Subcutaneous extraskeletal osteosarcoma of the gluteal region: a case report from a resource-limited setting

Background Extraskeletal osteosarcoma (ESOS) is a rare high-grade malignant mesenchymal tumor that produces osteoid in soft tissue without skeletal involvement. Subcutaneous ESOS is exceptionally uncommon and may mimic benign soft tissue lesions. Case presentation A 45-year-old Samoan man presented with a one-year history of a progressively enlarging, painless left gluteal mass. Contrast-enhanced CT showed a well-circumscribed subcutaneous lesion measuring 9.1 × 10.4 × 8.6 cm, with mild to moderate delayed enhancement and no osseous involvement. The tumor was surgically excised. Histopathology showed a spindle to pleomorphic high-grade sarcoma with osteoid produced by atypical tumor cells. Immunohistochemistry showed SATB2 positivity and negative staining for S100, SOX10, pan-cytokeratin, and ERG. The tumor involved the deep resection margin, consistent with R1 resection; the closest margin distance was not specified. The patient declined adjuvant therapy and was subsequently lost to follow-up. Conclusions Subcutaneous ESOS is rare and diagnostically challenging. This case highlights the importance of imaging, histopathology, immunohistochemistry, and multidisciplinary management, particularly in resource-limited settings.