How can an armpit lump be a sign of breast cancer?

Armpit lumps can be a sign of breast cancer, although it is usually not the first sign. Breast cancer can sometimes cause changes in the tissue around the armpit, which can lead to the formation of a lump or swelling. As breast cancer progresses, additional symptoms may appear including: persistent enlargement or thickening of an area located near or around your armpit, a change in size or shape of one or both breasts, nipple discharge (other than breast milk), redness and warmth over one specific area on your breast (indicative of inflammation), inverted nipples, puckering/dimpling in your skin that’s different from when you don't have any pressure applied to it (indicative of ridging caused by enlarged lymph nodes), and an unexplained lump found through self-exam.

If you do find an armpit lump that persists for more than two weeks and cannot be explained away by other causes such as infection or cysts then seeing a doctor is recommended so they can help rule out any signs of complications associated with more serious conditions such as breast cancer. Your doctor will perform a physical examination as well as screenings for other cancers like lymphoma to rule out potential diagnoses before reaching their conclusion about whether it could be due to early stages of breast cancer.