Diagnosis

The two most important parameters in the evaluation of thymomas are whether the tumor is encapsulated or invasive. If the tumor is invasive, pathological staging of the tumor needs to be determined by analyzing the extent of the invasion and/or whether the tumor is limited to the mediastinal region or involves structures such as the pleura, pericardium, lung, or other extrathoracic areas. Although, from the histopathological point of view, the diagnosis of thymoma may not represent a challenge, there is very limited information that can be provided in a small biopsy. The same can be said for fine needle aspirate cytology in which one cannot determine the invasion or encapsulation of the tumor. Thus, final diagnosis is best provided after complete surgical resection of the tumor. Histologically, the tumor may not pose a problem in diagnosis, and the vast majority of neoplasms will show similar histopathological features, namely the presence of epithelial cells in a background of lymphocytes, which recapitulates to some extent the normal cellular composition of the thymic gland. The presence of lymphocytes in thymomas may vary, and, in some cases, there is a predominance of lymphocytes, while in other cases, the lymphocytes are more inconspicuous. Thick fibrous bands composed of fibroconnective tissue separating the cellular proliferation are usually present giving the appearance of a nodular type of growth pattern. However, the essential component that must be evaluated with more detail is the tissue that is surrounding the tumor. In the great majority of tumors, there is a thick fibroconnective boundary, which is denominated “capsule.” If the tumor is confined within the boundaries of that capsule, then the tumor is diagnosed as encapsulated; in cases in which there is a breach of the capsule by the cellular proliferation, then the tumor is diagnosed as invasive. Although as stated above, the majority of tumors will display the characteristic histology, there is a group of tumors that may show unusual characteristics such as the presence of plasma cells, muscle cells, among others. In this setting, it is very important to be entirely familiar with these neoplasms in order to avoid an equivocal diagnosis. In terms of cytologic atypia and mitotic figures, thymomas do not display marked cellular atypia or increased mitotic activity. In some cases, areas of cystic degeneration, hemorrhage, or necrosis may be present, but it is still the cellular characteristics of the tumor and the presence of capsular integrity that are more significant in predicting outcome.

Classification

Treatment and Prognosis

In cases of thymomas, surgical resection is the treatment of choice. However, if the determination is that the tumor is invasive or that the tumor is not completely resected, adjuvant radiation or chemotherapy may be indicated. In cases in which the tumor is large and involves important adjacent thoracic structures, the use of chemotherapy may be necessary prior to surgical resection. The prognosis of patients with thymomas depends largely on the determination of the tumor being invasive or encapsulated. Three main features should be considered in predicting prognosis in patients with thymomas: encapsulation, invasiveness, and resectability of the tumor at the time of diagnosis.