RESUMEN
Phosphaturic mesenchymal tumor is an extremely rare disease and is frequently associated with oncogenic osteomalacia showing paraneoplastic syndrome, which is characterized by phosphaturia, hypophosphatemia, normocalcemia, and decreased levels of 1,25-dihydroxyvitamin D3 associated with a tumor. A 2-year-old boy, who had a soft tissue tumor on his right thigh and previously diagnosed as myositis ossificans at 9-months-old, was presented with rachitic rosary and mildly enlarged tumor. Biochemical investigations showed hypophosphatemia, hyperphosphaturia, and an increased alkaline phosphatase level of 440 U/l (25-100 U/l), suggesting rickets, which was resistant to vitamin D dietary supplementation. We were certain of intractable rickets because of oncogenic hypophosphatemia and thus decided to excise the soft tissue mass. We observed laboratory improvement of rickets after 2 weeks. On the basis of surgical and histopathological examinations, the tumor was finally diagnosed as the phosphaturic mesenchymal tumor.