An oncologist is a doctor whose work is cancer. They look at tumours, try to find out what kind of cancer a person has, and help choose the treatment — medicine, strong-ray treatment, or work with a surgeon. In everyday talk people say “cancer doctor.” Oncologist is the hospital word, and it sounds more formal. You meet this doctor in the cancer part of a hospital, not in a normal clinic. Though actually, one oncologist does not do every kind of cancer work: some look after cancers in the blood, some look after a lump you can point to, some work mainly with children. The science they work in is oncology — that is the part of medical science that deals with cancer and tumours. The -ist at the end is what turns the field into the person. The doctor and the science are different words.