Is axiliary breast tissue a symptom for breast cancer?

No: Normal breast has an extension into the axilla, axillary tail. While cancer can develop in the axillary tail, as it can elsewhere, the axillary tail is not a symptom of cancer.

Not usually: Some women have breast tissue under their arms that enlarges when they become pregnant, or occasionally during the menstrual cycle. Cancer is unusual, but not unheard of. If a lump persists, have it checked.