SAT-337 Ga-DOTATATE and Tumor Induced Osteomalacia: Finding the Culprit Lesion

Introduction Tumor-induced osteomalacia (TIO) is a very rare paraneoplastic disorder caused by tumors that produce fibroblast growth factor 23 (FGF23) resulting in phosphate wasting and inadequate bone mineralization. Complete resection can be curative; however, these tumors are typically difficult to identify due to size and location. Clinical case A 43-year-old man was referred to Endocrinology for evaluation of recurrent fractures and hypophosphatemia. First stress fracture was diagnosed at age 41, followed by musculoskeletal pain in several locations. Lower extremity MRI showed chronic left fibular stress fracture, new stress fracture in fifth metatarsal and right tibia. Tc99m-MDP bone scan revealed multiple foci of increased tracer uptake in bilateral ribs, hips, and lower extremities. Laboratory evaluation showed normal calcium (9.3 mg/dL, normal range [NR]: 8.7-10.2), low phosphorus (1.5 mg/dL, NR: 2.5-4.7), low 1,25-dihydroxyvitamin D (13 pg/mL, NR: 19.9-79.3), low 24-hour urine calcium (78mg), high phosphate excretion fraction in urine (27%, normal <5%), high ALP (163 U/L, NR: 38-126), and high FGF23 (238 RU/mL, NR<180). 25 OH vitamin D (36 ng/mL) and iPTH (5.7 pmol/L, NR: 1.6-6.9) were normal. Patient was started on calcitriol and phosphate supplements. Due to concern for TIO, a PET scan with 68Ga-DOTATATE was performed which showed multiple somatostatin avid lesions concerning for metastatic disease. However, after re-review with radiology, it was felt that other areas of uptake were due to fractures and not tumor given remarkably higher SUV in left acetabular lesion (SUV 20 vs 4-5). The left acetabular lesion was biopsied, followed by surgical resection. Pathology was consistent with phosphaturic mesenchymal tumor with uninvolved margins. FGF23 normalized within 24 hours after surgery (127 RU/mL) and calcitriol and phosphate supplements were discontinued on post-operative day 10. Clinical lesson TIO is a rare paraneoplastic syndrome commonly caused by phosphaturic mesenchymal tumors that secrete FGF23. Once the diagnosis of TIO is confirmed, the tumor is localized by anatomical or functional imaging. 68Ga-DOTATATE scan is currently the imaging modality of choice for localization. However, there are other pathologic processes, such as fractures, that could affect the interpretation of PET scans. Osteoblastic activity is increased in fractures, which results in increased uptake in Ga-DOTATATE PET scan since osteoblasts express somatostatin receptor. Our patient was initially thought to have multiple avid lesions concerning for metastatic disease, but culprit lesion was differentiated based on SUVs and confirmed with biopsy. Clinical and biochemical abnormalities resolved after surgery. Early recognition of culprit lesion in TIO is crucial, as successful surgery is curative and would lead to significant improvement in the quality of life of patients.