By Krystal Harris, DVM


As our pets age, they commonly develop lumps and bumps both on their skin and just under their skin. Some of these growths may be harmless, but how do you know which ones are safe and which ones may not be? Dogs and cats can develop many different types of skin cancer and many of these can have the same appearance as a non-cancerous mass to the naked eye (see Figures 1-3 below).

Common benign (non-cancerous) lumps and bumps include: lipomas (benign fatty masses), histiocytoma (usually affects young dogs), and sebaceous adenomas (wart-like masses). Some pets can develop multiple benign masses all over their body or may have both benign and malignant masses concurrently, so it is important to have all new masses checked by a veterinarian.

Common malignant masses include: Mast cell tumors (Figure 2), Soft tissue sarcomas (such as peripheral nerve sheath tumors or fibrosarcomas, Figure 3), hemangiosarcoma, melanoma, and squamous cell carcinoma. Unfortunately the appearance of malignant masses can mimic benign conditions such as a lipoma, a hot spot, a bug bite, or other forms of dermatitis. The only way to differentiate a benign bump from a malignant one is to have your veterinarian take a sample of the mass.

Photos: Withrow & MacEwen’s Small Animal Oncology 5th ed pages 614, 337, 366

lumps and bumps in dogs

Figure 1: Early cutaneous Lymphoma, beginning as an area of scaly skin and hair loss.
Figure 2: Mast cell tumor, the reddened appearance is due to histamine release by the tumor, similar to the body’s response to an insect bite.
Figure 3: Soft tissue sarcoma on the forelimb of a dog marked with ink in preparation for surgical removal.


Malignant masses may appear quickly and double in size rapidly but some remain small for weeks-to-months before entering a period of rapid growth. It is always best to have any new masses checked by your veterinarian while they are small and amenable to surgery if needed. Additionally, the longer that some masses remain on the body the more time they have to spread to other sites in the body.


Many skin masses can be diagnosed as benign or malignant based on a minimally-invasive test called a fine needle aspirate (FNA). This test can be done without having to sedate your animal and causes them very minimal to no discomfort. A small gauge needle (like the one used when your pet gets a vaccine) is inserted in to the mass so that individual cells from the mass are forced into the lumen of the needle, these cells are then expelled from the needle onto glass slides where they can be examined under a microscope. Your veterinarian may recommend sending the slides to a clinical pathologist for interpretation; this is referred to as “cytology.” Clinical pathologists are veterinarians who complete additional years of training specifically to examine specimens at the cellular level to aid your veterinarian in making an accurate diagnosis. This technique is fast and accurate in many cases with very minimal risk.

• Performing an FNA will NOT make the mass more aggressive or increase the risk of it spreading to other parts of the body

When an FNA is non-diagnostic, a biopsy may be necessary. A biopsy removes a piece of the mass (or the entire mass) in order to obtain a diagnosis, termed histology or histopathology. By providing the pathologist a piece of the mass instead of just cells (cytology), they can provide a more informative diagnosis. This technique does require sedation or general anesthesia, and can take several days (3-10 days) to get a report. This delay is due to the time needed for tissue fixation and processing before the pathologist can examine the sample.

In the majority of cases a fine needle aspirate is the first step.
It is important to know if a mass is malignant prior to surgical removal to give your pet the best possible outcome:

  1. Benign masses typically do NOT need to be removed, unless they are causing your pet discomfort or are limiting your pet’s mobility. If your pets mass is benign, they could avoid surgery all together.
  2. Malignant masses require different surgical planning than benign masses. Having this knowledge prior to surgery allows your veterinarian to formulate a surgical plan that allows the best chance of removing all the cancer cells in one surgery.
  3. Malignant masses may have already spread cells to local lymph nodes or distant organs. If local lymph nodes are involved they should be removed during surgery. If the tumor has spread beyond the local lymph nodes your pet may not benefit from surgery, and alternative treatment options should be discussed with a veterinary oncologist.
Treatment/ Histopathology

The best treatment for malignant masses of the skin is surgery. After surgery, the mass will be submitted for histopathology to provide the following information:

  1. Confirm the diagnosis
  2. Examine the tissue margins to see if the cancer cells were completely removed
  3. Provide information about how aggressive the cells appear microscopically

Histopathology is necessary in order for your veterinarian to explain how this mass may affect your pet’s life in the future. Many owners, when given the choice, decline histopathology, either to save money or avoid the potential bad news. If it is worth removing, then it is worth knowing what it is!

Owners often ask, “Why do I need a diagnosis? If it is cancer I am not going to treat” or they tell their vet “If it is cancer, I don’t want to know”
Even if you do not think you would pursue additional treatment following a cancer diagnosis it is important to gain as much information as you can about your pets disease. Knowing what type of cancer was removed from your pet helps give you the tools to best recognize signs that the disease may be progressing, or may help you rest better at night knowing your pets tumor was actually benign, or may have been cured with surgery. Even with a cancer diagnosis a veterinary oncologist can offer a range of therapy options that can fit your lifestyle and may improve your pet’s comfort.

Learn more about VETMED’s Oncology Services >

About  Krystal Harris, DVM

Dr. Krystal Harris completed her undergraduate work at Texas A&M University then earned her Doctorate of Veterinary Medicine from St. George’s University School of Veterinary Medicine in 2010. She completed an oncology internship and residency program at Oregon State University. Dr. Harris is inspired by the strong patient relationships she cultivates. As a veterinary oncologist, she helps her patients through the emotionally difficult decisions associated with cancer. By outlining clear treatment options, she is able to give pets the care they need.