Oncogenic rickets diagnosed at age 8 and the risk of persistent rickets: a rare case of pediatric-onset tumor-induced osteomalacia.

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excess fibroblast growth factor 23 (FGF23), leading to hypophosphatemia and osteomalacia. It typically manifests in adulthood, with pediatric cases being exceedingly rare. Early diagnosis is critical to prevent irreversible skeletal deformities. We report the case of a 19-year-old man with severe lower limb deformities and loss of ambulation since childhood. Initially misdiagnosed with X-linked hypophosphatemia (XLH) and treated with burosumab, genetic testing ruled out hereditary hypophosphatemic disorders. Further evaluation revealed an FGF23-secreting mesenchymal tumor in the right femur, confirming TIO. Surgical resection of the tumor led to biochemical remission, while burosumab treatment contributed to pain relief, functional improvement, and increased bone mineral density. Histological examination suggested potential tumor modifications linked to burosumab exposure. This case highlights the diagnostic challenge of pediatric-onset TIO, emphasizing the importance of considering oncogenic rickets in cases of early hypophosphatemic osteomalacia with severe deformities. The risk of persistent skeletal abnormalities despite treatment underscores the need for early recognition and intervention. Moreover, burosumab showed clinical efficacy in managing hypophosphatemia and symptoms, suggesting a therapeutic role in TIO when surgery is delayed or inoperable. Pediatric-onset TIO is an underrecognized entity that can lead to severe disability if not diagnosed early. This case underscores the importance of early tumor identification.