The lymphatic system is a similar system of vessels that carry waste from cells in a fluid called lymph. The waste is filtered through lymph nodes, which are distributed in groups throughout the body.

Lymph in the breast drains to the axillary, or underarm lymph nodes, internal mammary nodes, or to nodes near the collarbone called supraclavicular lymph nodes. If breast cancer spreads, it most often spreads to the lymph nodes first. The presence or absence of cancer cells in these nodes guide treatment recommendations.

When cancer cells spread through the blood and lymph and are found in other organs of the body, the process is called metastasis.