This is a method by which the spread of breast cancer can be determined.

A couple of lymph nodes in the armpit above the affected breast are removed (during the same procedure where cancerous lumps are removed from the breast - aka lumpectomy) and examined histologically for the presence of any cancer cells that may have spread outside the breat lump(s) and travelled up the lymphatic vessels.

Why bother? Detection of malignant cells in examined lymph nodes significantly reduces prognosis, and increases the likelihood that distant metastases exist elsewhere in the body.