Depends. It depends on what cancer or ailment. Where and how much radiation is given dictates how risky it is. Radiation oncologists have the knowledge of consenting the patient and giving all the risks and complications so the patient makes an informed decision. Everyday many patients are taking radiation treatments and the risks are considered low and reasonable.
Yes and no. Radiation therapy is a long established treatment for many cancers. The use of radiation for cancer treatment now goes back over one hundred years. The treatments that we give today are more focused and accurate today than even ten years ago. There are always risks to any cancer therapy, however, modern radiation therapy delivered by a board certified radiation oncologist is in most cases safe.