of external beam radiation therapy that delivers beams of radiation customized to the shape and size of the tumor. Unlike 3D-CRT, which delivers the same amount of radiation to both the tumor and the surrounding tissue, the intensity of the beams can be adjusted (modulated) for IMRT, enabling the radiation oncologist to deliver different amounts of radiation to different areas of the tumor and the surrounding tissue. This allows the radiation oncologist to deliver the maximum amount of radiation to the tumor while sparing the surrounding healthy tissue.