What is a mast cell tumour?
A mast cell tumour (MCT) is the most common skin tumour in dogs, accounting for 7–21 % of all canine skin tumours. It arises from mast cells — immune cells that store histamine and heparin and play a role in allergic and inflammatory reactions.
The appearance is very variable — it can resemble a benign cyst, lipoma or wart — making veterinary evaluation of any new lump essential.
Predisposed breeds
Boxer, English Bulldog, Boston Terrier, Shar Pei, Labrador, Golden Retriever, Pug and Beagle show higher incidence. Brachycephalic breeds tend to present lower-grade (I–II) tumours more frequently.
Symptoms and warning signs
- Skin or subcutaneous nodule of variable size, often appearing benign.
- Darier's sign: redness, urticaria and itch when the mass is manipulated (from histamine degranulation). Highly suggestive of MCT.
- Fluctuating size (the mass can grow and shrink).
- Systemic signs in advanced cases: vomiting, haemorrhagic diarrhoea, gastric ulcers, hypotension.
Diagnosis
- Fine-needle aspiration (FNA): first diagnostic step. Mast cells with metachromatic granules are characteristic and easily recognised.
- Biopsy + histopathology: determines tumour grade (Patnaik I–III or Kiupel low/high grade).
- c-Kit mutation PCR: relevant for selection of tyrosine kinase inhibitor therapy.
- Staging: blood panel, abdominal ultrasound, bone marrow biopsy in high-grade tumours.
Treatment
- Surgery: treatment of choice for localised MCT. A 2–3 cm margin in all directions is critical. Grade I–II with clean margins can be curative.
- Radiotherapy: used when surgical margins are incomplete or the location is challenging.
- Chemotherapy: for grade III or metastatic disease (lomustine, vinblastine).
- Tyrosine kinase inhibitors (toceranib/Palladia®, masitinib/Masivet®): for c-Kit-mutant or recurrent tumours. Oral treatment with reasonable tolerability.
- Antihistamines + H2 blockers: always perioperatively to control systemic effects of degranulation.
Prognosis
- Grade I (low grade): excellent. Surgery is usually curative. Median survival >2 years.
- Grade II (intermediate): good with appropriate treatment.
- Grade III (high grade): guarded. Median survival without treatment is months.
This is why any new lump should be evaluated — FNA is quick and relatively inexpensive.