Successful Superficial Blood Sampling to Localize a Fibroblast Growth Factor-23-Producing Tumor

Rare disease Background:Tumor-induced osteomalacia (TIO) is a paraneoplastic syndrome caused by aberrant fibroblast growth factor-23 (FGF-23)-producing tumors.Early surgical resection is the optimal strategy for preventing TIO progression.Thus, tumor localization is a priority for successful treatment.A simple and safe examination method to identify functional endocrine tumors is essential to achieve better outcomes in patients with TIO. Case Report:A 64-year-old Japanese man with recurrent fractures, hypophosphatemia, and elevated alkaline phosphatase and FGF-23 levels (109 pg/mL) was admitted to our university hospital and was diagnosed with FGF23-related hypophosphatemic osteomalacia.Notably, the superficial dorsal vein in the patient's left foot exhibited a high FGF-23 level (7510 pg/mL).Octreotide and 18 F-fluorodeoxyglucose (FDG) scintigraphy and systemic venous sampling revealed that the tumor in the third basal phalanx of the left foot was responsible for FGF-23 overproduction.Tumor resection resulted in a rapid decrease in serum FGF-23 levels and an increase in serum phosphorus levels. Conclusions:Octreotide scintigraphy, FDG-positron emission tomography, and systemic venous sampling are the standard methods for localizing functional endocrine tumors.However, the limited availability and invasive nature of these examinations hinder effective treatment.Here, we highlight the importance of peripheral superficial blood sampling as an alternative to conventional systemic methods for confirming the presence of FGF-23producing tumors.Clinicians should consider TIO as a potential cause of acquired hypophosphatemic osteomalacia.Furthermore, peripheral superficial vein blood sampling may be useful for confirming the localization of FGF-23-producing tumors.