PSAT198 Atypical Parathyroid Adenoma is an Unusual Cause of Primary Hyperparathyroidism

BACKGROUND: Atypical parathyroid adenomas (APA) are considered tumors of uncertain malignant potential and have a biochemical and clinical phenotype that is generally more aggressive than typical parathyroid adenomas (PA). Low incidence as an etiology of primary hyperparathyroidism (PHPT) presents a challenge in understanding the pathological features and natural history of APA. We present a case of PHPT due to an APA and review the published literature for these tumors. CASE: A 37 year-old male was referred for evaluation of hypercalcemia, with serum calcium level 13.6 mg/dL (8.6-10.3). Repeat serum calcium measurement was 12.7 mg/dL, and two measurements of intact PTH were 870 pg/mL and 919 pg/mL (12-88), respectively. The patient's laboratories raised concern for parathyroid carcinoma (PC), but there was no palpable neck mass and no enlarged parathyroid glands were visualized on ultrasonography. Parathyroid scintigraphy was notable for faint but persistent Tc-99 m sestamibi uptake in the left inferior neck. Additional preoperative evaluation revealed significantly reduced bone mineral density in the distal third of the radius (0.638 g/cm(2), Z-score -3.2), silent nephrolithiasis of the right kidney, and vitamin D deficiency (25-OH vitamin D 13.4 ng/dL, 30-100). An unequivocally enlarged (175 mg, 2.5 cm in largest diameter) left inferior parathyroid gland was removed, with the remaining glands unremarkable by inspection. Extensive nuclear atypia and cellular pleomorphism indicated an APA, and invasive features of PC were absent. One month after surgery, serum calcium improved to 10.2 mg/dL and intact PTH was 40 pg/mL. CONCLUSIONS: APA account for 1-2% of PHPT cases. This patient's case has several features that distinguish it from a PA including degrees of preoperative hypercalcemia and intact PTH elevation, low vitamin D level, significantly reduced bone density in the distal third of the radius, nephrolithiasis, relatively young age, and male gender. Fibrous bands, pseudocapsular invasion, and trabecular growth pattern are the most common histologic features of APA, but nuclear atypia and cellular pleomorphism are well established diagnostic criteria. APA may be biochemically similar to PC but are distinguished from PC by the absence of gross invasion into adjacent anatomic structures, distant metastases, and vascular or perineural invasion. Additionally, CDC73 mutations are present in nearly all cases of PC but not in APA. Over 90% of patients with APA are successfully treated by parathyroidectomy. Careful follow up of APA patients is recommended, though there are no specific guidelines regarding a minimum satisfactory duration of postsurgical monitoring. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.