Told today mastectomy of my right breast

Just a quick query , when I seen the oncologist at daffodil cottage she asked me why I was having radiation after chemo if they have already done the mastectomy . I presumed it was because there was minute traces of cancer cells in the first lymph node ? I'm seeing the radiologist Dr Tiver on Tuesday and back tot he breast clinic and DrTrinh straight after so I guess I'll ask them both. I'm not saying I'm not going to do the radiation I'll do what ever the doctors want but It has been in the back of my mind ever since the oncologist mentioned it yesterday .