Diagnosis of TIO: Is serum FGF23 Measurement always the answer?

Considering the limited availability of FGF-23 marker in our institutions, she was referred for a 68-Ga DOTANOC PET CT scan on suspicion of any neuroendocrine tumor that could be the cause of tumour-induced osteomalacia (TIO). Grossly the lesion was well circumscribed, 2 cm in maximum dimension which under light microscopy showed spindle cells in haphazard pattern, in a collagenous stroma [Figure 2]a and [Figure 2]b. By Immunohistochemistry, the spindle cells were positive for Vimentin [Figure 2]c and few clusters were positive for CD68 [Figure 2]d. Based on Histomorphological and immunohistochemical features, the diagnosis of Benign Fibrous Histiocytoma was rendered. In most cases of TIO, small, benign, mesenchymal soft tissue tumors are major determinants for increased FGF23 level production in the body. Besides causing phosphaturia, FGF23 inhibits renal 1α-hydroxylase, the enzyme that converts 25-hydroxy vitamin D to its active form, 1,25-dihydroxy vitamin D.[1] Clinical features of bone pains, fatigue, proximal myopathy, and fragility fractures are noticed in most cases encountered.