Phosphaturic mesenchymal tumour, connective tissue variant: a rare radiological presentation involving the radius mimicking fibrous dysplasia with oncogenic rickets.

Phosphaturic mesenchymal tumours (PMTs) are rare neoplasms that secrete fibroblast growth factor-23 (FGF-23), causing tumour-induced osteomalacia (TIO). Histological overlap with juvenile psammomatoid ossifying fibroma (JPOF) can lead to diagnostic difficulty, particularly outside the craniofacial skeleton. A 4-year-old girl presented with a painful forearm swelling. Radiographs demonstrated a fibro-osseous lesion in the radius and biopsy was initially non-diagnostic, leading to a presumptive diagnosis of fibrous dysplasia. Over the next 6 years, the lesion enlarged, and at age 10, the patient developed genu valgum with biochemical evidence of hypophosphataemic rickets (low phosphate, raised alkaline phosphatase, elevated FGF-23). Medical therapy corrected the rickets, though deformity required guided growth surgery. At age 13, a repeat biopsy of the enlarging lesion revealed a fibro-osseous tumour with spindle stroma and psammomatoid ossicles, negative for GNAS mutation, and consistent with a phosphaturic mesenchymal tumour, connective tissue variant. This case highlights a rare radiologic presentation of PMT involving a long bone, with imaging features closely mimicking fibrous dysplasia and histological overlap with JPOF. The rapid progression of imaging findings, discordant metabolic abnormalities and markedly elevated FGF-23 levels poses a diagnostic challenge. As such, fibrous dysplasia-like lesions of the extremities in the setting of hypophosphataemic rickets should prompt consideration of PMT.