The lymphatic system carries waste from cells in a fluid called lymph. Lymph nodes throughout the body filter waste. Breast cancer often spreads to the lymph nodes first. The lymph nodes usually involved are in the axillary or under arm lymph nodes, these nodes are used as an indicator for further treatments. Other areas can be the internal mammary nodes or the supraclavicular lymph nodes near the collar bone. When the cancer cells have spread the process is called metastasis.