The American Cancer Society estimates that one in 130 adults will be diagnosed with multiple myeloma (MM) at some point in their lifetime. Extramedullary plasmacytoma is a rare manifestation of multiple myeloma. Intracranial, extramedullary plasmacytoma is an extremely rare presentation of this condition that can lead to diagnostic confusion and may be confused to solid neoplasm metastasis, hemangiopericytoma, or even a subdural hematoma depending on the lesion’s location.

We report the case of a 79-year-old female with known MM who presented with four months of slowly progressive gait instability, falls, and dizziness. On presentation, the patient was found to have a non-focal neurologic exam and initial CT head revealed a 1.2 cm subdural collection with 1.4 cm of midline shift and associated sulcal effacement that was felt to most likely represent subdural mass vs. subdural hematoma. She was taken to the operating room where a subdural mass was encountered and intraoperative pathology was consistent with plasmacytoma. The patient underwent radiation therapy post-operatively, and was noted to be doing well until being found with decreased consciousness after three radiation treatments. CT head demonstrated severe worsening of cerebral edema for which she was transferred to the Neuro ICU and ultimately treated with hyperosmolar therapy until her mental status improved. She progressed well following this and was discharged to post-acute rehabilitation.

This case highlights a rare presentation of extramedullary plasmacytoma of the subdural space. A broad differential and high index of suspicion for neoplastic process were key in establishing the patient’s diagnosis in our case and helped to inform the treatment approach, which consisted of biopsy followed by radiation therapy. In addition, our case highlights the potential complications that can arise with radiation therapy for subdural plasmacytoma, namely the large degree of edema that was encountered after relatively few radiation treatments.