Lumps in and under the skin are a very common problem veterinarians are asked to assess. I’m not only saying this as a cancer specialist—skin lumps are common in primary care practice as well. According to a 2025 Clinician’s Brief Survey, primary care veterinarians evaluate an average of 2.7 masses each day in the clinic. What happens next—once a veterinarian examines a patient with a mass—is the subject of this blogpost.

Wait and Watch

One option for assessing a lump on your pet is to “wait and watch.” This is my least favorite approach because, in most cases, the mass will get bigger. No one—not even an expert oncologist—can reliably determine what a mass is based solely on how it looks or feels (called “palpation”).

Since an estimated 15% of skin lumps are malignant, there is a real danger of missing a malignant mass using the wait and watch approach. The only situation in which I favor the wait and watch approach is when the mass is simply too small to test. Masses smaller than about 1 mm are difficult to sample. For reference, 1 mm is roughly the thickness of a dime.

Fine Needle Aspiration

Fine needle aspiration (FNA) is a sampling procedure that takes cells from a mass, rather than a piece of tissue. The needle used is about the same size as one used for a vaccine. Instead of an injection, the needle is inserted into the mass and a gentle suction using a syringe removes cells from the mass.

The mass cells are then placed on a microscope slide and sent to the laboratory, where a clinical pathologist (like those in AMC’s in-house pathology lab) examines them to determine if the lump is benign or malignant. Fine needle aspiration is a helpful first step in planning further therapy, but it does not provide as much information as a biopsy.

A fine need aspiration is sometimes erroneously called a “needle biopsy,” but that is not technically correct. A true needle biopsy uses a special biopsy tool to remove a small core of tissue—about one inch long and the width of an angel hair pasta.

Biopsy

A biopsy involves taking a little piece of the lump and sending it to a laboratory for special processing and interpretation using a microscope. There are several different methods to obtain a biopsy, which I’ve described in a prior blogpost.

Once the biopsy is obtained, a veterinary anatomic pathologist interprets the sample and shares a report with the veterinarian. Unlike FNA, a biopsy provides not only a diagnosis, but often important information about prognosis and treatment options for the patient.

More Information About Lumps

If you are looking for more information about skin lumps and bumps in your pet, consider watching “Finding a Lump or Bump on Your Pet” presented by AMC’s Usdan Institute for Animal Health Education.