Anaplastic plasmacytoma: a rare tumor presenting as a pathological fracture in a younger adult

Solitary plasmacytoma is the rarest type of plasma cell neoplasm, and the anaplastic form is even more uncommon. Plasmacytoma most commonly originates in bone and predominantly affects older patients. We describe the case of a 35-year-old woman with solitary osseous anaplastic plasmacytoma that presented initially with a pathological fracture following minor trauma. The patient was immunocompetent and had no predisposing conditions for a plasma cell tumor. Left lower extremity radiographs revealed an oblique fracture of the distal femur, and CT imaging indicated a primary osseous lesion at the fracture site. MRI confirmed the diagnosis of pathological fracture. Initial surgical pathology of the lesion was concerning because it could have been an osteosarcoma. Further immunostaining demonstrated CD138 positivity and kappa light chain restriction, confirming the diagnosis of plasmacytoma. In addition, the presence of marked anaplastic cellular changes confirmed the anaplastic variant. Further workup showed no evidence of multiple myeloma. This case is unusual given the age and gender of the patient. Awareness of the anaplastic variant of plasmacytoma is important to avoid erroneous diagnoses.