Coop,
I am a MS4 going into radiology and doing a radiation oncology rotation right now. The two fields are actually pretty different.
Interventional radiology is, as you said, 5 years diagnostic radiology residency followed by 1-2 years of fellowship. The do a variety of procedures, with the majority being vascular cases, meaning they insert a long catheter into an artery and go from there. The vascular procedures include angiography, stenting of blood vessels (except in the heart), embolization of uterine fibroids, insertion of filters into the aorta to prevent pulmonary emboli, delivery of chemotherapy directly to tumors (liver, cervical, etc), and others. They also do kidney procedures with a needle and catheter, insert PICC lines, and probably others which I am forgetting. Very few interventional radiologists have a clinic, they are consulted by a clinician to do a procedure, do a very short history, and do the procedure. Most follow up is done by the clinician.
Rad onc is a completely different residency and is EXTREMELY competitive right now because there are less than 100 spots open in the country. Radiation oncologists use radiation therapy to treat mostly cancer. Many types of cancer can be treated, some for a cure and some for palliation. They also occasionally see people with graves eye disease, and some other benign conditions. Much of their work is clinical. They see patients in clinic, do a full history and physical exam if indicated, then recommend or don't recommend treatment. The patients then come in for several weeks to get treatment. The actual treatment is done by techs. It is the planning of the treatment that is done by the rad onc. Most rad oncs follow their patients in clinic at certain set intervals (every 3 months, 6 months, yearly, etc.) They work closely with cancer surgeons and medical oncologists to treat patients. They do very few procedures, but do need to know a lot of physics and treatment planning. There are lots of advances recently with CT planning of radiation therapy.
Similarities include knowledge of physics and use of technology, and the fact that they both treat disease. Otherwise, they are pretty different fields.