Abstract
BACKGROUND: Plasma cell neoplasms can present as solitary plasmacytomas or multiple myeloma. Osseous solitary plasmacytomas commonly affect the axial skeleton and are treated with radiation therapy; surgery is needed as an adjunct to address instability or progressive neurological symptoms. Traditional open spinal fixation poses risks of increased infection and wound dehiscence. Minimally invasive surgery may minimize these challenges and limit delays in definitive radiation therapy. OBSERVATIONS: A 61-year-old male presented with chronic low back pain with intermittent shooting pain along the S1 distribution to the bilateral lower extremities. MRI revealed a large, expansile sacral mass causing severe spinal canal stenosis, and CT demonstrated marked bony destruction. Biopsy confirmed plasma cell neoplasm. Because of his progressive instability, the patient underwent minimally invasive spinopelvic fixation using augmented reality (AR) guidance, with dual S2-alar-iliac screws for stabilization. Postoperatively, he received intensity-modulated radiation therapy, 4500 cGy, 180 cGy per fraction in 25 fractions per day for a month. Imaging at 5 months demonstrated unchanged bony erosion, stable instrumentation, and symptom improvement. LESSONS: Progressive pain due to instability in patients with osseous solitary plasmacytoma may necessitate surgical stabilization. Minimally invasive AR-guided lumbopelvic fixation offers an effective stabilization strategy, minimizing postoperative recovery and time to definitive therapy. https://thejns.org/doi/10.3171/CASE25634.