Transformation of a long-standing phosphaturic tumor-inducing osteomalacia into malignancy.

Key Clinical Message: Tumor-induced osteomalacia is a paraneoplastic syndrome characterized by renal phosphate wasting and deranged bone turnover. Clinicians should consider tumor-induced osteomalacia in unexplained hypophosphatemia and investigate for underlying tumors. Abstract: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by renal phosphate wasting, which leads to deranged bone turnover. TIO is usually associated with benign mesenchymal tumors, although it has also been reported in malignant tumors. We present the case of a 56-year-old individual who experienced a protracted 6-year clinical course characterized by hypophosphatemia, weakness, and kyphosis, alongside the presence of a foot tumor. Subsequently, this lesion displayed malignant behavior and was ultimately diagnosed as a high-grade sarcoma. To date, this case is among the 10 reported cases in the literature of a mesenchymal tumor associated with TIO undergoing malignant transformation. This report underscores the importance of a comprehensive evaluation of patients with unexplained hypophosphatemia and highlights the need for diligent follow-up to detect possible malignant transformation of the underlying tumor. Clinicians should consider TIO in the differential diagnosis of hypophosphatemia and promptly investigate for the presence of an underlying tumor, as early detection may improve the patient's prognosis.