Sarcomatoid plasma cell tumour: a rare entity

Abstract

Sarcomatoid plasma cell tumours are rare entities, and often present histopathologists with great diagnostic difficulty as they can exhibit a wide range of morphological and immunohistochemical profiles. Herein we present a case report of a 76-year-old man, with an enlarging knee mass, in whom an open biopsy showed solid sheets of pleomorphic cells with plasmacytoid, epithelioid and spindled morphology. Cytokeratin, melanocytic and mesenchymal markers were negative, so a provisional diagnosis of undifferentiated pleomorphic sarcoma (UPS), grade 3 was suggested. However, upon tertiary review of the CD138, there was focal weak positivity, triggering further haematological work up, including immunoglobulin heavy chain gene analysis which ultimately detected a heavy chain gene rearrangement. Thus, a diagnosis of a sarcomatoid plasma cell tumour was made. This case demonstrates the importance having plasma cell tumours within histopathologist's differential diagnoses when posed with a sarcomatoid tumour. It also highlights the need for judicious interpretation of IHC, and the use of a wide panel of IHC to ensure that correct histopathological diagnoses are made.