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Multifocal osteolytic lesions in an older adult, particularly in the presence of hypercalcaemia and a monoclonal paraprotein, raise strong suspicion of multiple myeloma.However, coexisting pathological processes may produce a convincing but misleading clinical picture.We describe a case involving a man in his 80s who presented with back pain, hypercalcaemia and multifocal lytic vertebral lesions, together with an elevated kappa/lambda free light-chain ratio and a monoclonal serum band.Multiple myeloma was initially considered the leading diagnosis.Bone marrow aspirate and trephine biopsy, however, demonstrated only a low-volume kappa-restricted plasma-cell neoplasm, with approximately 3%-4% plasma cells.Further examination identified small foci of cytokeratin-positive cohesive cells, initially raising concern for metastatic carcinoma.The discrepancy between the extensive radiological disease burden and limited plasma-cell findings prompted targeted biopsy of an L3 lytic lesion.This demonstrated a separate population of atypical epithelioid cells expressing pancytokeratin (AE1/3), ERG and CD31, which supported vascular differentiation despite aberrant cytokeratin expression.Following a multidisciplinary review of the clinical, radiological and histopathological findings, a likely diagnosis of epithelioid angiosarcoma was made.A low-volume kappa-restricted plasma-cell neoplasm was again identified in the background.The patient's condition deteriorated rapidly, and he died before the final diagnosis was confirmed.This case demonstrates how epithelioid angiosarcoma can masquerade as multiple myeloma, particularly when a coexisting plasma-cell neoplasm provides a plausible but incomplete explanation for extensive lytic bone disease.Discordance between radiological disease burden and pathological findings should prompt reconsideration of the working diagnosis and timely targeted biopsy of radiologically abnormal tissue.
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Fahy et al. (2026) studied this question.
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