Hypophosphatemic osteomalacia caused by diffuse tenosynovial giant cell tumor misdiagnosed as spondyloarthritis: a case report.

Tumor-induced osteomalacia (TIO) is an acquired hypophosphatemic osteodystrophy caused by increased renal phosphate excretion due to tumors. Tenosynovial giant cell tumor (TGCT) is a benign mesenchymal tumor but rarely causes hypophosphatemia. TIO is a clinically rare disease, and its clinical manifestations often overlap with rheumatic diseases such as spondyloarthritis (SpA), leading to misdiagnosis. A 32-year-old female had a prolonged misdiagnosis of undifferentiated spondyloarthritis due to chronic low back pain, limb weakness, and spinal deformity, with poor response to non-steroidal anti-inflammatory drugs and various biologic agents. After hospitalization, a systematic biochemical screening for metabolic bone disease revealed decreased serum phosphate, calcium, and 25-hydroxyvitamin D levels, along with elevated alkaline phosphatase. Subsequently, a previously undetected tumor in the right plantar region was successfully localized using 68Ga DOTA-TATE PET/CT. After complete surgical removal of the tumor, the patient's serum phosphate levels rapidly returned to normal, and bone pain symptoms were significantly alleviated. After histopathological staining of the tumor, it was considered to be diffuse tenosynovial giant cell tumor (D-TGCT). This case clearly demonstrates a TIO caused by the rare disease D-TGCT. Its diagnosis and treatment journey not only reveals the clinical relevance of D-TGCT as an atypical pathogenic tumor but also highlights the critical importance of multidisciplinary collaboration and advanced diagnostic techniques in tackling such rare diseases. This provides clear guidance for enhancing clinical awareness and optimizing treatment strategies.