Unveiling the unexpected: Skeletal sarcoidosis with an extraosseous soft-tissue mass in the ligamentous portion of sacroiliac joint.

Sarcoidosis is a chronic granulomatous, multisystem disease of uncertain etiology that affects various organ systems, although musculoskeletal involvement is rarely the dominant feature. We present the case of a woman in her 50s with a history of pulmonary sarcoidosis who developed new lower back and hip pain. Magnetic resonance imaging demonstrated an asymmetric soft-tissue mass in the ligamentous left sacroiliac joint, along with osseous involvement of the sacrum and ilium. These findings were suspicious for malignancy. An 18F-FDG PET/CT subsequently confirmed FDG-avid disease in this soft-tissue mass and the surrounding bone. However, a CT-guided biopsy of the lesion revealed non-caseating granulomas. This case highlights a rare presentation of skeletal sarcoidosis with an extraosseous mass, a diagnostic pitfall that can closely mimic malignancy or inflammatory sacroiliitis.