Incidental detection of spondyloarthritis on FDG-PET/CT performed for unintentional weight loss

18F-fluorodeoxyglucose (FDG)-positron emission tomography/computed tomography (PET/CT) is frequently used to evaluate malignancy. However, FDG uptake is not cancer-specific and may also be seen in inflammatory rheumatic diseases. Recognition of musculoskeletal uptake patterns can prevent misdiagnosis and prompt targeted evaluation. A 60-year-old woman developed unintentional weight loss. Malignant lymphoma was suspected based on elevated serum C-reactive protein, axillary lymphadenopathy, and elevated soluble interleukin-2 receptor. FDG-PET/CT demonstrated prominent uptake in the sternoclavicular joints, both shoulders, wrists, and the sacroiliac joints. She presented with long-standing inflammatory back pain, and examination showed reduced lumbar mobility. Pelvic radiography revealed grade 3 sacroiliitis in the right sacroiliac joint, and cervical radiography showed bulky asymmetric paravertebral ossification changes. Magnetic resonance imaging demonstrated short tau inversion recovery hyperintensity in the sacroiliac joints. Because FDG uptake was also observed in the sternoclavicular joints, synovitis-acne-pustulosis-hyperostosis-osteitis syndrome was considered; however, no characteristic skin lesions were present. She was diagnosed with spondyloarthritis. Ixekizumab was initiated, resulting in rapid clinical improvement. This case illustrates that FDG-PET/CT performed for suspected malignancy can incidentally reveal a characteristic musculoskeletal uptake pattern of spondyloarthritis. That prompt recognition of this pattern enables timely initiation of effective interleukin-17A inhibition, leading to rapid improvement in systemic inflammation and clinical symptoms.