ESPE Abstracts

Introduction: Hypophosphatemic rickets is usually due to genetic causes but in rare cases it can develop secondary to MAS, Fanconi syndrome, or oncogenic causes. Oncogenic osteomalacia, also called Tumor-Induced Osteomalacia (TIO), is a rare acquired paraneoplastic syndrome that develops as a result of excessive phosphate loss from renal tubules by FGF23 released from tumour tissue. In addition to the clinical features of rickets, gait disturbances, growth retardation, and skeletal deformities may develop.

Case: A 16-year-old male boy who had been completely healthy until 13 years of age underwent surgery for leg pain and limping that began 3 years ago and was found to have a cyst in the proximal right tibia. He developed severe deformities due to recurrent fractures after surgery and applied to us for further examination. He had painful deformities, tachypnea, and tachycardia on physical examination. The results of the laboratory tests are summarized in Table 1. Due to severe hypophosphatemia, oral phosphorus (up to 55 mg/kg/d), calcitriol (up to 60 ng/kg/d) and calcidiol (1200 U/day) were started. MRI of the right knee was taken to determine the possible location of the tumour and GA-68-DOTATATE-PET was performed due to the suspicious appearance of the mass and metastasis. An 8.5x7.5 cm heterogeneous mass with minimal involvement in PET was detected in the right knee. In biopsy, a phosphaturic mesenchymal tumour with osteoblastoma-like appearance was detected. Surgical resection was planned.

Conclusion: TIO is a rare paraneoplastic syndrome that presents clinical symptoms after overexpression of FGF-23 in benign mesenchymal tumours. TIO is usually diagnosed late and difficult to localize because of their slow progression and small size. Extensive resection of the tumour is very important for both the chances of cure and prevention of recurrence. It should be included in the differential diagnosis of acquired hypophosphatemia. This case presented due to its severity and large mass.