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Background: Oncogenic osteomalacia is a rare paraneoplastic syndrome, with only about 100 reported cases in the literature.  It is characterized by renal phosphate wasting, hypophosphatemia, low functional vitamin D, normocalcemia, normal PTH, and increased levels of alkaline phosphatase.  The majority of oncogenic osteomalacia is usually caused by phosphaturic mesenchymal tumors of mixed connective tissue. 

Case Description: In this case report, the authors present a case of oncogenic osteomalacia in a patient who initially presented with hip pain for bilateral femoral head osteonecrosis and was noted to have multiple insufficiency fractures of unknown cause.  The patient was eventually worked up with a PET/CT scan which showed a 2cm tumor in his right proximal femur.  A core biopsy of this lesion revealed it was a mesenchymal tumor.  Given the patient’s osteonecrosis of the bilateral proximal femurs, we chose to treat him with a right total hip arthroplasty.   As part of this surgery requires making a femoral neck cut and removing the head and neck of the femur, the femoral lesion was completely excised, and this led to resolution of his hypophosphatemia, and improvement in his symptoms.

Literature Review: There have been only three case reports of a similar lesion in the proximal femur causing oncogenic osteomalacia in the literature, and each was treated differently with varying outcomes.

Clinical Relevance: Although an extremely rare disease, clinicians and pathologists should be aware of oncogenic osteomalacia and phosphaturic mesenchymal tumors as a cause of unexplained insufficiency fractures in patients with hypophosphatemia.