A 73-year old woman with a history of multiple myeloma (MM) with lytic bone lesions, previously in complete remission 6 months after autologous stem cell transplant, presented with new nodular plaque-like lesions involving the skin, most significantly on the left forearm (panel A). Skin biopsy demonstrated a plasma cell neoplasm involving the dermis with irregular nuclei, finely granular chromatin, increased mitoses, and high nuclear-to-cytoplasmic ratio with small-to-moderate amounts of amphophilic-to-eosinophilic cytoplasm consistent with plasmablastic morphology (panels B-C;  hematoxylin and eosin stain; original magnification ×40 [B] and ×1000 [C]). Immunohistochemical and in situ hybridization (ISH) studies were positive for CD138 (panel D; original magnification ×200), BCL1 overexpression (panel E; original magnification ×200), λ-ISH (panel F; original magnification ×200), c-MYC overexpression, and Ki67 >90%; cells were negative for κ-ISH (panel G; original magnification ×200), Epstein-Barr virus–encoded RNA–ISH, CD20, and CD117. Cytogenetics/fluorescence in situ hybridization from bone marrow biopsy at initial diagnosis demonstrated complex cytogenetics including gain(1q21), del(17p), and t(11;14), corroborating the observed BCL1 overexpression. She received daratumumab, carfilzomib, pomalidomide, and dexamethasone with resolution of her skin lesions 1 month later (panel H).

Cutaneous plasmacytomas are rare complications of MM and can develop in extramedullary relapse. Plasmablastic morphology can be observed in these skin infiltrates, associated with a high proliferative index and clinically aggressive disease as illustrated in this case. Extramedullary relapses are more commonly associated with high-risk cytogenetics as observed and soft tissue extramedullary relapses, in particular, portend a poor prognosis with reported median survival of <6 months.

A 73-year old woman with a history of multiple myeloma (MM) with lytic bone lesions, previously in complete remission 6 months after autologous stem cell transplant, presented with new nodular plaque-like lesions involving the skin, most significantly on the left forearm (panel A). Skin biopsy demonstrated a plasma cell neoplasm involving the dermis with irregular nuclei, finely granular chromatin, increased mitoses, and high nuclear-to-cytoplasmic ratio with small-to-moderate amounts of amphophilic-to-eosinophilic cytoplasm consistent with plasmablastic morphology (panels B-C;  hematoxylin and eosin stain; original magnification ×40 [B] and ×1000 [C]). Immunohistochemical and in situ hybridization (ISH) studies were positive for CD138 (panel D; original magnification ×200), BCL1 overexpression (panel E; original magnification ×200), λ-ISH (panel F; original magnification ×200), c-MYC overexpression, and Ki67 >90%; cells were negative for κ-ISH (panel G; original magnification ×200), Epstein-Barr virus–encoded RNA–ISH, CD20, and CD117. Cytogenetics/fluorescence in situ hybridization from bone marrow biopsy at initial diagnosis demonstrated complex cytogenetics including gain(1q21), del(17p), and t(11;14), corroborating the observed BCL1 overexpression. She received daratumumab, carfilzomib, pomalidomide, and dexamethasone with resolution of her skin lesions 1 month later (panel H).

Cutaneous plasmacytomas are rare complications of MM and can develop in extramedullary relapse. Plasmablastic morphology can be observed in these skin infiltrates, associated with a high proliferative index and clinically aggressive disease as illustrated in this case. Extramedullary relapses are more commonly associated with high-risk cytogenetics as observed and soft tissue extramedullary relapses, in particular, portend a poor prognosis with reported median survival of <6 months.

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