Two rare primary tumours of the thymus with differential immunohistochemical characterisation and a potential new entity

Adenoid cystic carcinoma (ACC) is a rare and aggressive biphasic malignant tumour with both ductal and myoepithelial components. Though mostly considered a tumour of the salivary gland (accounting for 10% of all salivary tumours) its exocrine origin means that it is rarely also described at other sites with exocrine glandular components such as the respiratory tract, breast, skin, and lacrimal glands. Thymic carcinoma with adenoid cystic carcinoma-like features (TCACC) is an extremely rare thymic adenocarcinoma variant that has been described nine times in the literature to date. Until now, primary ACC in the thymus is yet to be described. We present two recent cases for comparison - the first is a classic example of a primary ACC of the lung; the second is a proposed case of primary ACC of the thymus - and consider the morphological, immunohistochemical, and genetic similarities, differences, as well as difficulties in diagnosis.