Clinical history: 68 year-old female with left anterior chest pain and fever. Treated with antibiotics as a pneumonia. Follow-up thoracic imaging revealed an anterior mediastinal mass.
What is the diagnosis?
Type AB thymoma
Thymomas are neoplasms arising from or exhibiting differentiation towards thymic epithelial cells, regardless of the presence and relative numbers of non-neoplastic lymphocytes. Thymomas are classified based on whether the neoplastic epithelial cells and their nuclei have a spindle or oval shape, and are uniformly bland (type A) or whether the cells have a predominantly round or polygonal appearance (type B). Type B thymomas are further divided on the basis of the extent of the lymphocytic infiltrate and the degree of atypical of the neoplastic epithelial cells into three subtypes: B1- richest in lymphocytes, B2, and B3- richest in epithelial cells. Thymomas combining type A with B1 or B2-like features are designated type AB.
Type AB thymoma accounts for 15-43% of all thymomas and is either the most common or second most common type. The affected patients’ ages range from 29-82 years with a mean of 55 years. A slight male predominance has been noted. Approximately 14% of type AB thymomas are associated with myasthenia gravis and paraneoplastic red cell aplasia has also been reported.
Grossly, type AB thymoma is usually encapsulated and the cut surface shows multiple tan coloured nodules of various size separated by white fibrous bands. Average size is 7.7 cm.
Histologically, AB thymoma shows a nodular growth pattern with diffuse areas and is composed of a variable mixture of a lymphocyte poor type A component and a more lymphocyte-rich type B component. The type B areas are different from either B1, B2 or B3 thymoma and the cells in the B component are composed predominantly of small polygonal epithelial cells with small round, oval nuclei showing dispersed chromatin and inconspicuous nucleoli. The type A and type B components either form discrete nodules or intermix together. Lymphocytes are more numerous than in the type A component.
The patterns of cytokertain expression are essentially similar to those of type A thymoma except that the epithelial cells in type B areas are usually CK14 positive. The associated lymphocytes are T cells positive for CD3, CD5 including varying proportions of CD1a and CD99 immature T cells.
The overall survival rate of patients with type AB thymoma is 80-100% at 5 and 10 years. They are generally regarded as clinically benign tumours because they can usually be cured after surgical resection. Recurrence and metastasis are rare.
WHO Classification of Tumours. Tumours of the Lung, Pleura, Thymus and Heart. 2004.