Page 182 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

Round Cell Tumours
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10.1 Canine Mast Cell Tumour (Canine MCT)
VetBooks.ir Neoplasia of mast cells.

Clinical features
• Common neoplasm of the dog, accounting for approximately 21% of canine skin
tumours.
• Age: mostly occurs in adult and old animals, with an average of approximately
9 years. However, it can be observed in dogs of any age, including puppies.
• Cutaneous mast cell tumour (MCT) is the most common form, followed by subcuta-
neous MCT. The differentiation between the two is based on clinical and histological
examination.
• Cutaneous MCT: can develop anywhere in the body. Macroscopically, it ranges
from nodular rashes to diffuse swellings or, most commonly, hairless raised ery-
thematous solid lesions. Masses can be ulcerated. Multiple forms can also occur,
especially in Boxer, Boston Terrier, Golden Retriever and Pug.
• Subcutaneous MCT: arises in the subcutaneous adipose tissue and presents as
soft, fleshy masses. Legs, back and thorax are the most common sites.
• Variable biological behaviour: this mostly depends on histological grading (for cuta-
neous MCT), proliferation indexes (e.g. Ki67), c-Kit mutation and Kit receptor
expression. High‐grade cutaneous MCTs have an aggressive behaviour, with metasta-
ses starting from regional lymph nodes and often progressing to the spleen and liver
in advanced stages. Peripheral blood involvement may also occur (mastocytaemia).
• MCTs in the oral mucosa or muzzle usually are more aggressive.
• As a general rule, subcutaneous MCTs tend to be less aggressive than cutaneous forms.
• Local and/or systemic paraneoplastic signs may be observed and are linked to the
release of histamine, heparin and proteases contained within the mast cell granules.
Local signs include oedema, ulceration and swelling at the primary tumour site, and
possibly delayed wound healing and local coagulation abnormalities. The most com-
mon systemic effects are gastrointestinal signs and include vomiting, GI haemorrhage,
anorexia and abdominal pain. Acute anaphylactic reactions are rare.
• Over-represented canine breeds: Boxer, Labrador Retriever, Golden Retriever, Shar
Pei, Bulldog, Boston Terrier, Staffordshire Bull Terrier, Fox Terriers, Weimaraner,
Cocker Spaniel, Rhodesian Ridgeback, Dachshund, Australian Cattle Dog, Beagle,
Schnauzer and Pug.

Cytological features
• Cellularity is variable, often high.
• Background: often clear, variably haemodiluted and with free purple granules from dis-
rupted mast cells. It may contain collagen fibrils.
• Cells are round with a moderate N:C ratio.
• Nuclei are often obscured by the granules, especially in well-differentiated forms. When
visible, they are medium sized, round to oval, mostly paracentral, with uniform to slightly
clumped chromatin and variably visible nucleoli.