Radiation therapy is a cancer treatment in which high-energy radiation is directed at cancer cells via an external or internal source. It works by causing damage to cells’ DNA so that those cells die or cease dividing. While this treatment can cause a tumor to shrink and/or even disappear, it can also cause damage to healthy surrounding cells. Radiation oncologists are very careful to ensure that the radiation is precisely targeted to abnormal cells and to limit the exposure of others. This is accomplished through detailed imaging of the tumor and its perimeter via CT, PET, MRI and/or ultrasound scans. Patients may also be partially covered with protective material over nearby body parts to shield them from radiation exposure. Different types of radiation therapy include external-beam radiation therapy, intensity-modulated radiation therapy, 3D-CRT (three-dimensional conformal radiation therapy), stereotactic radiosurgery, stereotactic body radiation therapy and proton therapy. Your doctor will discuss which treatment options may be right for you.