The goal of this observational study is to create, manage and analyze a multicenter national database of patients with parathyroid carcinoma or atypical parathyroid adenoma, aimed at collecting and studying anamnestic, diagnostic, genetic, clinical, histological, and therapeutic data in a relatively wide number of patients in Italy. The study will include 33 specialist clinical centers of endocrinology and endocrine surgery, located throughout the Italian territory, and to which patients refer from all the 20 regions of Italy. Data will be collected over time, both in retrospective and prospective manners, during the 10-year duration of the study, starting from the recruiting visit (basal visit) and then during each follow-up visits patients will undergo for the control of disease progression at the recruiting clinical centers. Collected data will include both the most classic traits of the pathology and the less common ones, with the final goal of refining and deepening medical knowledge in the field of these two extremely rare parathyroid cancers and be able to define optimal clinical and therapeutic management of patients, improving their quality and life expectancy. The main aspects this observational study aims to assess and clarify are: 1. Evaluation of prevalence and incidence of parathyroid carcinoma and atypical parathyroid adenoma in Italy, both as sporadic disease or in the context of genetic disorders. 2. Clinical, histological, and biochemical characterization of parathyroid carcinoma vs atypical parathyroid adenoma, to assess features and aspects that distinguish these two kinds of tumors and may help in the differential diagnosis. 3. Evaluation of short-term and long-term response to therapies in patients with parathyroid carcinoma and in patients with atypical parathyroid adenoma to assess possible differences between these two types of cancer and, thus, to design tailored clinical and therapeutic managements for patients. 4. Evaluation of short-term and long-term response to therapies in sporadic forms vs genetic forms of parathyroid carcinoma and atypical parathyroid adenoma, to assess possible differences between these two different forms of diseases and, thus, to design tailored clinical and therapeutic managements for patients. The study will include two independent cohorts of female and male patients of any age, one including patients who developed parathyroid carcinoma (cohort 1) and one including patients who developed atypical parathyroid adenoma (cohort 2), both as sporadic cancer or in the context of genetic diseases. The study does not include either any control group/comparison group or healthy volunteers. The study itself does not involve any medical intervention or drug administration. Treatments (surgery and/or drugs), for which data on response to therapy will be collected in the database, are those that are conventionally used in patients with parathyroid carcinoma or atypical parathyroid adenoma for treatment of their pathology, regardless of their inclusion in this observational study.