A mass on a dog is any abnormal lump or growth of cells found on or under the skin. The good news: roughly 65% of skin masses in dogs turn out to be benign, meaning they won’t spread or threaten your dog’s health. The other 35% are malignant, which is why any new lump deserves a closer look from your vet.
Mass, Tumor, and Cyst: What’s the Difference?
“Mass” is a general term for any abnormal lump or growth. It doesn’t tell you whether something is harmless or dangerous. Your vet might also call it a “tumor,” which simply means an abnormal growth of cells. A tumor can be benign or malignant. The word itself doesn’t mean cancer.
A cyst is a specific type of mass: a sac filled with fluid or semi-solid material. Many cysts in dogs form from blocked hair follicles and contain thick, grayish or yellowish material. They’re almost always benign. Cysts can sometimes rupture, which causes local pain and inflammation, but they rarely pose a serious health risk.
Common Benign Masses
Most lumps you’ll find on your dog fall into a few familiar categories.
Lipomas are fatty tumors that sit just under the skin. They’re round, soft, and movable, almost like a small water balloon beneath the surface. Lipomas are most common in middle-aged and older dogs. They grow slowly and rarely cause problems unless they get large enough to interfere with movement, in which case surgical removal is an option.
Sebaceous adenomas are small, wart-like growths from the oil-producing glands in the skin. They’re usually under 1 centimeter, white to pink, and sometimes ooze a greasy white material. They look a little alarming but are harmless. Occasionally one gets bumped or scratched and becomes inflamed or infected.
Follicular cysts develop when a hair follicle or oil gland gets blocked. They’re typically small, round, and solitary, though some dogs develop several. If a follicular cyst ruptures, the body treats its contents as foreign material, triggering noticeable swelling and soreness around the area.
Common Malignant Masses
Mast cell tumors are the single most common malignant skin tumor in dogs, accounting for nearly 23% of all skin tumors in one large study. They’re tricky because they can look like almost anything: a small pink bump, a raised red lump, or even something that resembles an insect bite. They tend to appear on the hindlimbs, belly, and rib area. Some dogs develop more than one. Mast cell tumors range from low-grade (slow-growing, very treatable) to high-grade (aggressive, requiring more intensive treatment).
Soft tissue sarcomas are a group of cancers that develop in connective tissues like fat, muscle, and nerves. They usually show up on the limbs of older dogs as firm, poorly defined masses that feel attached to deeper tissue. They tend to invade locally rather than spread to distant organs, but they can be difficult to remove completely because of how they grow into surrounding structures.
Melanomas make up a smaller percentage of skin tumors (about 3.6%) but deserve attention, especially when found on the paws or in the mouth. Skin melanomas in haired areas are often benign, but those on the toes, nail beds, or lips are more likely to be malignant.
Breeds With Higher Risk
Certain breeds are genetically predisposed to developing skin masses, particularly mast cell tumors. Boxers carry the highest risk and have been flagged in studies across multiple countries. Other breeds with elevated risk include Shar-Peis, Labrador Retrievers, Golden Retrievers, Boston Terriers, Bulldogs (including French Bulldogs), Pugs, Staffordshire Bull Terriers, and Weimaraners.
Golden Retrievers face a broader cancer burden overall, with increased rates of melanomas, soft tissue sarcomas, and lymphomas on top of mast cell tumors. Labrador Retrievers tend to develop higher-grade mast cell tumors compared to Boxers, whose tumors are more often low-grade and carry a better prognosis.
How Vets Diagnose a Mass
You can’t tell whether a mass is benign or malignant just by looking at it or feeling it. Even experienced veterinarians need lab results to be sure.
The most common first step is a fine needle aspirate. Your vet inserts a small needle into the lump, draws out a sample of cells, and examines them under a microscope. It takes seconds, requires no sedation, and gives useful information in most cases. Studies show excellent correlation between fine needle aspirate results and more invasive biopsy results, making it a reliable and low-stress starting point.
If the aspirate isn’t conclusive, or if the mass looks suspicious, your vet may recommend a biopsy. This involves removing a piece of the mass (or the entire thing) and sending it to a pathologist for detailed analysis. A biopsy provides a definitive diagnosis, including the tumor’s grade, which predicts how aggressively it’s likely to behave.
For masses that turn out to be malignant, staging comes next. This means checking whether the cancer has spread beyond the original site. Staging typically involves imaging like X-rays or ultrasound of the chest and abdomen, and sometimes sampling nearby lymph nodes.
Treatment for Malignant Masses
Surgery is the primary treatment for most malignant skin masses in dogs. The goal is to remove the tumor along with a margin of healthy tissue around it to reduce the chance of recurrence. For mast cell tumors, veterinary surgeons historically aimed for 3-centimeter margins on all sides, though newer approaches may use narrower margins for low-grade tumors.
When a tumor can’t be fully removed, or when it’s high-grade, additional treatment options include radiation therapy and chemotherapy. Radiation can be delivered before or after surgery, and protocols range from several sessions per week over a few weeks to once-weekly palliative schedules designed to shrink the tumor and control pain. Chemotherapy is typically reserved for aggressive tumors or those that have already started to spread.
Tumor removal surgery generally costs between $250 and $1,800 or more, depending on the tumor’s size, location, and how invasive the procedure needs to be. Diagnostic workups, imaging, and pathology add to the total. If radiation or chemotherapy is needed, costs increase significantly.
What to Watch for at Home
Get in the habit of running your hands over your dog’s body regularly, especially as they age. Most skin masses are first discovered during petting or grooming. When you find a new lump, note its size, location, and how it feels. A soft, movable lump under the skin is more likely (though not guaranteed) to be something benign like a lipoma. A firm mass that feels attached to deeper tissue, or one that’s growing quickly, warrants prompt attention.
Signs that a mass needs a vet visit sooner rather than later include rapid growth over days to weeks, changes in color or texture, ulceration or bleeding from the surface, discharge or odor, and any lump that seems to bother your dog. Even lumps that seem harmless are worth mentioning at your dog’s next checkup so your vet can note them in the record and track any changes over time.