A radiation oncologist is a specialist physician who uses ionizing radiation (such as megavoltage X-rays or radionuclides) in the treatment of cancer. Radiation oncology is one of the three primary specialties, the other two being surgical and medical oncology, involved in the treatment of cancer. Radiation can be given as a curative modality, either alone or in combination with surgery and/or chemotherapy. It may also be used palliatively, to relieve symptoms in patients with incurable cancers. A radiation oncologist may also use radiation to treat some benign diseases, including benign tumors. In some countries (not the United States), radiotherapy and chemotherapy are controlled by a single oncologist who is a 'clinical oncologist'. Radiation oncologists work closely with other physicians such as surgical oncologists, interventional radiologists, internal medicine subspecialists, and medical oncologists, as well as medical physicists and technicians as part of the multi-disciplinary cancer team.. Radiation oncologists undergo four years of oncology specific training whereas oncologists who deliver chemotherapy have two years of additional training in cancer care during fellowship after internal medicine residency in the United States.In the United States, radiation oncologists undergo four years of residency (in addition to an internship), which is more dedicated towards oncology training than any other medical specialty. Currently, radiation oncology is among the most competitive specialties in terms of residency match rates. During the four year post-graduate training, residents learn about clinical oncology, the physics and biology of ionizing radiation, and the treatment of cancer patients with radiation. After completion of this training, a radiation oncologist may undergo certification by the American Board of Radiology (ABR). Board certification includes two written tests and a practical oral exam. Successfully passing these tests leads to the granting of a time limited board certification. Recertification is obtained via a series of continuing medical education and practice qualifications including a written exam, clinical practice parameter evaluation, continuing medical education credits, and meeting community practice standards.