Plasma Cell Neoplasms Mimicking Solitary Fibrous Tumour

While plasma cell neoplasms typically display overt plasmacytoid morphology, unusual histologic variants can mimic soft tissue tumours such as solitary fibrous tumour (SFT). Critically, SFTs lack plasmacytoid differentiation, which is a key distinguishing feature. We present two cases in which plasma cell neoplasms mimicked SFT histologically, underscoring the importance of recognizing this pitfall.
Case 1: A L1-L2 lytic lesion in a 46-year-old woman was diagnosed on biopsy as SFT due to the presence of a "patternless" spindle-cell proliferation with hemangiopericytoma-like vessels. Excision revealed focal plasmacytoid cells, a feature that is not in keeping with SFT. This prompted immunohistochemistry (IHC) testing, which showed that the tumour cells were CD138+ plasma cells, confirming a plasma cell neoplasm.
Case 2: A 41-year-old man presented with a L2 fracture, that on histology exhibited a vascular tumour with staghorn vessels and diffuse plasmacytoid morphology. Further evaluation with IHC (CD138+) and serum analysis led to a diagnosis of multiple myeloma.
These cases demonstrate that plasmacytoid morphology, even if focal, should prompt consideration of plasma cell neoplasms. Pathologists must recognize this distinction, particularly in vascular-rich spindle-cell lesions, as misdiagnosis delays appropriate treatment.