Lymph nodes in the armpit are the first location breast cancer would normally invade. Even though from staging standpoint, this is not technically considered metastatic or stage IV or M1 their involvement is an indication for radiation therapy. Technically speaking metastatic breast cancer suggests invasion of other organs by breast cancer. Organs commonly invaded by breast cancer include bone, lung, liver and brain but, also rare, breast cancer can metastasize to almost any other organ.
Due to Blood Brain Barrier; most of chemotherapy agents are filtered out of brain, therefore treatment of choice for brain metastasis is radiation. Radiation for the metastatic breast cancer to bone is indicated if the lesion is either causing pain, or has caused fracture or is creating risk of a fracture. Otherwise we would not recommend radiation to every metastasis in the bone. Most of metastatic breast cancers to liver and lung respond very well to chemotherapy but after a while some of these lesions may become resistant to chemotherapy agents. In that case, radiation therapy would be indicated.