Mast cell tumours are among the most common skin cancers in dogs, and they are the single best argument for having every new lump checked rather than monitored.
The reason is simple: they can look like absolutely anything.
Why they are hard to spot
A mast cell tumour can present as:
- A small raised pink lump
- A soft lump under the skin that feels exactly like a fatty lipoma
- A wart-like growth
- A flat, reddened patch
- Something resembling an insect bite
- A hairless area
- An ulcerated sore
They can also change size from week to week, sometimes dramatically, because the cells release histamine and other inflammatory mediators. A lump that swells after being handled, then shrinks again, is a classic mast cell behaviour.
No vet can identify one by feel or by looking. Neither can you. That is not a criticism of anyone’s judgement — it is the nature of the tumour.
The test that gives the answer
Fine needle aspirate. A small needle draws cells from the lump and they are examined under a microscope.
- Usually done conscious, in a consultation
- Takes about a minute
- Relatively cheap
- Mast cells have a very distinctive granular appearance and are usually easy to identify
This is why “let’s keep an eye on it” without sampling is an incomplete answer for any new lump. The aspirate is the whole point.
Biopsy and grading follow if it is a mast cell tumour, because grade drives everything afterwards.
Grading and staging
Grade describes how the tumour looks under the microscope.
The two-tier system — low grade and high grade — is now widely used and predicts behaviour reasonably well. Low-grade tumours are usually cured by complete surgical removal; high-grade tumours are more likely to spread and have a poorer outlook.
An older three-tier system (grades 1, 2 and 3) is still sometimes used, and grade 2 in that system was famously unpredictable, which is part of why the two-tier system was developed.
Staging looks for spread — usually assessment of the local lymph node, abdominal ultrasound, and sometimes bone marrow or chest imaging depending on grade and location.
Location matters. Tumours on the muzzle, in the mouth, in the groin, on the prepuce, on the nail bed, and those that are ulcerated or fast-growing tend to behave more aggressively.
Certain breeds are over-represented: Boxers, Boston Terriers, Bulldogs, Pugs, Labradors, Golden Retrievers, Weimaraners, Shar Peis. Interestingly, mast cell tumours in Boxers and Pugs are more often low grade.
Treatment
Surgery is the mainstay. The key is margins — the surgeon takes a wide area of normal-looking tissue around and beneath the tumour, because mast cell tumours extend microscopically well beyond what is visible.
This is why a mast cell tumour removal leaves a much bigger scar than the lump suggested, and why it matters that the diagnosis is known before surgery rather than discovered afterwards. A lump removed with narrow margins as a “simple lump removal” frequently needs a second, larger operation.
Histopathology of the removed tissue tells you the grade and whether margins were complete.
Radiotherapy for incompletely excised tumours where further surgery is not possible.
Chemotherapy for high-grade tumours, those with spread, or those that cannot be completely removed. Dogs tolerate chemotherapy considerably better than people do — the protocols are designed for quality of life, and severe side effects are uncommon.
Targeted therapy. Tyrosine kinase inhibitors, which block a specific mutation present in a proportion of mast cell tumours. A genuine advance in this disease.
Supportive medication — antihistamines and stomach protectants — because circulating histamine from the tumour can cause gastric ulceration.
The prognosis picture
Low-grade, completely excised, no spread: frequently cured. Many dogs never have another problem from that tumour.
High-grade, or with spread: more guarded, and treatment aims at control and quality of life. Outcomes vary considerably.
Dogs who develop one mast cell tumour are more likely to develop another, which is the argument for continuing monthly checks for life.
What you can do
The monthly body check. Two minutes, once a month:
- Run both hands over the whole body — head, ears, muzzle, neck, body, legs, between the toes, belly, under the tail
- Feel the lymph nodes: under the jaw, in front of the shoulders, behind the knees
- Lift the lips and check the mouth
- Note anything new, measure it, photograph it beside a coin, and write the date
Map it. Draw a rough outline of your dog and mark each lump with a number. In a dog with several lumps, a map is the only way to notice that one of them has changed.
Get every new lump aspirated. Not monitored — aspirated.
Go sooner if a lump is growing, changing, ulcerated, firmly attached, painful, licked persistently, in one of the higher-risk locations, or if your dog has other signs such as vomiting or lethargy.
What not to do
- Do not squeeze or manipulate a suspected mast cell tumour. Handling can cause degranulation, releasing histamine, which makes the lump swell and can cause a systemic reaction
- Do not apply black salve or other caustic “natural” remedies. They cause severe tissue damage and are genuinely dangerous
- Do not assume a soft lump is a lipoma. Some mast cell tumours feel exactly like one
- Do not wait to see if it grows, unless it has already been sampled and your vet has advised monitoring
The asymmetry that decides it
Most lumps turn out to be nothing. The cost of checking is a consultation and an aspirate.
The cost of not checking, in the minority of cases where it matters, is a tumour that was straightforward to remove in March and complicated in September.
That asymmetry is the entire argument, and it is why vets ask to see lumps that owners think are probably fine.
