Rad Oncs do not get sued often - in fact we are in the lower tiers among physicians who are frequently sued. There are several reasons for this:
1. Many of our patients are palliative or have a poor prognosis
2. Sometimes it can be challenging to prove recurrent tumor vs. radiation necrosis
3. We often spend more time with patients; it is well known that open communication with patients mitigates litigation
4. We have robust long- and short-term data to accurately prognosticate risk to organs at risk which are irradiated
5. Since what we treat is most often malignant, many patients feel that bad side effects are 'worth it' when you consider the alternative to not being treated aggressively
However, one thing not in our favor is the multi-disciplinary nature of Rad Onc. If one the members of your treatment team, MA, RN, Dosimetrist, Therapists, Physicist, or PA/NP screws up and it is not caught in time, then you are on the hook. This is because the Rad Onc is the 'Captain of the Ship.' Also if there is a failure of the hardware (a la the SRS disaster published in the NYT several years ago) then patients will come after you AND the manufacturer.
With regard to the cost of malpractice insurance, that is extremely variable based on the terms, location of your practice, and whether you are purchasing malpractice individually, as a group, or as part of a large institution.