Metastases to the Parathyroid Glands: A Comprehensive Literature Review of 127 Reported Cases.

Metastases to the head and neck organs are uncommon, potentially representing the initial presentation of an occult malignancy. Single case reports and small series report metastases to the parathyroid gland, but there is no large review of the literature on secondary tumors involving the parathyroid glands. A review of the English literature between 1950 and 2017 was performed of all metastases or secondary involvement of the parathyroid glands. One hundred and twenty-seven cases of metastatic tumors were reported, although potentially significantly unrepresented in autopsy series (parathyroid glands are not routinely examined) and due to reporting bias. Women were affected more commonly than men (5.8:1; 99 vs. 17, respectively), with a mean age at presentation of 58.5 years, when reported. The most common primary sites of malignancies that metastasized to the parathyroid glands were breast carcinomas (66.9%, n = 85), melanoma (11.8%, n = 15), and lung carcinoma (5.5%, n = 7), with carcinomas representing 86.6% of metastases. Metastases were nearly always identified as part of widely metastatic disease, with only five (3.2%) cases reported as isolated metastases. Tumor-to-tumor metastases comprised 5.5% of all metastases to the parathyroid glands (metastases to parathyroid adenoma). A significant clinical finding of metastases to the parathyroid glands was the development of deranged calcium homeostasis, well beyond the 9 (7.2%) cases with primary parathyroid gland disease present. Although concurrent conditions (renal disease; bone metastases) may partially affect calcium metabolism, the onset of calcium derangement seemed to coincide with parathyroid gland metastases and not systemic disease. In summary, metastases to the parathyroid glands are uncommon, potentially under-recognized in patients who have otherwise widely metastatic tumors. Women are affected more often than men, with breast carcinomas (66.9%) and melanoma (11.8%) the most common primary tumors. Calcium homeostasis is affected, probably as a result of parathyroid gland parenchymal destruction.