Dog Lump or Mast Cell Tumour? The Right, Safe Next Step

A mast cell tumour (spelled “mast cell tumor” in US sources) is the most common skin cancer in dogs, and it can look and feel almost exactly like a harmless cyst, wart or fatty lump. That is why no owner, and no vet, can tell what a lump is just by looking at it or touching it: a quick needle sample, called a fine-needle aspirate, examined under a microscope is the usual first step. If you find a new lump, record it and book a vet check rather than waiting to see what it does.

This guide explains the common types of lumps and bumps on dogs, why mast cell tumours are called “the great pretenders”, how to record a lump so your vet has useful information, and a simple urgency guide for booking. It does not diagnose or suggest treatment; that is always your vet’s job.

At a glance

• Most lumps on dogs turn out to be benign, but you cannot know which ones without testing.
• Mast cell tumours are the most common skin tumour in dogs and can mimic harmless lumps.
• A fine-needle aspirate is a quick, usually simple test that often gives a first answer.
• A widely shared veterinary oncology rule of thumb is: if a lump is pea-sized (about 1 cm) or larger and has been there for a month, have it aspirated.
• Record each lump’s size, location, date found and a photo, and recheck monthly.

Common lumps and bumps on dogs

Dogs can develop many kinds of lumps, especially as they age. The table below describes common ones in general terms so you know what your vet may be considering. Appearance alone is not a reliable guide: several of these can look the same.

Type of lump	What it often is	What it can feel or look like	Why it still needs a check
Lipoma	Benign fatty lump, common in older dogs	Soft, rounded, moves under the skin	Some mast cell tumours and other tumours feel similar
Sebaceous or follicular cyst	Blocked gland or hair follicle	Small, round, may ooze or discharge	Can look like other skin growths
Wart-like growths (papillomas, sebaceous adenomas)	Often benign growths	Small, cauliflower-like or raised	Some tumours start small and wart-like
Histiocytoma	A usually benign growth, more common in young dogs	Small, raised, often hairless and pink	Looks very similar to a mast cell tumour
Abscess	Pocket of infection, often after a bite or wound	Warm, painful, may burst	Needs treatment and can make a dog unwell
Mast cell tumour	A skin cancer that ranges from low to high grade	Anything: raised, flat, soft, firm, red, itchy, or perfectly normal-looking	Early testing and removal are important
Engorged tick	A parasite, not a growth	Grey or brown, attached to the skin; legs may be visible at the base	Ticks can carry disease and some cause paralysis in some regions

If you think a lump might be a tick, check carefully and speak to your vet about safe removal. Year-round parasite control is covered in the guide to flea and tick prevention for dogs.

Mast cell tumours in dogs: what owners should know

Mast cells are part of the immune system and contain chemicals such as histamine, the same substance involved in allergic reactions. When mast cells grow out of control, they form a mast cell tumour.

How common are they?

VCA Animal Hospitals describes mast cell tumours as the most common skin tumour in dogs, making up roughly 7 to 21 percent of canine skin tumours. Most affected dogs develop a single tumour, and they are most often seen in middle-aged to older dogs, though younger dogs can be affected.

Breeds more often affected

Any dog can develop a mast cell tumour. VCA lists breeds including Boxers, Boston Terriers, Pugs, English Bulldogs, Golden Retrievers, Labrador Retrievers, Cocker Spaniels, Schnauzers, Staffordshire Terriers, Beagles, Rhodesian Ridgebacks, Weimaraners and Shar Peis as more commonly affected. If you share your life with one of these breeds, regular lump checks are especially worthwhile.

Why they are called “the great pretenders”

Mast cell tumours can look like almost anything: a small raised bump, a soft lump under the skin that feels like a fatty lump, a red or itchy patch, or something that looks like an insect bite. Some stay the same for months; others grow quickly. Because of this, veterinary sources consistently stress testing rather than guessing.

Lumps that change size

One feature owners sometimes notice is a lump that seems to swell and shrink, sometimes from day to day. VCA explains that handling, pressure, licking or chewing can make mast cells release histamine, causing the area to swell, redden, itch or bleed. A lump that changes size like this should be checked promptly. Avoid squeezing or repeatedly prodding any lump; let your vet handle it.

Signs that can go beyond the skin

Chemicals released by mast cell tumours can sometimes affect the stomach and gut. VCA lists vomiting, reduced appetite, lethargy and black, tarry stools as possible signs. These signs have many causes, but in a dog with a known or suspected lump they are a reason to contact your vet the same day.

How vets find out what a lump is

Fine-needle aspirate

A fine needle is placed into the lump to collect a sample of cells, which is spread on a slide and examined under a microscope, either in the clinic or by a pathologist. VCA notes that this is the usual first test for a suspected mast cell tumour. It often gives a quick indication of whether a lump is fatty tissue, a cyst, inflammation or tumour cells.

Biopsy and grading

If the aspirate shows a tumour, or does not give a clear answer, your vet may recommend a biopsy (a small piece of tissue) or removal of the whole lump for testing. For mast cell tumours, the tissue is graded, which tells your vet how aggressive the tumour is likely to be and helps them plan next steps. Your vet may also suggest checking nearby lymph nodes or doing further tests.

Why “watch and wait” can be risky

Veterinary oncologist Sue Ettinger’s “See Something, Do Something” campaign, widely shared in the veterinary press, puts the rule of thumb simply: if a lump is the size of a pea (about 1 cm) and has been present for one month, take your dog to the vet for an aspirate or biopsy. The idea is that small lumps found early are generally easier to deal with, whatever they turn out to be.

Found a lump? How to record it

Good notes help your vet see whether a lump is changing. A few minutes now saves guesswork later.

1. Note the date you first found it.
2. Record the location as precisely as you can, for example “left side of chest, two finger-widths behind the elbow”. A quick sketch of a dog outline with numbered lumps works well.
3. Measure it. Use a ruler or tape measure across the widest point, in millimetres or centimetres. Comparing it to a pea, a grape or a coin is less accurate but better than nothing.
4. Take a photo with a coin or ruler beside it for scale. Part the fur so the lump is clearly visible.
5. Describe the feel gently: soft or firm, moves under the skin or feels fixed, smooth or bumpy, warm or not, and whether your dog minds it being touched.
6. Note any changes in colour, hair loss, itching, licking, ulceration, bleeding or size, and the date you saw them.
7. Recheck monthly on the same day each month, and bring your notes and photos to every vet visit.

Monthly grooming is a perfect time for a nose-to-tail lump check. The guide to home grooming by coat type shows how to build handling into your routine, and older dogs benefit most from regular checks, as covered in caring for a senior dog.

How urgently should you book?

What you notice	Suggested timing
Lump that is bleeding heavily, very swollen and painful, or your dog seems unwell, weak, vomiting, or has black tarry stools	Contact your vet the same day
Facial swelling, hives, or breathing difficulty in a dog with a lump	Emergency: go now
Lump that is growing quickly, changing size, ulcerated, red or itchy	Book within a few days
New lump about pea-sized (1 cm) or bigger, or any lump present for a month	Book a vet appointment for assessment and an aspirate
Small new lump that seems unchanged	Record it and mention it at a vet visit soon; do not wait for the next annual check if it persists
Lump already tested and identified by your vet as benign	Keep measuring monthly and report any change

If you are unsure how quickly to act, the guide on when to call the vet explains how to describe symptoms over the phone so the clinic can triage properly.

Sudden lumps on a dog under the skin

A lump that appears overnight is often an insect bite or sting, an allergic reaction, an abscess or a bruise, but a mast cell tumour can also appear to come up quickly, especially if it swells after being bumped. Hives or a swollen face, especially with breathing difficulty, needs emergency care. A single sudden lump that does not go away within a few days, or that keeps changing size, should be checked. Itchy skin and hives can also be linked to allergies, which the guide to dog allergies covers.

Signs of cancer in dogs to know

A lump is only one possible sign. Your vet will want to know about any of the following, especially if they last more than a week or two:

• A lump or swelling that persists or grows
• Sores that do not heal
• Unexplained weight loss or loss of appetite
• Bleeding or discharge from any body opening
• Persistent lameness or stiffness
• Difficulty breathing, eating, swallowing, urinating or passing stools
• Low energy or reluctance to exercise
• An unusual smell, for example from the mouth

All of these have many possible causes, most of them not cancer. That is exactly why your vet’s examination matters.

Common mistakes

• Assuming a soft lump is “just fat”. Some mast cell tumours feel like fatty lumps; only testing can tell.
• Waiting for a lump to grow before acting. Smaller lumps are generally easier to deal with.
• Squeezing or popping lumps. This can cause infection and, with mast cell tumours, swelling and irritation.
• Not measuring. “It seems about the same” is hard to judge; a ruler and photo are much more reliable.
• Forgetting where lumps are. Dogs with several lumps need a simple map so new ones are not mixed up with old ones.
• Using home remedies or human creams on a lump. They will not tell you what it is and may delay proper care.
• Letting cost delay the first check. Ask your vet about the cost of an aspirate up front; it is often one of the simpler tests. Insurance basics are covered in is dog insurance worth it.

Monthly lump-check checklist

• Pick a set day each month and a calm time, with treats ready.
• Run your hands slowly from nose to tail, including legs, paws, belly, armpits, groin, tail and inside the ears.
• Gently check inside the lips and gums if your dog is comfortable with it.
• Measure and photograph any lump, and update your lump map.
• Book a vet visit for any new lump that persists, or any lump that changes.
• Reward your dog throughout so checks stay a pleasant routine.

Sources this guide draws on: VCA Animal Hospitals owner information on mast cell tumours in dogs and the “See Something, Do Something” lump-awareness campaign by veterinary oncologist Sue Ettinger as reported by dvm360.

Frequently asked questions

What does a mast cell tumour look like on a dog?

There is no single look. A mast cell tumour can be a raised lump on or just under the skin, a soft lump that feels like fat, or a red, itchy or ulcerated patch. Some look completely harmless, which is why a needle test is needed.

Can a vet tell if a lump is cancer just by feeling it?

No. Feeling a lump gives useful clues, but many lumps feel alike. A fine-needle aspirate or biopsy examined under a microscope is needed to know what a lump is.

Are most lumps on dogs cancerous?

Many lumps on dogs turn out to be benign, such as fatty lumps and cysts. But some harmless-looking lumps are tumours, so every new lump that persists deserves a vet check rather than a guess.

What is a sebaceous cyst on a dog?

It is a small, usually benign lump formed when a skin gland or hair follicle becomes blocked. It may ooze a thick or cheesy material. Because cysts can look like other growths, ask your vet to confirm what it is, and do not squeeze it.

Why does my dog’s lump keep getting bigger and smaller?

Lumps that swell and shrink can be caused by mast cell tumours releasing histamine when bumped or licked, as well as by inflammation or infection. Any lump that changes size should be checked by your vet promptly.

How quickly should I take my dog to the vet for a new lump?

Go the same day if your dog is unwell, the lump is bleeding heavily or there is facial swelling or breathing trouble. Otherwise, book within days for a lump that is growing or changing, and book an aspirate for any lump that is pea-sized or larger or has been there a month.

Finding a lump on your dog is worrying, but you now have a clear plan: record it, avoid squeezing it, and let your vet test it. Regular, kind check-ups at home mean you are far more likely to spot changes early, which is one of the most valuable things you can do for your dog’s long-term health.