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Case of the Month Diagnosis - Cutaneous Plasmacytomas

Betsy Aird DVM PhD DACVP

Cutaneous plasmacytomas are uncommon in dogs (~2%) and rare in cats. They are described clinically as a typically solitary (rarely multiple), well-circumscribed, round, raised, pink, cutaneous mass that may be alopecic or ulcerated and typically 1-2 cm in diameter. Common sites are digits, ears (pinnae) and mouth (i.e., tongue and gingiva); however, they can appear at any site on the body.

Median age at diagnosis is 9-10 years with no sex predisposition reported. A breed disposition for American Cocker Spaniels, English Cocker Spaniels and West Highland White Terriers has been reported. Golden and Labrador retrievers have been described as common breeds for this neoplasm; however, this may be due to the popularity of these breeds rather than breed predisposition.

Cytologically, aspirates are moderate to markedly cellular and individual cells have variable amounts of basophilic to bluish cytoplasm with distinct borders. Cells exhibit mild to moderate anisocytosis and anisokaryosis. Nuclei are round to oval with fine to moderately coarse chromatin and indistinct nucleoli. The nuclei are often eccentric with indistinct nucleoli and cells frequently are multinucleated (Figure 2).

Cutaneous Mass Figure 2
Figure 2.

Cutaneous plasmacytomas are typically benign neoplasms without systemic involvement, but rarely, a cutaneous plasmacytoma can be a metastatic lesion of multiple myeloma. Primary cutaneous tumors almost always have good long-term prognoses and low metastatic potentials. Treatment of these primary tumors involves wide surgical excision that is often curative, although local recurrences can occur. Surgery, radiation therapy, or chemotherapy (i.e., melphalan and prednisone) have been recommended for recurrent tumors.

Primary cutaneous plasmacytomas should be considered in the differential diagnosis of solitary or multiple cutaneous tumors of dogs. They are discrete cell tumors that readily exfoliate for cytology via fine needle aspiration. Cellularity is typically high and cells remain intact when gently smeared on a slide. The distinct features of this neoplasm (discrete cell population, eccentric nuclei, distinctly bluish cytoplasm) often allow for a diagnosis to be made with cytology.

It should be noted however, that not all cutaneous plasmacytomas are as well-differentiated as in this case example. Some poorly differentiated plasmacytomas can resemble histiocytic neoplasms or agranular mast cell tumors. Therefore, differential considerations can include other discrete cell tumors such as histiocytic neoplasms, undifferentiated/agranular mast cell tumor, cutaneous lymphoma, and amelanotic melanoma. For this reason, histopathologic confirmation may be recommended for a definitive diagnosis.

Plasma cell tumors also occur in other soft tissue sites. Plasma cell tumors of the oral cavity can be aggressive but they have low metastatic potential. Plasma cells tumors of the esophagus, stomach, intestine or rectum can be aggressive and metastasize to local lymph nodes. Plasma cell tumors in the spleen can be primary tumors that do not metastasize or they can be metastatic lesions from primary multiple myeloma.