Giant Cell Tumor of the Sacrum With Secondary Changes of an Aneurysmal Bone Cyst: A Case Report

Giant cell tumor (GCT) of bone is a typically benign but locally aggressive neoplasm that occurs between the ages of 20 and 40 years, and most commonly arises in the epiphysis of long bones, with sacral involvement being rare and diagnostically challenging due to its insidious onset and nonspecific symptoms. We report the case of a 21-year-old woman who presented with progressive low back pain radiating to both lower limbs, associated with perianal numbness and bilateral heel pain aggravated by physical activity. Imaging revealed a well-defined, expansile osteolytic lesion involving the S1-S2 vertebrae with features suggestive of a GCT. Magnetic resonance imaging (MRI) demonstrated an avidly enhancing mass extending into the spinal canal, causing compression of the thecal sac and adjacent nerve roots, more pronounced on the right side, along with fluid-fluid levels consistent with secondary aneurysmal bone cyst-like changes. Histopathological examination confirmed the diagnosis of GCT with secondary aneurysmal bone cyst transformation. Sacral GCTs are rare, locally aggressive lesions that often present with neurological symptoms, and imaging typically shows an expansile "soap-bubble" osteolytic lesion with possible soft-tissue extension and secondary cystic changes. Surgical excision remains the mainstay of treatment, with preoperative transarterial embolization playing an important role in reducing tumor vascularity and intraoperative blood loss. This case highlights the importance of including GCT in the differential diagnosis of sacral lesions in young adults and underscores the role of timely imaging and biopsy in establishing an accurate diagnosis and guiding appropriate management.