Tumor-Induced Osteomalacia in a Patient With FGF23-Amplified Lung Adenocarcinoma and FGF23-Deleted SCLC: Case Report

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by ectopic fibroblast growth factor-23 (FGF23) production. We report a unique case of a 78-year-old female patient with refractory hypophosphatemia, ultimately diagnosed as TIO, in the context of two metastatic primary lung cancers: adenocarcinoma and SCLC. Molecular analyses of tumor samples highlighted <i>FGF23</i> amplification in one adenocarcinoma sample and <i>FGF23</i> deletion in one SCLC sample, suggesting a potential link between tumor <i>FGF23</i> molecular alterations and elevated serum FGF23 levels. This case underscores the complexity of diagnoses and management of TIO when associated with solid tumors and highlights the need for awareness of this condition to prevent diagnostic delays. Future research should explore the mechanisms linking <i>FGF23</i> alterations and cancer progression and evaluate targeted therapies for TIO in the context of resistant metastatic cancers.