My wife had breast cancer approx. ten years ago and had radiation and chemotherapy. After her treatments were ending her oncologist discovered she had a very high heart rate and she was subsequently diagnosed with atrial fibrillation . Cardioversion has not been successful in restoring normal rhythm for more than a few days. Atenolol and Tykosyn were somewhat successful until recently. She had cancer in her other breast last year and underwent radiation treatments. The Tykosyn has now ceased being effective at all. What is the likelihood of the radiation having affected her heart and precipitated her atrial fibrillation?