Mast cell tumour (mastocytoma) in a dog
Comprehensive care of a dog with a mast cell tumour — from cytology and staging, through surgery with an appropriate margin, to interpretation of the grade and a follow-up plan. The most common skin tumour in dogs.
What it involves
A mast cell tumour (mastocytoma) is a cancer arising from mast cells — cells of the immune system that contain histamine. It is the most common malignant skin tumour in dogs and at the same time one of the most variable: it can look like a harmless lump, a wart, a swelling or an ulcer, and its behaviour — from a lesion curable by surgery alone to disseminated disease — depends on the grade of malignancy, which cannot be judged “by eye”.
A characteristic feature is variability: the tumour can grow, turn red and become itchy when touched, and then shrink. The histamine released can also cause general signs — vomiting, stomach ulcers, black tarry stools. Predisposed breeds are boxers, bulldogs, Labradors, golden retrievers, Boston terriers, pugs and Shar-Peis (in the latter the course can be more aggressive).
We proceed in stages: cytology (a mast cell tumour is usually easy to recognise on it), staging, surgery with a margin, histopathology with determination of the grade (Patnaik I–III, Kiupel low/high) and proliferation indices, and on that basis — a decision on further treatment or on check-ups only.
When it is used
For any skin lesion of changeable appearance, for a mast cell tumour confirmed by cytology, after an operation performed at another practice with an unclear margin, and for a recurrence. We do not recommend “watching” a suspicious lesion without cytology — an untreated mast cell tumour grows and worsens the chance of a clean margin.
Indications: symptoms that should prompt a visit
- A lump on my dog’s skin is sometimes larger and reddened, sometimes smaller.
- When the lesion is touched, the skin around it swells and turns red.
- My boxer, bulldog or Labrador has a new lump that has grown over a few weeks.
- The skin lesion has ulcerated and the dog is scratching it.
- My dog with a lump on its skin is vomiting or has dark, tarry stools.
- A lump has been removed and the histopathology result is a mast cell tumour — what next?
Conditions for which we use it in particular
Diagnostics beforehand
Cytology of the lesion in the on-site laboratory (mast cell granules are characteristic). Staging: sampling of the regional lymph node (even when it is not enlarged), abdominal ultrasound with assessment and, if changes are seen, cytology of the liver and spleen, a blood count with a differential, and biochemistry. We take a chest X-ray at the pre-anaesthetic assessment, although lung metastases are uncommon in mast cell tumour. Before the procedure, the STD-01 assessment; we start antihistamine and gastroprotective drugs a few days before the operation to limit the effects of histamine release during handling of the tumour.
How it proceeds
We perform the procedure under general anaesthesia with monitoring (STD-02): we remove the tumour with a 2 cm lateral margin (for lesions that are low grade on cytology, narrower, proportionate margins are possible) and one layer of fascia in depth, without squeezing the lesion, using electrocautery. We remove an involved or suspicious lymph node during the same procedure. Duration: 45–90 minutes, depending on the location and the need for skin reconstruction. We mark the specimen with ink and send it for histopathology with a request for grading and the Ki-67/mitotic index.
The patient wakes up in the in-patient hospital with pain control and usually goes home the same day or the next morning. Once we have the result (7–14 days), we agree a plan: check-ups, wider re-excision of the scar or additional treatment (targeted drugs, chemotherapy — we confirm the scope available on site individually).
Aftercare
A collar until the stitches are removed (10–14 days), restricted activity for 2 weeks, and antihistamines, gastroprotectants and pain relief as set out in the medication schedule. Wound check after 2–3 days and at suture removal. After treatment ends, check-ups with examination of the skin and lymph nodes every 3 months for the first year, then every 6 months; we sample any new lumps with a needle straight away.
Call us urgently if there is vomiting, tarry stools, swelling or redness of the wound, the stitches coming apart, or if new lesions appear.
Frequently asked questions
Can a mast cell tumour be cured?
With a low grade and clear margins, removal is often enough and recurrence is rare. With a high grade the prognosis is poorer and further treatment is needed — we do not promise an outcome, but we discuss the chances clearly.
Why can't we simply watch the lump?
Because without cytology there is no way to know whether it is a mast cell tumour, and over time it grows and infiltrates the surrounding tissue, which makes removal with clear margins harder. Taking a needle sample takes around ten to fifteen seconds.
Will the dog develop further mast cell tumours?
In some dogs, yes — especially in predisposed breeds. Each new lesion is a new tumour, not a metastasis, and needs a separate assessment. Hence the regular skin checks.
Why are antihistamines given before surgery?
Handling the tumour releases histamine, which can cause a drop in blood pressure, swelling and stomach ulcers. Protective medication limits these reactions.
We manage every case individually. The scope, indications and course described above are indicative; we agree the exact diagnostic and treatment plan during the consultation. This content is for information only and does not replace a veterinary consultation.
Do you have questions about mast cell tumours?
Call us or book a consultation. We will set out the diagnostic and treatment plan in writing, before you decide.