Purpose <p>To report a rare case of lumbosacral instability following a right external hemipelvectomy for malignant peripheral nerve sheath tumor (MPNST), successfully treated with rigid sacropelvic fixation using a combination of a unilateral iliac screw and an S2 alar-iliac screw.</p> Methods <p>MPNST is a malignant soft tissue sarcoma with a high recurrence rate and poor prognosis. A 44-year-old woman was diagnosed with MPNST in the right pelvis. Despite multiple courses of chemotherapy, the tumor lesion continued to grow. A right external hemipelvectomy was performed, and histopathological analysis confirmed the diagnosis of spindle cell sarcoma consistent with MPNST. Although the tumor was radically resected, the patient developed severe low back pain due to instability at L5-S1.</p> Results <p>Sacropelvic fixation was performed using a unilateral iliac screw and an S2 alar-iliac screw. Intraoperative computed tomography (CT) navigation and virtual reality (VR) images proved invaluable for preoperative planning and for identifying anatomical landmarks during the surgery. The patient had a favorable outcome, with no recurrence observed six years postoperatively.</p> Conclusion <p>Rigid sacropelvic fixation using a combination of a unilateral iliac screw and an S2 alar-iliac screw was successfully performed for lumbosacral instability following the external hemipelvectomy for MPNST, aided by intraoperative CT navigation and VR imaging technologies. Stable fixation was achieved, with no recurrence of MPNST.</p>