Breast tissue can occur in areas other than just the breast. This can be called “ectopic” breast tissue, “accessory” breast tissue, or “axillary” breast tissue.
Do you have an extra nipple? (Remember Chandler on friends with the third nipple?) Have you ever had your armpits swell when you get your period? Do your armpits ache during your period? Did your armpits swell or ache when you breastfed?
Ah. There is a thing called the mammary ridge in fetal development. It goes in a line from the armpit, through the breast, onto the abdomen, down to the groin. When a fetus is developing, this band forms at five weeks of gestation. In normal development, the breasts form in the area in front of the pectoral muscle, and the rest of the ridge regresses. But in some cases, this regression does not occur, so you will get an accessory breast or accessory nipple.
This breast tissue is real breast tissue. It can lactate, form cancers, have pain, and grow.
What do you need to do?
The answer is nothing needs to be done unless you have a pathologic finding: mass, pain, or skin changes associated with breast cancer. I have had women who did not know they had axillary breast tissue, or what they thought was a “mole” was really an accessory nipple. They discovered it was accessory breast tissue when it started to lactate when they breastfed.
Treatment?
This depends on how much it is and if it bothers you. If it is growing, painful, unsightly, or has any signs worrisome for cancer (you can’t mammogram the tissue there, though you could do ultrasound), you can cut it out. This usually leaves a scar in the armpit. The tissue removed is sent to pathology for analysis, like any breast tissue removed.