Page 188 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

Round Cell Tumours
175
• Well-differentiated form:
• Most common form of feline MCT.
VetBooks.ir • Neoplastic cells show little pleomorphism and resemble normal mast cells.
Eosinophilic infiltrates are rare or absent.
•
• Small lymphocytes are often found.
• Pleomorphic form:
• Uncommon form of feline MCT.
• As per name, the cellular pleomorphism in this form is often prominent. Cytomegalic
cells can be seen and anisokaryosis and anisocytosis can be marked. Multinucleated
cells and cells with bizarre nuclei are observed.
• Cell pleomorphism and nuclear atypia do not seem to correlate with a malignant behaviour.
• Numerous eosinophils are often present.
• Atypical (histiocytic) form:
• Rare form of feline MCT.
• Seen mostly in young cats as multiple nodular lesions.
• Associated with a benign clinical course or spontaneous regression.
• Siamese cats may be predisposed.
• Cytologically, neoplastic mast cells resemble histiocytes. They are large, polygonal to
round, with lightly basophilic cytoplasm lacking the typical intracytoplasmic granules.
Nuclei are large, round, often indented and paracentral. Mitotic activity is low.
• Small lymphocytes are frequently present.

Differential diagnoses
• Mast cell rich inflammatory lesions (e.g. insect bite reaction)
• Eosinophilic granuloma

Pearls and Pitfalls
• Cytoplasmic granules are not as easy to see as in canine MCTs, especially if slides are
stained with Diff-Quik. Hence, when possible, Wright-Giemsa stain is preferable.
• Feline MCTs are often referred to as compact or diffuse, mainly depending on their de-
gree of invasiveness into the deeper subcutis. The use of these terms has not been consist-
ent in the previous literature.
• The cellular pleomorphism is not associated with the clinical behaviour of feline MCTs.
The only confirmed prognostic factor for feline MCTs is the mitotic activity. It is still
unclear if other variables such as tumour diameter, presentation with multiple masses and
histological parameters have a prognostic significance.