Case of the month #5
by Ioannis Oikonomidis (DipACVP/ECVCP)
This is a fine needle aspiration (FNA) from a small wart-like mass on the elbow of a 5-year-old, mixed breed, male neutered dog. What’s your diagnosis?
Fig 1-3 Elbow mass in a dog, Wright Giemsa, 50x.
The smears were of moderate to high cellularity and the cell morphology was adequately preserved. Low numbers of erythrocytes and moderate numbers of naked nuclei were present in a heavy, tigroid, amphophilic background. There was a predominant population of medium sized to large, round to polygonal to spindle-shaped cells, exfoliating individually or in small, loose or occasionally more cohesive aggregates. These cells had a round to ovoid nucleus with finely to occasionally coarsely stippled chromatin and 1-3 small, round, prominent nucleoli. The cytoplasm was moderate to large in amount and amphophilic to lightly to moderately basophilic with ill-defined borders. It occasionally contained small, discrete, clear vacuoles or small, smooth, pink globules. Anisocytosis was marked and anisokaryosis was moderate. Bi- or multi-nucleated cells and nuclear crowding were often noted. Occasional mitotic figures were observed. A small amount of amorphous, pink, extracellular material was occasionally seen in association with the cell population. Low numbers of macrophages were also noted; they often contained a few haemosiderin granules inside their cytoplasm. Rare non-degenerate neutrophils and small lymphocytes were found.
Malignant neoplasm suggestive of clear cell adnexal carcinoma.
The possibility of another malignant neoplasm, primarily an amelanotic melanoma, or less likely a liposarcoma or a poorly differentiated carcinoma or sarcoma cannot be excluded in this case. The mass was excised and sent for histopathological examination, which confirmed the cytological diagnosis of clear cell adnexal carcinoma.
Clear cell adnexal carcinoma has recently been described in dogs. It is a rare cutaneous neoplasm of adnexa without conclusive evidence of follicular, apocrine, or sebaceous differentiation. The cell origin is likely a cutaneous epithelial stem cell. Its name stems from the histological appearance of the neoplastic cells, which have a clear or vacuolated cytoplasm. Clear cell adnexal carcinoma is likely the same entity as previously described clear cell hidradenocarcinoma or follicular stem cell carcinoma. Histopathology is usually diagnostic without the need for immunohistochemistry. When in doubt, the combination of histological appearance and positivity of neoplastic cells to pancytokeratin and vimentin will likely be diagnostic. Variable proportions of the neoplastic cells express pancytokeratin and vimentin, while rare neoplastic cells appear to express Melan-A or PNL-2. Immunoreactivity to protein S-100 has been inconsistently reported. Despite the pleomorphic cytological appearance of clear cell adnexal carcinoma, surgical excision is expected to be curative in most cases. Local recurrence and metastasis to regional lymph nodes is not common and distant metastasis appears to be a rare event.
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