We report a case of tumor-induced osteomalacia (TIO) caused by a massive phosphaturic mesenchymal tumor (PMT) of the acetabulum. A 68-year-old woman presented with progressive bone pain of the rib cage, and polyarthralgia and back pain for 3 years. She was diagnosed with hypophosphatemic osteomalacia because laboratory testing was remarkable for low serum phosphorus and a low level of 1,25(OH)2 vitamin D. Three years later, her hip radiograph revealed an osteolytic lesion of the acetabulum. Magnetic resonance imaging of the acetabulum showed a massive lesion. Laboratory data showed hypophosphatemia and an elevated serum level of fibroblast growth factor 23 (FGF-23). Samples obtained with open biopsy showed a low-grade spindle cell neoplasm with FGF-23 positivity, identified by using immunohistochemical staining, confirming the diagnosis of a PMT mixed connective tissue variant. Curettage of the tumor was performed, and the defects were filled with bone allografts. The hip joint was reconstructed with total hip arthroplasty using a Muller support ring. To our knowledge, this report represents the first documented case of massive PMT of the acetabulum causing TIO.
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