What would determine the target of radiation therapy after a mastectomy depends on the pathological findings at the time of mastectomy if patient has not received any chemotherapy prior to her mastectomy and the clinical findings prior to the mastectomy if patient has received chemotherapy prior to the mastectomy. The clinical findings prior to the mastectomy as well as the pathological findings in the surgical specimen would suggest what would be the areas at highest risk of a recurrence. This would be an educated guess based on natural history of the disease and years of research and therefore data and statistics.
Generally speaking the most common sites of recurrence after a mastectomy are mastectomy scar, followed by supraclavicular nodes, followed by the axillary nodes. So the minimum area covered by radiation would be the chest wall including the mastectomy scar. Whether the regional lymph nodes including supraclavicular nodes and axillary nodes need to be irradiated or not depends on individual patient and subject to review of each patient’s clinical presentation and review of pathology and details of surgical procedure including whether the patient had undergone sentinel lymph node biopsy or a full axillary dissection and many other factors including biological markers defining level of aggression of the disease, etc. One size does not fit all and multidisciplinary conferences are where medical teams discuss the best approach for each patient and offer individualized care.