Sinonasal Phosphaturic Mesenchymal Tumor Presenting With Tumor-Induced Osteomalacia: A Rare Case and Review of the Literature.

Phosphaturic mesenchymal tumor (PMT) is a rare tumor involving bone and soft tissues, usually benign and solitary. They mainly affect the extremities and are rarely seen in the head and neck region. They are most commonly associated with tumor-induced osteomalacia (TIO). TIO, also known as oncogenic osteomalacia, is a paraneoplastic syndrome in which tumor cells secrete hormones that reduce mineralization of mature bone, thereby decreasing osteoblastic activity and causing renal phosphate wasting. One of the most common hormones involved is fibroblast growth factor-23 (FGF-23), which belongs to a group of phosphatonins. These tumors are seen in middle-aged individuals and present with symptoms and signs suggestive of stress fractures. The investigative modalities include blood investigations, such as serum FGF23, which is highly suggestive of PMT. Serum phosphorus levels are also indicative. Conventional imaging reveals multiple insufficiency fractures and decreased osseous density. Molecular imaging, such as positron emission tomography/computed tomography (PET/CT), can help detect occult lesions. The confirmatory test is histopathology. Once diagnosed, the treatment of choice is surgical excision. We present a case of a 61-year-old male who presented with multiple stress fractures and was initially treated for arthropathy. However, on further investigations, a diagnosis of sinonasal PMT was made, and the tumor was removed surgically. Post-surgery, the patient improved in terms of symptoms and biochemical parameters, with no paraneoplastic syndrome. The histopathology report confirmed the diagnosis.