Phosphaturic mesenchymal tumor of the right thigh

Phosphaturic mesenchymal tumors (PMTs) are rare neoplasms often associated with tumor-induced osteomalacia (TIO). They present diagnostic challenges due to their rarity and nonspecific symptoms. A 44-year-old male presented with a one-year history of back pain, eight-month history of difficulty walking, and swelling over the right upper thigh. MRI of the dorsolumbar spine revealed subtle disc bulges and mild vertebral hypodensity. A bone scan suggested seronegative spondyloarthropathy. Positron emission tomography - Computer tomography (PET-CT) showed a somatostatin receptor-expressing lesion in the right proximal thigh. MRI confirmed a soft tissue lesion. Surgical excision and histopathology confirmed PMT. Preoperative serum phosphorus was low (1.6 mg/dL), and fibroblast growth factor 23 (FGF23) was elevated (352.4 pg/mL). Postoperative serum phosphorus normalized, and the patient received denosumab treatment. Follow-up over one year showed the patient remained asymptomatic with normal serum markers. This case highlights the importance of considering PMT in patients with persistent musculoskeletal pain and hypophosphatemia. Complete surgical excision is curative, and adjunct therapies like denosumab can be beneficial.