If you feel a lump in your breast, you know enough to take it seriously. But did you know that a lump in your armpit should be taken just as seriously? Read the following FAQS for informatiton about the types and causes of armpit lumps, and guidance on whether/when to see a doctor if you feel one.

Q. I recently noticed a small lump under my left arm, in my armpit. Could this be a sign of breast cancer?

A. It could indeed be a sign of breast cancer, for two reasons. First, breast tissue sometimes extends up into the armpit region. A lump felt in your underarm could in reality be a lump in your breast; you just never realized your breast extended that far.

Second, the lymph nodes in your armpit filter out any abnormalities (infections, reactions to drugs, cancer) in the same-side chest wall, arm, or breast. So a lump under your arm may indicate that your lymph nodes have identified, and are trying to fight, cancer cells that have reached them from your breast.

Q. Well, now I’m really scared. Should I see the doctor right away?

A. You should see the doctor if the lump doesn’t disappear on its own within a couple of weeks; or if it appears to be getting worse. That said, don’t panic! There are many, many causes of underarm lumps, and the vast majority have nothing to do with cancer.

The most prevalent cause of a lump in the armpit is infection. An infection can be localized to the armpit itself (perhaps a nick from shaving); or it can be more widespread. It’s possible to get a localized infection from using an antiperspirant rather than a deodorant; antiperspirants prevent your sweat glands from releasing germs that can cause infections. Or you may have a boil or abscess just under the skin.

Viral infections, like shingles, chickenpox, infectious mononucleosis, and HIV often cause underarm lumps. You may also get a lump under your arm as a reaction to a vaccination, such as smallpox, typhoid or, rarely, measles/mumps/rubella. Sometimes, an allergic reaction to penicillin or iodine produces an underarm lump.

Q. So, can I tell if the lump is from an infection rather than cancer?

A. Not absolutely. But in general, a lump that’s moveable and painful signifies an infection. And, while a painless lump that’s hard and fixed in place is more likely to be cancer than one that’s painful, softer, and moveable, such a lump doesn’t automatically signify cancer. In fact, an unexplained lump (i.e., one that’s not quickly identified as being from illness or injury) has about a 4% chance of being malignant, if you’re over the age of 40; and about a .4% chance of malignancy if you’re under 40. 

In addition, a lump that seems to increase in size within a few days is probably due to infection, rather than malignancy. Redness/soreness of the skin is also often a sign of a lymph node infection, not cancer.

Q. I think I can rule out infection. And I haven’t had any vaccinations. What now?

A. If the lump persists for a couple of weeks, see a doctor. Be prepared: the doctor will likely ask you the following: