Burosumab-responsive micro tumor-induced osteomalacia caused by a 6-mm phosphaturic mesenchymal tumor of the femoral head: a case report with review of literature on small (≤10 mm) FGF23-producing tumors.

A woman in her thirties presented with progressive bone pain, muscle weakness, recurrent stress fractures, and delayed fracture healing. Laboratory investigations demonstrated hypophosphatemia, renal phosphate wasting, elevated fibroblast growth factor 23 (FGF23), and inappropriately low 1,25-dihydroxyvitamin D, leading to a diagnosis of FGF23-mediated tumor-induced osteomalacia (TIO) approximately one year after symptom onset. Bone scintigraphy demonstrated multiple fractures involving the ribs and pubic bones. [18F]-fluorodeoxyglucose positron emission tomography/computed tomography ([18F]-FDG PET/CT) identified mild uptake in the left femoral head/femoral neck region, and targeted magnetic resonance imaging (MRI) confirmed a well-defined 6-mm lesion with low signal intensity on T1- and T2-weighted images and high signal intensity on short tau inversion recovery images. Somatostatin receptor scintigraphy (SRS) showed no abnormal uptake. Systemic venous sampling (SVS) demonstrated elevated FGF23 levels in the left pelvic venous drainage region. CT-guided biopsy revealed a phosphaturic mesenchymal tumor (PMT) with FGF23 immunohistochemical positivity. Complete resection would likely have required hemiarthroplasty or total hip arthroplasty because of the tumor location, and the patient declined surgery. Burosumab was therefore initiated. At 24 weeks, serum phosphate increased from 1.5 to 4.1 mg/dL, alkaline phosphatase decreased from 145 to 100 U/L, generalized bone pain and gait disturbance improved markedly, pseudofracture-related uptake disappeared on bone scintigraphy, and lumbar-spine and femoral-neck bone mineral density (BMD) improved. This case shows that multimodal localization can identify micro-TIO lesions that may escape conventional strategies, and that FGF23 blockade can be clinically effective in a localized FGF23-positive 6-mm PMT when surgery is anatomically challenging or declined.