Page 178 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

Mesenchymal Tumours and Other Neoplasms

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• The cytoplasm is moderate in amount, basophilic and contains variable numbers of mel-
anin granules; these may be few or absent in poorly differentiated (amelanotic) forms.
VetBooks.ir • Anisocytosis, anisokaryosis and atypical mitotic figures are often seen in melanomas.
Melanin-laden macrophages (melanophages) are often seen. They can be difficult to differ-
•
entiate from the melanocytes.
Variants
Several histopathological variants of melanomas have been described in dogs and cats. These
variants do not carry any prognostic significance; however, the epithelioid type of malignant
melanoma in the cat may behave in a more aggressive and malignant fashion. The balloon cell
variant has a characteristic appearance.
• Balloon cell variant:
• Rare variant. Also called clear cell melanoma.
• Cytologically, it is characterized by a population of large, round to polygonal cells with
abundant clear or pale basophilic cytoplasm containing clear intracytoplasmic vacuoles.
Cytological features of atypia can be marked.

Differential diagnoses
• Melanocytoma:
• Melanoma
• Pigmented trichoblastoma and basal cell tumour (especially in cats)
• Sweat gland adenoma (may contain melanin)
• Melanoma:
• Melanocytoma
• Soft tissue sarcoma (for spindle cell variant only)
• Histiocytic sarcoma (for amelanotic form only)
• Sebaceous cell carcinoma, liposarcoma, clear cell adnexal carcinoma (for balloon cell
variant only)

Pearls and Pitfalls
• In dogs, as a general rule, melanocytic neoplasms arising from the haired skin are most frequently
benign, whereas those arising from nail beds, mucucutaneous junctions and oral mucosa are
more commonly malignant.
• When undertaking cytological examination of regional lymph nodes for metastasis, care must
be taken to differentiate melanophages from neoplastic melanocytes.
• For the diagnosis of amelanotic and poorly melanotic melanomas, special cytochemical
(e.g. Fontana-Masson) or immunohistochemical (e.g. Melan A) stains can aid in the
diagnostic process.