Page 104 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

Epithelial Tumours
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Trichoepithelioma
VetBooks.ir Follicular tumour that shows differentiation to all three segments of the hair

follicle: infundibulum, isthmus and inferior segment.

Clinical features
• In dogs the overall incidence varies from 0.4% to 4% of all skin tumours. It is uncommon
in cats, ranging from 0.3% to 0.8% of all skin tumours.
• Malignant trichoepithelioma is rare and is described only in dogs.
• Age:
• Trichoepithelioma: peak of incidence between 5 and 11 years in dogs and 4
and 11 years in cats.
• Malignant trichoepithelioma: between 8 and 12 years of age.
• Lesions can be single to multiple, round to ovoid or dome-shaped. Usually < 2 cm;
larger masses (up to 15 cm) are less common. Malignant trichoepithelioma presents as
a nodular infiltrative mass.
• It occurs more frequently on the dorsal trunk and legs but may affect other areas of
the body.
• Trichoepithelioma carries a good prognosis. Malignant trichoepithelioma can metasta-
size to the draining lymph nodes and lungs.
• Over-represented canine breeds:
• Trichoepithelioma: Basset Hound, Bullmastiff and Soft-coated Wheaten Terrier.
• Malignant trichoepithelioma: Basset Hound and Airedale Terrier.

Cytological features

• Cellularity is variable, from low to moderate.
• Background: usually lightly basophilic and granular with variable numbers of anucleated
squamous epithelial cells. Cholesterol crystals and hair shafts may be present.
• Tumours are composed of epithelial islands and cystic structures containing keratinized
squames. For this reason, aspirates may only yield sheets of keratin without nucleated cells.
• The cystic cavities contain anucleated squames and keratinized material, which can derive
from all sections of the hair follicle. A mixture of the following structures is often observed
cytologically:
• Large polygonal corneocytes and/or liquid keratinized material: infundibulum.
• Dense and homogeneous amorphous keratin: isthmus.
• Ghost cells: inferior segment.
• Neoplastic cells are cuboidal (basaloid epithelial cells) and arranged in uniform cohesive
clusters.
• Nuclei are small to medium sized with clumped chromatin and inconspicuous nucleoli.
• The cytoplasm is scant and pale basophilic.
• The N:C ratio dis high and anisokaryosis and anisocytosis are minimal.