SAT206 A Case Of Parathyroid Carcinoma Presenting As A Parathyroid Adenoma

Disclosure: P. Subarajan: None. A. Boutsicaris: None. C.A. Poku: None. N. Lopez: None. Distinguishing between parathyroid adenoma (PA) and parathyroid cancer (PC) in patients with primary hyperparathyroidism (PHPT) can be difficult to determine before pathology results. Estimates vary between studies, but roughly 85% of cases of PHPT are PAs. We present a case of a PC that presented clinically as a PA. A 59 year old female presented to Endocrinology for hypercalcemia that was incidentally noted to be 13.8 and 12.8 on initial recheck. Endocrinology completed a secondary workup of the hypercalcemia which noted a PTH of 147 in the setting of a calcium (Ca) of 12.6. Lab review showed that her corrected Ca had been elevated over the preceding 2 years. She had a replete 25-OH vitamin D of 43 and a GFR of 29. A neck ultrasound revealed a 2.1 cm mass suspicious for a parathyroid mass. A sestamibi scan was consistent with increased focal uptake at the inferior pole of the right thyroid lobe. She had renal dysfunction, hypercalcemia and newly diagnosed osteoporosis which prompted referral to endocrine surgery. Intraoperatively, an enlarged right inferior parathyroid gland and a large mass encompassing the inferior thyroid pole extending into the anterior mediastinum with lymphadenopathy were noted. En Bloc resection was completed and final pathology was consistent with high grade parathyroid carcinoma infiltrating the adjacent thyroid gland. Genetic testing for HPT-JT was negative and ultrasound monitoring has remained negative. PC is rare and typically has a clinical presentation and course that sets it apart from PA. The PTH is typically 3-10 times above the upper limit of normal in PC and only 1-2 times above the upper limit of normal in PA. As a result of the variation in PTH, serum Ca is more elevated in PC than PA. PC patients typically present with a parathyrotoxicosis that is profound. A palpable neck mass can be present in 40-70% of individuals with PC. Our patient had an equivocal presentation that appeared more consistent with PA, which can be problematic when deciding on the optimal surgical approach. Amino-PTH is hypersecreted in PC and reacts with 3rd generation PTH assays, but not second generation assays. Clinically, using a third/second generation PTH assay ratio >1 can support a possible clinical suspicion of PC over PA. En Bloc resection is the surgical approach of choice for suspected PC. En Bloc resection may not always be done for a patient with a presentation and evaluation consistent with PA: data shows that only 12.5% of PC patients underwent En Bloc resection, as opposed to 78.6% who underwent parathyroidectomy alone. This puts patients at risk of requiring repeat surgical intervention and poses a higher risk for recurrence. It brings to question whether every PA should be addressed with an En Bloc resection to avoid this inherent risk. This case adds to the literature of PC presenting as PA and supports the argument for standard En Bloc resection in all cases. Presentation: Saturday, June 17, 2023