7149 Asymptomatic Tumor-Induced Osteomalacia: Reverse Diagnosis Of Hypophosphatemia Based On Pathologic Findings In A Tumor

Abstract Disclosure: K.G. Romo: None. B.L. Blazer, MD, PhD: None. E.W. Brien, MD: None. A. Khosravi, MD: None. Phosphaturic mesenchymal tumors (PMT) are rare and often occult tumors associated with a renal phosphorus wasting paraneoplastic syndrome known as tumor-induced osteomalacia (TIO) caused by tumor secretion of fibroblast growth factor (FGF) 23. FGF23 causes hypophosphatemia by increasing phosphate loss from the kidneys as well as lowering production of 1,25 dihydroxyvitamin D. As a result of hypophosphatemia, PMTs are associated with symptoms of fatigue, muscle weakness, bone pain, and fractures. Most occur in middle-aged adults with the extremities being a common location, though they may occur in any soft-tissue or osseous site. Novel chromogenic in situ hybridization assays for FGF23 have higher sensitivity and specificity than former PCR methods for detecting PMTs. The following presents a case of an asymptomatic man with a distal forearm mass who was found to have a PMT initially identified on biopsy of the lesion. A 67-year-old-man presented for evaluation of a right (R) distal forearm lesion. He noticed the mass 2 years prior to evaluation when his wrist ached while playing golf. He had no further pain around the lesion, no muscle weakness, no issues with ambulation, fractures, height loss, or fatigue. Vital signs were normal. Physical exam noted a palpable 1.5 cm firm mass on the volar aspect of his distal R forearm, no kyphosis, and normal strength. Initial radiograph was unrevealing, subsequent MRI noted R distal forearm 1.5 x 1.0 x 2.2 cm lesion with post-contrast peripheral enhancement. Biopsy revealed chondromyxoid neoplasm with features suggestive of PMT with positive chromogenic in situ hybridization assay for FGF23. Labs revealed slightly low to low-normal serum phosphorus, but normal serum creatinine, calcium, PTH, 1,25 vitamin D, alkaline phosphatase and (inappropriately normal) FGF23. He had normal urinary calcium and a slightly low tubular maximum reabsorption of phosphate to glomerular filtration rate ratio (TMP/GFR). Bone density scan was normal. Status-post wide margin resection was successful with post-operative normalization of serum and urine phosphorus and excision pathology consistent with PMT, no fusion identified. A whole-body PET/CT scan with dotatate was deferred given location of PMT was known and risk of multiple PMTs or malignancy was deemed low given serum phosphorus normalized post-resection and patient remained asymptomatic on follow-up. Typically, the constellation of symptoms of hypophosphatemia leads to the investigation/diagnosis of PMT. Here we present a case of reverse diagnosis of hypophosphatemia based on pathologic findings in a tumor. Additionally, with increased awareness of PMT and better methods for detecting FGF23 by pathology, we are now seeing earlier manifestations of TIO and associated hyperphosphaturia in which serum phosphorus may be still normal or mildly low and patients may have no or mild symptoms. Presentation: 6/1/2024