ESPE Abstracts

Objectives: This report presents the unusual case of an 8-year-old boy with tumor-induced osteomalacia (TIO) characterized by suppressed FGF-23 levels.

Case Presentation: An 8-year-old boy presented with a six-month history of left knee pain, exacerbated by walking. Physical examination revealed genu valgus without signs of inflammation. X-rays indicated rickets, prompting a referral to a pediatric endocrinology clinic. Laboratory tests revealed normal calcium and magnesium, hypophosphatemia, hyperphosphaturia, markedly elevated alkaline phosphatase, and suppressed FGF-23 levels. Whole exome sequencing revealed no pathogenic mutations. Despite treatment with phosphate supplements, vitamin D, and calcitriol, phosphate levels remained low. Advanced imaging with 68Ga-DOTATE PET-CT and MRI identified a focal DOTATE-avid lesion measuring 0.6x0.2x2.6 cm in the intracortical region of the anterior cortex of the distal right femur. The patient underwent surgical resection of the lesion using O-arm navigation, followed by reconstruction with a fibular allograft. Pathological findings reported proliferation of blood vessels and spindle cell lesions in the distal femur, with staining for CD56 and SATB2 confirming TIO from a phosphaturic mesenchymal tumor. Postoperatively, phosphorus levels normalized within four days, and clinical symptoms improved significantly within four weeks, with complete recovery observed at six months.

Conclusions: This case highlights that tumor-induced osteomalacia can occur with suppressed FGF-23 levels. Therefore, in instances of acquired hypophosphatemic rickets, the absence of elevated FGF-23 does not exclude TIO, as other phosphatonins can contribute to hypophosphatemia, although this is rare. TIO, in addition to overproducing FGF-23, can also secrete other phosphatonins such as secreted frizzled-related protein-4 (SFRP4), FGF7, dentin matrix acidic protein 1 (DMP1), and matrix extracellular phosphoglycoprotein (MEPE), resulting in suppressed FGF-23 levels.