Plasmablastic myeloma (PBM) is a blastic morphologic variant of plasma cell myeloma with less favorable prognosis than those with non-blastic morphology. PBM is rare, without clear-cut definition and detailed clinicopathologic features in the literature. PBM may mimic plasmablastic lymphoma (PBL) as they share nearly identical morphology and immunophenotype. Using the criteria of ≥ 30% plasmablasts in tissue sections, we retrospectively recruited PBM cases and analyzed their clinical, imaging, and pathologic findings, with emphasis on extramedullary involvement. We performed immunohistochemistry, in situ hybridization for Epstein-Barr virus (EBER), and fluorescence in situ hybridization (FISH) for lymphoma- and myeloma-associated genetic alterations. Of the 25 recruited cases, 15 (60%) had extramedullary involvement, which occurred as initial presentation in nine cases. The most common extramedullary sites were soft tissue and/or skin (10/15, 67%), followed by pleural effusion, the lungs, and lymph nodes. Immunohistochemically, tumor cells expressed MYC (74%; 17/23), CD56 (56%; 14/25), and cyclin D1 (16%; 4/25), while CD117 was all negative (n = 25). Of the 20 cases stained with p53, four (20%) cases were diffusely positive, and the remaining 16 cases showed a heterogeneous pattern. EBER was negative in all 24 cases examined. Of the 13 cases examined with FISH, the genetic aberrations identified included del(13q14)(92%; 12/13), gain of chromosome 1q (90%; 9/10), loss of chromosome 1p (60%; 6/10), IGH-FGFR3 reciprocal translocation (23%; 3/13), rearranged MYC (15%; 2/13), and rearranged CCND1 (8%; 1/13), while there were no cases with TP53 deletion (n = 10) or rearrangement of BCL2 (n = 13) or BCL6 (n = 13). The prognosis was dismal regardless of the presence or absence of extramedullary involvement. In conclusion, PBM in Taiwan frequently presented as extramedullary and extranodal lesions, particularly in soft tissue and/or skin, mimicking PBL. FISH for targeted genetic alterations such as del(13q14), gain of chromosome 1q, loss of chromosome 1p, and IGH-FGFR3 might be helpful for the differential diagnoses. Larger studies are warranted to investigate the genetic alterations between PBM and PBL.
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