Extramedullary plasmacytoma of the tibial nerve: illustrative case.

backgroundExtramedullary plasmacytomas (EMPs) are rare plasma cell tumors forming outside the bone marrow. EMPs spread to soft tissues with a predilection for the head and neck region. Therefore, EMPs are rarely included in the differential for peripheral nerve sheath tumors (PNSTs). The authors discuss a case of tibial nerve sheath invasion by an EMP in the context of multiple myeloma (MM). OBSERVATIONS: A 70-year-old female with MM presented with right leg pain and a suspected PNST of the tibial nerve. Imaging revealed a T1-hypointense lesion in the popliteal fossa. Resection was attempted, but frozen pathology revealed a plasma cell neoplasm with a high Ki-67 proliferation rate. The resection was halted due to the tumor's infiltrative and hemorrhagic nature and its extensive encasement of the tibial nerve. Postoperatively, the patient received 5 fractions of radiation therapy and three courses of teclistamab, which she continued as an outpatient without complications. LESSONS: This case highlights a rare occurrence of peripheral nerve invasion by an EMP with the first documented involvement of the tibial nerve. This report suggests that EMPs should be considered in the differential diagnosis of PNSTs, particularly in MM patients. The present case underscores the need for intraoperative biopsies in PNST resection, as imaging features alone may not be fully diagnostic. https://thejns.org/doi/10.3171/CASE25104.