Common thymomas: classification, histology, staging and prognosis

Thymomas are rare tumors that together with thymic carcinomas and thymic neuroendocrine tumors form part of the spectrum of thymic epithelial neoplasms. They arise from neoplastic epithelial cells of the thymic gland in the anterior mediastinum and predominantly affect patients in the 5^th-7^th decades of life. Thymomas can be associated with autoimmune disorders, especially myasthenia gravis, the diagnosis of which may ultimately lead to the detection of the tumor. Histologically, thymomas are characterized by a wide morphologic spectrum, although their classic biphasic nature, consisting of a mixture of neoplastic epithelial cells and non-neoplastic T lymphocytes, is retained throughout all types. The diagnosis of thymomas is relatively straightforward if sufficient histologic material is available from surgical resection specimens but may be difficult or impossible in small core biopsies. The immunohistochemical phenotype of thymomas is non-specific and no specific marker exists that would unequivocally identify thymic epithelial cells. Investigations into the molecular profiles of thymomas have recently identified characteristic aberrations in certain thymoma subtypes. While these may be of diagnostic use, they do not represent therapeutic targets and treatment still consists of a combination of surgery, radiotherapy and chemotherapy. In this article, a review of the current thymoma classification, histologic features, immunohistochemical and molecular profiles, differential diagnosis, treatment, prognosis and staging is offered.