When cancer patients are treated with radiation, it's possible that the therapy itself may cause new tumors to form in the body later. Radiation kills cancer cells by damaging their DNA, but if the treatments cause genetic damage to normal cells near the radiation target, there's a small risk that these secondary malignancies may arise over time.Just over 10 years ago, Canadian researchers set out to assess the risk of secondary malignancy among men with prostate cancer who were treated with a type of radiation called brachytherapy. Unlike radiation delivered from sources outside the body, brachytherapy is accomplished by implanting dozens of radioactive pellets, or "seeds," directly into the tumor site. Those seeds, which are never removed, emit radiation at a dose that declines toward zero over the course of a year.Brachytherapy has the advantage of convenience. Instead of traveling for repeat sessions of radiation, men need only one treatment, usually given in an outpatient setting. But brachytherapy is also falling out of favor, in part because newer types of external beam radiation deliver high-precision doses with fewer side effects.