When the pathology report says "cancer free," it really means, "We didn't see any cancer."  It doesn't mean that there might not still be some cancer cells hanging around.  I was a Stage IIIB breast cancer patient, and my doctors included radiation in the treatment plan from Day One.  When surgery showed that I had 16 positive lymph nodes, they added in some extra radiation to the lymph nodes above the collarbone and behind the sternum.  My doctor shared some studies with me that showed significantly better survival rates for patients who added radiation to their chemo.  My philosophy in treating an aggressive cancer is hit it hard from the beginning.  

My treatment was in 1998, so it obviously worked for me, but there may be some new research, and it is true that radiation will increase the risk for lymphedema.  So if a second opinion would make you feel more confident in going ahead with radiation, you should ask for a consult with a doctor at a comprehensive cancer center.  However, for a patient at age 30, I think it makes sense to zap any stray cells that may have lived through the chemo with radiation.  The words, "complete pathological response" are a very good predictor for a long survival time, so that's good news.