August 2003: 47-year-old female with a 7-year history of osteomalacia and hypophosphatemia.

The August 2003 COM. A 47-year-old woman presented with a long history of muscle pain, weakness, and visual disturbances. Over the last year, she developed diplopia and left sixth nerve palsy. No other neuro-ophthalmologic abnormalities were found. Past medical and family history was unremarkable. Laboratory investigation disclosed hypophosphatemia, phosphaturia, elevated serum alkaline phosphatase activity, and normal serum calcium levels. CT scans showed a lobulated mass arising on the meningeal surface of the cavernous sinus, measuring 3x 2 x 2 cm. The lesion was partially resected and microscopic examination revealed a highly vascularized tumor composed of primitive mesenchymal cells arranged whether in a patternless-pattern or surrounding thin-walled, branching vascular spaces and thick-walled hyalinized vessels. Other eye-catching features were microcystic areas, multinucleated osteoclastic-like giant cells, scattered islands of mature adipocytes, foci of hemorrhage, thrombosed medium-sized-to-large vessels, and hemosiderin-laden macrophages. After surgery, the patient recovered from the muscle pain and weakness, with a significant improvement of serum calcium and alkaline phosphatase levels and phosphate blood levels. Taken together, the clinical and pathologic features were consistent with a diagnosis of phosphaturic mesenchymal tumor (mixed connective tissue variant) of the cavernous sinus in a setting of oncogenic osteomalacia. No further treatment was carried out. The patient has been followed for the last 4 years with no evidence of recurrent disease. Oncogenic osteomalacia is a rare clinical entity characterized by hypophosphatemia, phosphaturia, and a low concentration of 1,25-dihydroxyvitamin D(3) caused by a neoplasm. Pathologists should be aware of this entity, because surgical excision of the tumor is usually curative.