Skin cancer in dogs doesn’t have one single look. It can appear as a raised bump under the skin, a red ulcerated sore, a dark pigmented mass, or even a firm lump that blends in with surrounding tissue. Skin tumors account for roughly 20 to 30 percent of all tumors found in dogs, making them one of the most common cancers veterinarians diagnose. Knowing what to look for across the three most common types can help you catch a problem early.
Mast Cell Tumors: The Most Common Type
Mast cell tumors make up about 21 percent of all canine skin tumors, making them the single most frequent skin cancer in dogs. What makes them tricky is that they’re highly variable in appearance. Some look like raised bumps sitting within or just below the skin’s surface, easy to mistake for a harmless cyst or fatty lump. Others show up as red, ulcerated, bleeding, bruised, or swollen growths that are harder to ignore.
One hallmark of mast cell tumors is their unpredictable behavior. A lump might stay the same size for months or even years, then suddenly grow rapidly over days or weeks. Some tumors actually fluctuate, swelling up and then shrinking back down. This happens because mast cells release histamine, which causes the surrounding tissue to swell and become inflamed. If your dog keeps scratching, licking, or biting at a particular spot on their skin, it could be the irritation from a mast cell tumor driving that behavior.
Melanoma: Often in the Mouth or on the Toes
Melanoma in dogs looks different depending on where it grows. On haired skin, it typically appears as a raised, darkly pigmented mass without much redness or inflammation around it. These skin melanomas are sometimes benign. The more dangerous versions tend to show up in places you might not think to check: inside the mouth (on the gums, lips, tongue, or palate), on the toes, around the nail bed, or on the footpads.
About 62 percent of canine melanomas occur in the oral cavity, 27 percent on the skin, and the rest on digits and nail beds. Oral melanomas are particularly aggressive, with a high rate of spreading to other parts of the body. They can appear as darkly pigmented masses on the gums or palate, but some are “amelanotic,” meaning they have little or no dark coloring and may look pink or flesh-toned instead. Dogs with these non-pigmented melanomas tend to have worse outcomes, partly because the tumors are easier to miss.
On the toes, melanoma often presents as a swollen, painful digit or a sore near the claw that doesn’t heal. If your dog suddenly starts limping or you notice a toe that looks puffy and tender, it’s worth having it examined.
Squamous Cell Carcinoma: Crusty, Ulcerated Sores
Squamous cell carcinoma tends to look more like a wound than a lump. These tumors can range from pink, raised, irregular masses to thickened, ulcerated plaques that may crust over. They often develop in areas with less hair or lighter pigmentation, where sun exposure is greater, such as the belly, inner thighs, or nose. In the mouth, they can resemble rough, raw patches on the gums or tongue.
Because squamous cell carcinoma can look like a sore that simply won’t heal, it’s easy to dismiss as an injury or skin irritation. The key difference is that these lesions persist and gradually worsen rather than improving over a week or two the way a normal wound would.
Hemangiosarcoma: Blood Vessel Tumors on the Skin
Hemangiosarcoma is a cancer of blood vessel cells, and while it’s most often associated with internal organs like the spleen, it also develops in the skin. The surface version typically appears as one or more small, reddish-to-blackish nodules in the skin. These may bleed intermittently because of the blood vessel involvement. Sun-exposed areas with thin hair are common sites.
Deeper versions of this tumor grow in the tissue below the skin or within muscle. These feel firmer, may be either fixed in place or movable, and don’t always have that reddish or dark purple color on the surface. That makes them easier to confuse with other types of lumps. The deeper the tumor, the more aggressive it tends to be, with a greater chance of spreading.
Warning Signs That a Lump Needs Attention
Not every bump on your dog is cancer. Dogs develop lipomas (fatty lumps), cysts, warts, and skin tags throughout their lives, especially as they age. But certain changes should prompt a veterinary visit sooner rather than later:
- Rapid growth: A lump that noticeably increases in size over days or weeks, rather than staying stable for months.
- Changing appearance: Any shift in size, shape, color, or texture of an existing bump.
- Ulceration or bleeding: A mass that breaks open, oozes, or bleeds without an obvious injury.
- A sore that won’t heal: Any wound or lesion that persists beyond two to three weeks.
- Swollen toes or limping: Especially if a single digit looks puffy or a nail bed appears abnormal.
- Masses in the mouth: Dark spots, raised lumps, or raw patches on the gums, lips, tongue, or palate.
How Skin Cancer Is Diagnosed
A veterinarian can’t determine whether a lump is cancerous just by looking at it. The standard first step is a fine needle aspirate, where a small needle is inserted into the mass to collect cells for examination under a microscope. This is quick, usually doesn’t require sedation, and can often identify mast cell tumors on the spot.
For a definitive diagnosis and to understand how aggressive a tumor is, a biopsy is typically needed. This involves removing a tissue sample (or sometimes the entire mass) and sending it to a pathologist. It’s worth knowing that pre-treatment biopsies agree with the final diagnosis about 59 percent of the time when it comes to grading aggressiveness. Nearly 30 percent of the time, the initial biopsy underestimates how serious the tumor actually is. This is one reason veterinarians sometimes recommend removing the entire mass for testing rather than sampling a small piece.
How to Check Your Dog at Home
A monthly hands-on check is one of the simplest things you can do. Run your hands over your dog’s entire body during grooming or a belly rub, feeling for any new lumps, bumps, or areas of thickened skin. Pay special attention to areas you don’t normally see: the belly, inner legs, armpits, between the toes, inside the lips and gums, and around the nail beds. Lift the ears and check underneath.
If your dog has multiple existing lumps (common in older dogs), keep a simple chart noting their location and approximate size. This makes it much easier to spot new growths or track changes in existing ones over time. The more familiar you are with your dog’s body on a normal day, the faster you’ll notice when something isn’t right.