Mast Cell Tumour

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Also known as: Mastocytosis

Introduction

Mast cells are a cell population that originate in the bone marrow and mature in connective tissue.

Mast cell tumours are ubiquitous in domestic animal species. The tumours can be focal or multicentric in the skin and may occasionally involve internal viscera such as the spleen, liver and intestine.

Canine mast cell tumours

Mast cell tumours are the most common cutaneous tumour in the dog, primarily a disease of older dogs. Boxers, Labrador Retrievers, Beagles and Schnauzers are reported to be predisposed.

Boxers are more predisposed, but generally develop the low or intermediate grade forms of the disease which have a more favourable prognosis.

There is no gender predilection.

All tumours should be considered malignant as each tumour has metastatic potential.

Clinical signs

Mast cell tumours vary widely in appearance.

The most common gross appearance is a small, raised, well circumscribed mass that may be hyperaemic, alopecic, ulcerated or look like normal skin. They may also appear as a poorly-defined, soft, fluctuant lesion. The tumours may become quite large, up to 30 cm, or may occur as a diffuse, inflamed, dermal thickening.

They are more commonly found on the caudal half of the body.

Regional lymph node involvement is common, up to 75%, but widely disseminated metastasis is less common.

Clinical signs reflect the extent of systemic involvement and the presence or absence of paraneoplastic disorders. Signs vary from: pruritus and excoriation to nausea, vomiting, diarrhoea, melena, abdominal pain and microcytic, hypochromic anaemia.

More than 50% of patients with systemic mastocytosis have systemic signs, but it is rare for patients with cutaneous MCTs to have systemic signs.

The release of mast cell granule contents lead to: gastric ulceration, peritumour inflammation, cardiopulmonary abnormalities, prolonged bleeding and delayed wound healing.

Aggressive palpation of masses can lead to histamine release and immediate peritumour erythema, pruritus, oedema and swelling.

The metastatic pattern typically includes lymphatic spread to regional lymph nodes, spleen, liver and bone marrow.

Diagnosis

Cytological examination of the mass is usually sufficient to diagnose a mast cell tumour and should be performed before surgery in all cases.

Mast cell tumours have blue to purple granules which differentiate them from other round cell tumours.

Histology is necessary to confirm the diagnosis, to grade the tumour, and to determine that the surgical margins are free of mast cells.

There are three histological grades for mast cell tumours:

Grade I: well-differentiated, mature cells
Grade II: moderate or intermediate differentiation
Grade III: poorly-differentiated, anaplastic cells

Clinical staging of the disease should also be performed as it is strongly associated with prognosis:

Modified World Health Organisation clinical staging format:

Stage 0: one tumour incompletely excised from the dermis, identified histologically, without regional lymph node involvement
Stage 1: one dermal tumour without regional lymph node involvement
Stage 2: one dermal tumour with regional lymph node involvement
Stage 3: multiple dermal tumours or large, infiltrating tumours with or without regional lymph node involvement
Stage 4: any tumour with distant metastasis (including bone marrow) or recurrence with metastasis.
a: no clinical signs
b: clinical signs

All palpable lymph nodes should be sampled regardless of their size.

Abdominal palpation, radiographs and ultrasound will help determine the presence of an enlarged spleen or liver. Fine needle aspirates should be obtained from any enlarged organ.

Bone marrow aspiration cytology is also useful if bone marrow involvement is suspected.

Complete haematology and biochemistry, urinalysis and faecal occult blood test should be performed to check for paraneoplastic syndromes and the animal's ability to withstand treatment.

Treatment and prognosis

The behaviour and response to treatment of mast cell tumours is often unpredictable.

Surgical excision is the treatment of choice where possible, usually Stage 1 tumours. Well localised tumours may be completely resected. Wide excision is necessary, with 3cm laterally and one fascial plane deep to the tumour.

All margins should be examined for residual tumour.

All incomplete surgical resections need additional local treatment. Either re-excision or radiotherapy should be performed.

The grade of the tumour is important in predicting post-surgical survival, and dogs with well-differentiated tumours are more likely to survive longer than dogs with poorly-differentiated tumours, which are more likely to metastasise and cause early death.

Radiation therapy is also an effective mode of treatment.

Treatment for Stage 2-4 mast cell tumours varies. For stage 2 tumours, options include excision of the mass and of the regional lymph node, prednisolone and radiotherapy, either singly or in combination.

Treatment of stage 3 or 4 is generally palliative and may involve chemotherapy agents such as L-asparaginase, prednisolone, chlorambucil, cyclophosphamide, vincristine and doxorubicin, alone or in combination.

Supportive therapy should be given, with histamine receptor antagonists such as cimetidine or ranitidine, to treat the systemic effects of histamine release.

Feline mast cell tumours

Mast cell tumours are less common in cats than in dogs, but they are more commonly multicentric.

Siamese cats are most are risk, and also develop a histiocytic variant of the disease.

Mast cell tumours occur in different forms in the cat. In one form, one or more tumours originate in the skin and may metastasise to regional nodes and other viscera. In the other, more common form, the neoplasm arises in internal organs without obvious cutaneous involvement. There is also a systemic and a leukaemic form.

Clinical features

Most mast cell tumours are of the 'mast cell form' and are similar to the cutaneous mast cell tumours of dogs. They usually appear on the head and neck of cats over the age of 4. Most are benign and appear as well-differentiated dermal tumours.

Cats that develop histiocytic variants are usually Siamese cats under the age of 4, but they can be of other breeds and older. Lesions are usually multiple, small subcutaneous papulonodules. The overall histological appearance is that of granulomatous inflammation with eosinophilic and lymphoid aggregates.

Special stains may be required to diagnose the uncommon variants of the disease.

For the visceral and systemic form, signs will include: vomiting, enlarged lymph nodes, depression, anorexia and possibly ascites.

Treatment

Complete excision of the cutaneous mast cell tumours is curative, unless there are multiple tumours which may metastasise or recur.

For the histiocytic variant, the lesions often regress spontaneously and no treatment is usually necessary. The prognosis is good.

Mast cell tumours in other species

Horses

Most cases are in male horses and occur as focal masses on the head or legs. They respond to complete excision and are invariably benign.

In younger horses the tumours may spontaneously regress.

Cattle

The majority of bovine mast cell tumours are malignant and have high metastatic potential.


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References

Morrison, W. (2002) Cancer in dogs and cats Teton NewMedia

Meuten, D. (2002) Tumors in domestic animals Wiley-Blackwell

Withrow, S. (2007) Withrow and MacEwen's small animal clinical oncology Elsevier Health Sciences

Merck and Co (2008) Merck Veterinary Manual Merial




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