What do oncologists do when they get cancer? A medical oncologist treats cancer using chemotherapy or other medications, such as targeted therapy or immunotherapy. ... He or she also performs certain types of biopsies to help diagnose cancer. A radiation oncologist treats cancer using radiation therapy. What is the role of a medical oncologist ? The Role of a Medical Oncologist. ... Medical oncologists and radiation oncologists work side by side to treat patients diagnosed with cancer and help them maintain the best quality of life possible throughout the treatment process.