Is atypical parathyroid tumor a different clinical entity than parathyroid adenoma and carcinoma? A retrospective review of a large single-center case series

Primary hyperparathyroidism is primarily caused by single-gland pathology (80-85% of cases). According to the 2022 World Health Organization (WHO) guidelines (Erickson et al. in Endocr Pathol, 2022), single-gland pathologies include parathyroid adenoma, atypical parathyroid tumor and parathyroid carcinoma (Gurrado in J Clin Med 12:6297, 2023). The aim of this study is to identify differences or similarities of both pre-, intra- and post-operative characteristics between atypical parathyroid tumor and parathyroid adenoma/carcinoma, thereby establishing an appropriate follow-up protocol for atypical parathyroid tumor. We retrospectively analyzed 437 patients who underwent parathyroidectomy for primary hyperparathyroidism between 2012 and 2022 at the Thyroid Unit of ASST Santi Paolo e Carlo in Milan focusing our analysis on 352 patients with single-gland disease. Several pre-, intra-, and post-operative variables, including follow-up, were analyzed and compared using non-parametric statistical methods. Histological analysis identified 316 cases of PA (90%), 27 cases of atypical parathyroid tumor (7.7%), and 9 cases of parathyroid carcinoma (2.3%). Patients with atypical parathyroid tumor had significantly higher pre-operative PTH levels, intermediate calcium levels, falling between those of parathyroid adenoma and parathyroid carcinoma patients and larger gland diameter. No cases of disease persistence or recurrence were observed in patients with atypical parathyroid tumor after a mean follow-up of 42.8 months. APT exhibits biochemical and pathological features overlap with both PA and PC. However, the lack of recurrence or persistence suggests that APT behaves more similarly to PA than PC. The favorable evolution of APT in our case series could be a factor in favor of reducing the follow-up time for atypical tumors to a shorter period than the one recommended for carcinomas.