Phosphaturic Mesenchymal Tumor: A Rare Cause of Severe Osteomalacia and Debilitation in A 44-YEAR-OLD Man

ABSTRACT Objective: Tumor-induced osteomalacia (TIO), caused predominantly by phosphaturic mesenchymal tumor (PMT), is a rare paraneoplastic syndrome characterized by renal phosphate wasting and 1,25-dihydroxyvitamin D deficiency. Resection is curative; however, diagnosis is frequently delayed or missed due to the inherent characteristics of the tumor and poor recognition. Methods: We report the case of a 44-year-old male with PMT, with a focus on work-up progression and the elusiveness of diagnosis. Results: The patient presented with hip pain and difficulty in ambulation and was found to have numerous skeletal fractures and avascular necrosis of the hips. Serum laboratory studies showed very low phosphorus, normal calcium, and high parathyroid hormone levels. Ultrasound and nuclear imaging showed no parathyroid adenoma. 25-Hydroxyvitamin D level was low, suggesting a secondary hyperparathyroidism. This might have been the leading differential diagnosis; however, a mass was noted on the volar aspect of the patient's left hand. This was biopsied, and pathology demonstrated features consistent with PMT. A fibroblast growth factor 23 (FGF-23) level returned extremely elevated, confirming the diagnosis of TIO secondary to PMT. The mass was resected. Six weeks postresection, FGF-23 and phosphorus had returned to within normal limits, and the patient was improving clinically. Conclusion: PMT causes severe osteomalacia and debilitation in relatively young individuals; skeletal fractures and hypophosphatemia result in extreme pain and weakness. Resection is curative if the tumor is identified; however, TIO/PMT can be missed despite a thorough work-up if not specifically suspected. PMT should be considered any time a patient presents with osteomalacia and hypophosphatemia. Abbreviations: CT computed tomography; FGF-23 fibroblast growth factor 23; MRI magnetic resonance imaging; PMT phosphaturic mesenchymal tumor; PTH parathyroid hormone; TIO tumor-induced osteomalacia