CONDITION

Mast Cell Tumours in Cats

Mast cell tumours are growths that arise from mast cells, a type of immune cell normally found in skin and other tissues. In cats, these tumours most often appear as single or multiple skin nodules or lumps, though they can also develop in internal organs, particularly the spleen and intestines. The appearance and behaviour vary considerably: some remain small and cause no trouble, whilst others grow, spread, or release chemicals that affect surrounding tissues. Owners often discover a new lump during grooming, or notice a bump that has changed in size or texture. Because mast cell tumours in cats can look similar to many other skin growths, and because their behaviour ranges from entirely benign to more concerning, biopsy is typically needed to understand what is present. The tumour's location, appearance under the microscope, and certain laboratory markers help clarify what to expect. This page explores what mast cell tumours look like when they first appear, what drives their development, how they are investigated and graded, and the range of approaches used to manage them. The goal is to provide a clear picture of what this diagnosis can mean and what questions may be useful to explore.

Why this matters now

Mast cell tumours in cats tend to appear in middle-aged to older animals, with most diagnosed between eight and twelve years of age, though they can occasionally develop in younger adults or even kittens. Certain breeds, including Siamese and related breeds, appear in some studies more frequently than others, though mast cell tumours can arise in any cat. There is no clear link to sex or neuter status, and unlike some other tumours, no strong environmental or dietary associations have been identified.

The behaviour of mast cell tumours in cats varies considerably. Many remain confined to the skin, grow slowly over weeks or months, and can be managed with local treatment. A proportion—approximately one in four or five—behave more aggressively, spreading to local lymph nodes or developing as multiple skin nodules, or occasionally involving internal organs. The pattern in any individual cat depends on the site of the tumour, its appearance under the microscope, and factors that are not yet fully predictable at the time of diagnosis.

Signals & patterns

Early signals

Single lump on the skin

A firm or soft nodule, often on the head, neck, or body, that may be a few millimetres to a couple of centimetres across. The lump may have been present for some time before being noticed, and may feel movable or fixed to surrounding tissue.

Lump with hair loss or colour change

The skin over the lump may lose hair, appear pink or red, or look slightly thickened. These changes reflect the presence of mast cells and the substances they contain, which can affect the surrounding tissue.

No other visible signs

Many cats with a mast cell tumour on the skin appear otherwise well, with normal appetite, activity, and behaviour. The lump itself may be the only observable change for weeks or months.

Occasional small patches or nodules

Some cats develop a few small lumps rather than a single large one. These may appear close together or in different areas of the body, and may vary in size or texture.

Later signals

Enlarging or ulcerated lumps

A lump that grows more quickly, becomes irregular in shape, or breaks through the skin surface, sometimes weeping or bleeding. This can indicate more active tumour behaviour or changes in the local tissue.

Multiple new lumps appearing

The development of several new nodules over time, which may suggest spread within the skin or the presence of mast cells in multiple sites. This pattern is less common but can occur in a proportion of cases.

Swelling near the original site

Thickening or firm swellings in the area of a lymph node, such as under the jaw, in the armpit, or behind the knee, which may reflect involvement of the draining lymph node. This is more often seen in tumours that have a higher tendency to spread.

Click to read about the biological mechanisms

How this is usually investigated

Investigation typically begins with a physical examination to characterise the lump—its size, location, texture, whether it is fixed to deeper tissues, and whether other lumps are present. The next step is usually to sample cells from the mass to identify what type of tumour it is. As the picture becomes clearer, further tests may be used to assess whether the tumour has spread, to classify its microscopic appearance more precisely, and to evaluate the cat's overall health before any treatment decisions are made.

Cytology

Purpose: A fine needle is used to collect cells from the lump, which are then examined under a microscope. This often confirms that the mass is composed of mast cells and can distinguish mast cell tumours from other types of skin growth.
Considerations: Cytology is quick and minimally invasive, and mast cells tend to yield diagnostic samples readily. It does not, however, provide information about how the cells are organised within the tissue, their degree of invasion, or their microscopic grade, all of which require a tissue sample.

Histopathology

Purpose: A surgical biopsy or removal of the entire lump provides tissue for detailed examination, allowing the pathologist to assess the microscopic architecture, the degree of cellular abnormality, the mitotic rate, and whether the tumour has been completely excised. This information is used to assign a histological grade, which helps predict behaviour.
Considerations: Histopathology requires anaesthesia and a surgical procedure, and results take several days. The grading systems used in cats are not as predictive as those in dogs, and many feline cutaneous mast cell tumours behave indolently regardless of grade.

Physical examination

Purpose: The clinician palpates regional lymph nodes to assess their size and texture, examines the skin for additional lumps, and checks the spleen and liver if they are palpable. This helps identify whether the tumour may have spread beyond the original site.
Considerations: Physical examination provides useful initial information but cannot detect small internal metastases or confirm whether an enlarged lymph node contains tumour cells. Further sampling or imaging may be needed to clarify the findings.

Abdominal ultrasound

Purpose: Ultrasound allows visualisation of internal organs, particularly the spleen, liver, and abdominal lymph nodes, to check for masses or changes in size and texture that might suggest spread of the tumour or involvement of internal organs.
Considerations: Ultrasound is non-invasive and does not require sedation in most cats, but it depends on the skill of the operator and the cooperation of the animal. It can identify abnormalities but cannot confirm whether they are mast cell tumours without sampling.

Complete blood count

Purpose: A blood sample is examined to look for circulating mast cells, changes in white blood cell populations, or anaemia. Occasionally, mast cells appear in the bloodstream if there is widespread or systemic disease.
Considerations: Most cats with solitary skin mast cell tumours have normal blood counts. Circulating mast cells are uncommon and tend to be associated with more advanced or systemic disease, so a normal result does not exclude the presence of a tumour.

Options & trade-offs

Management is shaped by the tumour's location, its microscopic appearance, whether it has spread, and what is practical for the individual cat and owner. Many feline cutaneous mast cell tumours can be managed with surgery alone, while others may benefit from additional treatments to reduce the risk of recurrence or to address tumours that cannot be fully removed. Different combinations suit different situations, and the approach often evolves as more information becomes available.

Surgical excision

The tumour is removed with a margin of surrounding normal tissue to reduce the chance of leaving abnormal cells behind. For many cutaneous mast cell tumours in cats, complete surgical removal is associated with a low rate of recurrence, particularly if the tumour is well differentiated. The width of the margin required is less clear in cats than in dogs, and narrower margins may still be effective for less aggressive tumours.

Trade-offs: Surgery requires general anaesthesia and may not be straightforward if the tumour is in a location where wide margins are difficult to achieve, such as the face or lower limbs. Incomplete excision increases the chance of local regrowth, though some incompletely excised tumours remain stable for long periods.

Monitoring without immediate intervention

Some mast cell tumours in cats, particularly small, solitary, well-differentiated cutaneous masses, grow very slowly or remain stable over months to years. In these cases, the tumour may be monitored with periodic examination and measurement, with treatment deferred unless the mass enlarges, ulcerates, or causes discomfort. Occasionally, small cutaneous mast cell tumours regress spontaneously.

Trade-offs: Monitoring avoids the risks and costs of anaesthesia and surgery, but it requires regular reassessment and carries the uncertainty of not knowing how the tumour will behave over time. It is less suitable for tumours that are growing, symptomatic, or located internally.

Radiation therapy

Radiation is sometimes used after incomplete surgical excision to target any remaining tumour cells, or as a primary treatment if surgery is not feasible due to the tumour's location. It involves a series of sessions, typically under short anaesthesia, delivered over several weeks. Radiation can help control local disease and delay or prevent regrowth.

Trade-offs: Radiation therapy is available only at referral centres, requires repeated anaesthesia, and may cause temporary or permanent changes to the treated skin, such as hair loss or pigmentation changes. Its role in feline mast cell tumours is less well defined than in dogs, and not all tumours respond equally.

Systemic therapy

Drugs that target rapidly dividing cells or specific molecular pathways may be used for mast cell tumours that have spread, are not amenable to surgery, or have recurred after local treatment. Options include conventional chemotherapy agents and tyrosine kinase inhibitors, which can interfere with signals that promote mast cell growth. Responses vary, and treatment is often continued for weeks to months.

Trade-offs: Systemic therapy can reduce tumour size and slow progression, but it does not reliably cure mast cell tumours and may be associated with side effects such as digestive upset, bone marrow suppression, or fatigue. The evidence base for efficacy in cats is smaller than in dogs, and individual responses can be unpredictable.

Supportive care for mast cell mediator effects

Antihistamines, gastric acid suppressors, or corticosteroids may be used to manage symptoms caused by substances released from mast cells, such as itching, swelling, or gastrointestinal ulceration. These treatments do not address the tumour itself but can improve comfort and quality of life, particularly if the tumour is large or if systemic signs are present.

Trade-offs: Supportive care can relieve discomfort and allow time for other treatments to take effect, but it does not slow tumour growth or prevent spread. It is often used alongside other approaches rather than as a standalone strategy.

Common misconceptions

Misconception:

"All mast cell tumours in cats are aggressive and spread quickly."

Reality:

The behaviour of mast cell tumours in cats varies considerably, and many cutaneous tumours are relatively indolent, growing slowly or remaining stable over long periods. Tumours involving internal organs or those with poorly differentiated cells tend to follow a more aggressive course, but solitary, well-differentiated skin masses often have a favourable outlook, particularly if completely excised.

Misconception:

"If the lump has been there for months without growing, it cannot be a mast cell tumour."

Reality:

Mast cell tumours can remain static or grow very slowly for extended periods, and some are only noticed incidentally during examination for other reasons. A stable lump does not exclude the diagnosis, and the behaviour of the tumour over time is not always predictable from its initial appearance or rate of change.

Misconception:

"Genetic mutations in feline mast cell tumours are rare compared to dogs."

Reality:

Mutations in the c-KIT receptor are actually reported more frequently in feline cutaneous mast cell tumours than in canine tumours. What is less well characterised in cats is the clinical and prognostic significance of these mutations—their relationship to tumour behaviour and prognosis remains less clearly established than in dogs.

Related conditions

Mast Cell Tumours in Dogs

Mast cell tumours in dogs share the same cellular origin as those in cats, though their behaviour and patterns differ markedly—canine tumours tend to be more aggressive, particularly in the skin, while feline cutaneous mast cell tumours are often less aggressive, though visceral forms in both species can behave more unpredictably.

Cutaneous Lymphoma

Cutaneous lymphoma can present as skin nodules or plaques in cats and may appear similar to mast cell tumours on examination, making histopathology and immunohistochemistry important in distinguishing between these two round-cell tumours of the skin.

Eosinophilic Granuloma Complex in Cats

Eosinophilic granuloma complex lesions contain mast cells alongside eosinophils in the inflamed tissue, and the raised, ulcerated appearance of eosinophilic plaques can occasionally be mistaken for mast cell tumours on initial examination, though the underlying mechanisms differ.

Splenic Mass

Mast cell tumours can develop in the spleen as part of visceral disease in cats, and a splenic mass discovered on imaging may represent a mast cell tumour among other possibilities, with definitive identification requiring tissue sampling.

Lymphoma in Cats

Lymphoma and mast cell tumours can occasionally occur together in the gastrointestinal tract or other organs in cats, and both conditions can involve similar visceral sites such as the spleen, liver, or intestine, making careful tissue diagnosis important when internal masses are found.

Once a mast cell tumour has been identified and characterised, the questions that shape next steps often involve how the tumour fits into the broader picture of the cat's health and how different approaches align with what is practical and acceptable. The relationship between microscopic appearance, molecular findings, and clinical behaviour in feline mast cell tumours is still being clarified, and individual cases can follow different trajectories. This is a useful conversation to have once diagnostic information is available.