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A 26-year-old man presented with right upper hip pain. A CT scan showed a 6.0 × 6.0 × 7.0 cm3 lytic lesion in the right iliac wing with significant soft tissue involvement. A fine-needle aspiration of the lesion revealed numerous atypical plasma cells consistent with a plasmacytoma. Flow cytometry confirmed the diagnosis. A radiological bone survey revealed a single 7-mm well-defined lucent focus at the front aspect of the lateral skull. By serum protein electrophoresis and immunofixation, the patient had an IgG kappa M-spike estimated at 2.76 g/dl. A bone marrow biopsy demonstrated an increased number of plasma cells by CD138 immunohistochemistry (approximately 10–15% of the total marrow space). On the aspirate smears and touch preparations, the plasma cells were remarkable for large basophilic globules that markedly compressed the nuclei (Image 1). Bone marrow core biopsy sectionsrevealed numerous refractile globular structures (Image 2).

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Illustration Image 1.. High power view of the bone marrow aspirate with signet-ring plasma cells. Note the nuclear compression and displacement by a single, large immunoglobulin droplet in the upper right (1000× magnification).

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Illustration Image 2.. The bone marrow core biopsy with signet ring plasma cells.

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The patient's bone marrow findings, imaging, and laboratory findings are consistent with the diagnosis of a plasma cell myeloma. The morphologic presentation of plasma cell myeloma varies from benign appearing mature plasma cells to anaplastic variants. The unusual plasma cell morphology seen in this biopsy, with large immunoglobulin inclusions, has been referred to in the literature as “signet-ring” or “clear cell” plasma cell myeloma. Signet-ring myeloma is exceptionally rare with only a handful of reported cases in the literature [1–6]. The accumulation of intracellular immunoglobulin is most likely from defective immunoglobulin assemblage. In one case of plasmacytoma with signet ring features, the inclusions were amyloid [1].

Awareness of this morphologic variant is crucial to avoid the misdiagnosis of other neoplasms with signet ring features, such as metastatic adenocarcinoma, liposarcoma [3], or B-cell non-Hodgkin lymphoma [7]. It should be noted that adenocarcinomas can express CD138, therefore this immunohistochemical stain cannot be used solely to rule out metastatic carcinomas [8].

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