Background: Extramedullary plasmacytoma (EMP) of the thyroid is a rare neoplasm that may present either as a solitary plasmacell tumor or involvement of plasma cell myeloma (PCM). In this report, we present a unique case of PCM presented as a thyroid mass without previous history, and initially misinterpreted as medullary carcinoma on fine-needle aspiration cytology (FNAC).
Case Presentation: A 77-year-old female presented with shortness of breath and palpitation. Ultrasonography-guided FNAC from the large thyroid mass revealed hypercellular smears composed of monomorphic plasmacytoid cells, focally binucleated, scattered diffusely. Subsequent histopathological examination and immunohistochemistry of thyroidectomy specimen showed infiltration of sheets of plasma cells and diagnosed as EMP. Histology of the bone marrow displayed Lambda positive neoplastic plasma cells. A combination of singly distributed cells and aggregates with plasmacytoid morphology on a proteinous background without clinical suspicion and previous history leads to misinterpretation of FNAC.
Conclusion: Clinical correlation, immunocytochemistry, and complementary cytological approach including various preparation techniques of staining are crucial for avoiding diagnostic pitfalls and the patients' proper treatment.
Key words: Plasma cell myeloma, case report, thyroid, cytology, medullary carcinoma, FNAC.
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