CONDITION
Mast Cell Tumours
Why this matters now
Mast cell tumours are the most common skin cancer in dogs, typically appearing in middle-aged to older animals though they can occur at any age. Certain breeds, including Boxers, Boston Terriers, Labrador Retrievers, and Golden Retrievers, appear to have increased susceptibility. Tumours may develop anywhere on the body and can vary considerably in their behaviour.
Mast cell tumours range widely in their biological behaviour, from relatively benign masses that remain localised to aggressive cancers that spread. A single tumour's behaviour cannot be predicted by appearance alone. Some grow slowly over months or years, whilst others enlarge rapidly. Local spread to lymph nodes and distant spread to organs like the liver and spleen can occur with more aggressive forms.
Signals & patterns
Early signals
A new lump in or under the skin
Many mast cell tumours first appear as raised masses, though they can resemble various benign lumps.
A skin mass that changes size
Mast cell tumours may swell and shrink due to the release of histamine and other chemicals from the cells.
Redness or irritation around a skin lump
The substances released by mast cells can cause local inflammation and reactions in surrounding tissue.
Itchiness around a skin mass
Histamine release may cause the dog to lick, scratch, or pay attention to the area.
Later signals
Rapid growth of a previously stable mass
Changes in growth rate may indicate biological changes within the tumour.
Ulceration or bleeding from a skin mass
Surface breakdown can occur with some tumours, particularly larger or more aggressive ones.
Enlarged lymph nodes near the tumour
Regional lymph node involvement may indicate local spread.
Gastrointestinal signs
Histamine release from tumour cells can cause stomach ulceration, leading to vomiting, reduced appetite, or dark stools.
Click to read about the biological mechanisms
How this is usually investigated
Investigation of suspected mast cell tumours aims to confirm the diagnosis, determine the tumour's characteristics, and assess whether spread has occurred. This information helps guide treatment discussions.
Fine needle aspirate
Surgical biopsy or excision
Lymph node assessment
Staging tests
Molecular testing
Options & trade-offs
Treatment approach depends on tumour location, grade, stage, and individual circumstances. Various options may be appropriate for different situations.
Surgical excision
Removal of the tumour with wide margins of normal tissue.
Trade-offs: Often the first-line treatment for localised tumours. Achieving adequate margins depends on tumour location. Complete excision of lower-grade tumours often provides excellent outcomes.
Radiation therapy
Targeted radiation to destroy remaining tumour cells.
Trade-offs: May be used when surgical margins are incomplete or when surgery alone is unlikely to be curative. Requires multiple treatment sessions under anaesthesia.
Chemotherapy
Medications targeting rapidly dividing cells.
Trade-offs: May be recommended for higher-grade tumours or when spread is present. Various protocols exist with different treatment intensities and schedules.
Targeted therapy
Medications designed to block specific molecular pathways, such as tyrosine kinase inhibitors for c-kit positive tumours.
Trade-offs: Can be effective for tumours with certain mutations. May be used alone or combined with other treatments.
Supportive medications
Antihistamines and acid reducers to manage effects of histamine release.
Trade-offs: Often used alongside other treatments to reduce side effects from mast cell degranulation.
Common misconceptions
"All skin lumps in dogs are harmless fatty tumours"
Whilst lipomas are common, many different tumour types occur in dogs. Mast cell tumours in particular can mimic the appearance of other masses, making sampling important.
"Mast cell tumours are always aggressive cancers"
These tumours span a wide spectrum of behaviour. Many lower-grade tumours have excellent outcomes with appropriate treatment.
"If a mast cell tumour is removed, it will not return"
Outcome depends on factors including tumour grade, completeness of excision, and stage. Follow-up monitoring remains important.
Monitoring existing skin lumps for changes in size, shape, or behaviour provides useful information. Noting whether masses swell and shrink, become itchy, or cause any changes in the dog's wellbeing can be helpful. Recording when lumps first appeared and any changes observed over time supports veterinary assessment.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS