MON-920 Pediatric Parathyroid Carcinoma Presenting With Acute Progressive Genu Valgum

BACKGROUND: Parathyroid carcinoma (PC) is exceptionally rare in children and its clinical features are poorly understood. Available reports have identified that very high serum calcium and parathyroid hormone levels would be expected in this disease. Definitive therapy for PC requires en bloc resection of the parathyroid tumor to reduce the risk of disease recurrence. Unfortunately, there are very few treatment options for metastatic PC and patients with recurrent PC have a high risk of mortality related to their disease. Because of the rarity of the condition, there are very limited data including pediatric patients. Case: A previously healthy 13-year-old girl initially presented to primary care with bilateral foot and ankle pain in the context of acute and progressive genu valgum, referred to the orthopedic clinic. Lab evaluation prompted by imaging demonstrated severe hypercalcemia (15.6 mg/dL) and hyperphosphatasia (Alkaline phosphatase 2056 units/L). She was urgently referred to our clinic where we found her to have a firm right sided neck mass within the thyroid. Lab testing at that time confirmed profound hyperparathyroidism (PTH 2970 pg/mL). Neck ultrasound confirmed the presence of a 5.8 cm hypoechoic lesion adjacent to the right lower lobe of the thyroid. The patient received IV fluid resuscitation before undergoing urgent parathyroid surgery. The parathyroid lesion was grossly adherent to the thyroid gland and she received en bloc resection of the tumor and right lobe of the thyroid. Post-operatively, PTH rapidly declined to <3.4 and she developed a prolonged hypocalcemic phase. PTH function improved, but she continued to require high doses of calcium carbonate to address hungry bone syndrome over subsequent weeks. Comprehensive evaluation included testing for genetic mutations, deletions, and duplications in CASR, CDC73, CDKN1B, MEN1 and RET, all of which were negative. Extensive genetic testing of the parathyroid tumor did not identify any specific mutations which have been previously associated with PC. Six months post-operatively, her labs have vastly improved, although she has clinical evidence of PTH resistance and her alkaline phosphatase has not yet normalized. Neck ultrasound, F-18 FDG PET/MRI imaging and Sestamibi scans show no evidence of persistent disease. Conclusions: While parathyroid carcinoma is extremely rare in pediatric patients, the diagnosis must be considered in cases of hypercalcemia with marked elevations in PTH and palpable neck mass. Although rarely described, bone changes related to PC may be dramatic and may result in lasting morbidity - including genu valgum requiring surgical intervention as well as prolonged hungry bone syndrome and metabolic bone disease. Genu valgum is a unique pediatric feature of this disease because this may only evolve in individuals whose physes are open.