To compare how often cancer recurs (comes back) after 3 weeks of radiation compared to 5 weeks of radiation in patients who receive radiation therapy delivered to the lymph nodes near the breast. The side effects that can develop during or after radiation treatment, including how often arm swelling (edema) happens, will also be studied

The primary objective of the trial is to assess whether there is no difference in disease-free survival for patients treated with a 1cm excision margin when compared to a 2cm margin for stage II primary melanomas (AJCC 8th edition; pT2b-pT4b).