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Background: Atypical parathyroid adenoma is a rare entity that share some pathology features of parathyroid carcinoma such as fibrosis. Little is known about the clinical behavior of atypical parathyroid adenomas. Pathologically, it is defined as a hypercellular parathyroid lesion with intratumoral banding fibrosis, mitotic figures, trabecular growth and presence of tumor in the surrounding capsule but lacks unequivocal evidence of invasion, such as invasion to peritumoral vessels, perineural invasion and surrounding structures. Methods: A retrospective study of 15 consecutive patients with atypical parathyroid adenoma treated at a single center between 2010 and 2020 was performed. Patient demographics, clinical characteristics, biochemical profile, indications for surgery, preoperative localization studies, intraoperative findings, histopathological characteristics, disease recurrence or persistence and survival were collected. Results: 7 of 15 were female with a median age of 62 (IQR: 52–67). Five of the 15 patients (33%) were re-operative. No patients presented with palpable neck mass. Presentations were consistent with usual primary hyperparathyroidism. Average calcium on presentation was 11.2, and average PTH was 199. One patient had known MEN1 syndrome and one patient had family history of hyperparathyroidism but had negative genetic testing. Most patients 12/15 had correctly localizing imaging pre-operatively with the other 3 having equivocal or non-localizing studies. Two patients did not have biochemical resolution of hyperparathyroidism, both were re-operative. Of the patients with biochemical cure, 6 did not have follow up beyond 6 months, and 7 patients had long term follow up with persistent biochemical resolution and no recurrence of disease for a median of 4 years (IQR: 3.75–9.25). On review of pathology, no patients had invasive features and all patients had presence of thick fibrous bands or capsule. Conclusion: Patients with atypical adenoma have good response to surgery and low recurrence rates. Reoperation with associated scarring and fibrous bands can confound pathological findings. Our experience shows that patients found to have atypical parathyroid adenoma at their primary operation with resulting biochemical cure can be followed long-term with seemingly indolent and nonaggressive behavior.

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