Highly Variable Presentation, Natural History, and Treatment in Sinonasal Phosphaturic Mesenchymal Tumors: Two Markedly Contrasting Cases

Background Phosphaturic mesenchymal tumor (PMT) is a polymorphous neoplasm frequently associated with tumor-induced osteomalacia (TIO), an FGF-23-mediated paraneoplastic syndrome that manifests as hypophosphatemia and bone demineralization secondary to urinary phosphate wasting. PMTs are notoriously difficult to localize and often require extensive work-up with delayed diagnosis. Sinonasal PMTs are extremely rare, representing <5% of cases. Only 17 cases have been described in the literature prior to 2014, with only two diagnosed without overt symptoms or signs of TIO (Deep et al). Here, we present two reports of sinonasal PMT, illustrating its highly variable clinical course.

Case Series Case 1 is a 64-year-old man with a history of colon cancer and bilateral nontraumatic hip fractures (both separately necessitating intramedullary nailing within the 3 preceding years) who presented to our institution with diffuse bone pain and weight loss. Pathologic fracture secondary to bony metastasis was suspected; however, NM bone scan showed diffuse increased uptake, pathologic fractures, and progressive osteoporosis without localization. Bone marrow biopsy was negative for hematologic malignancy or metastasis. Further work-up revealed inappropriate renal phosphate wasting and elevated FGF-23, consistent with PMT. Initial PET and octreotide scan did not reveal focal secreting tumor source, yet subsequent repeat imaging revealed a 1-cm tumor in the right frontal sinus with bony erosion and dural abutment. Definitive surgical management, notably 5 years after first suggestive manifestation (femur fracture), included combined endoscopic and open subcranial resection with abdominal fat graft and pericranial flap reconstruction. The patient had subsequent rapid and dramatic improvement of symptoms. His phosphorus level and imaging have remained stable through 16 months of follow-up.

Case 2 is a 68-year-old woman who presented with clear rhinorrhea, nasal obstruction, and new bifrontal headache. A bluish mass filling the right nasal cavity was identified on examination, and CT revealed a 6.9-cm heterogenous mass in the right nasal cavity with anterior skull base defect. MRI excluded intracranial communication, and biopsy of the mass revealed benign low-grade spindle cell neoplasm, most consistent with PMT. The patient underwent endoscopic tumor resection via ethmoidectomy, right sphenoidotomy, and right frontal sinusotomy just 5 weeks following her initial presentation and had an uneventful postoperative course. Notably, the patient had no history of osteoporosis or hypophosphatemia. Postoperative laboratories show normal phosphorus, with very mildly elevated FGF-23.

Discussion Complete surgical resection of PMT via endoscopic and/or open approach provides drastic relief of TIO-related symptoms and is potentially curative. Definitive diagnosis can be confirmed by elevated FGF-23 or tissue pathology. However, disease presentation is often nonspecific leading to frequent misdiagnosis, especially if TIO is mild, absent, or erroneously attributed to another disease process. In challenging cases, a multidisciplinary approach to diagnosis and treatment is critical, yet a minority of cases present without symptoms or are identified incidentally. Tumor characteristics predicting disease severity remain to be determined.

Conclusion Sinonasal PMT is rare with variable presentation ranging from asymptomatic to severe/prolonged phosphorus wasting, osteoporosis, and symptoms secondary to mass effect. This challenging diagnosis warrants low threshold for consideration among other more common pathologies.