How to deal with parathyroid carcinoma: case report, histological difficulties and literature review

The histological diagnosis of parathyroid carcinoma is often difficult due to the existence of many potentially misleading similarities, between this malignant entity and its main differential diagnosis: parathyroid adenoma. Several histological criteria of malignancy have been proposed in order to solve this diagnosis problem: the presence of fibrous bands emerging from a thick capsule dividing the tumor proliferation into lobules, tumor cells arranged in clusters and trabeculae, moderate to clear cytonuclear atypia and low mitotic activity. Unfortunately, these criteria are not exclusive to carcinomas and can also be seen in cases of adenomas. The more relevant histological criteria of malignancy are proposed, such as atypical mitosis, capsular invasion, vascular invasion or the exceptional perineural invasion. But these criteria are rarely found. Infiltration of the thyroid gland, adjacent soft tissue and the occurrence of metastases remain the only indisputable signs of malignancy. Immunohistochemistry can contribute to the differential diagnosis, especially with the loss of expression of Parafibromin, commonly found in carcinomas unlike adenomas, or the expression of parathormone, allowing the elimination of a thyroid tumor.