Type AB thymoma is an organotypical thymic epithelial neoplasm composed of a mixture of a lymphocyte-poor type A thymoma component and more lymphocyte-rich type B areas.¹ There is a great variation in the proportion of the two components, and while usually both components are present in most sections, either type A or type B areas can be scanty. The cellular origin of the type A component, like in type A thymoma, has been postulated to derive from or differentiate toward thymic medullary epithelial cells.^2,3 The type B component ultrastructurally resembles epithelial cells at the corticomedullary junction but is similar to thymic subcapsular epithelial cells in expression of CK14; thus, its normal counterpart is uncertain.^1,4

Type AB thymoma is either the most or the second most common type of thymoma and accounts for 15% to 43% of all thymomas.¹ The patients’ ages range from 29 to 82 years. A slight male predominance has been noted in most reports.¹

The clinical presentation is similar to that of type A thymoma. Approximately 14% of type AB thymomas are associated with myasthenia gravis.^5,6,7 Paraneoplastic pure red cell aplasia has also been reported.⁸ Other tumors manifest by local symptoms or can be asymptomatic and are found incidentally upon x-ray, CT, or MR imaging examination.

Grossly, type AB thymoma is usually encapsulated and the cut surface shows multiple tan-colored nodules of various sizes separated by white fibrous bands.

Histologically, type AB thymoma shows a nodular growth pattern with diffuse areas and is composed of a variable mixture of a lymphocytepoor type A thymoma component and lymphocyte-rich type B areas (Figs. 103.1 and 103.2). A type B3 thymoma component is not part of AB thymoma. All histologic features of type A thymoma can be seen in the type A component. The tumor cells in the type B component are composed predominantly of small polygonal epithelial cells with small round, oval, or spindle pale nuclei showing dispersed chromatin and inconspicuous nucleoli (Fig. 103.3).⁹ In contrast to type B2 thymoma, large epithelial cells with prominent nucleoli and vesicular nuclei and nucleoli are uncommon.⁹ The type A and type B-like components either form discrete separate nodules or intermix together.² The type A component in the latter areas may form bundles of elongated fibroblast-like spindle cells. Type B areas harbor lymphocytes in variable numbers, and medullary differentiation is rarely observed. In particular, Hassall corpuscles are absent. There is a great variation in the proportion of both components, and in particular, type A areas can be extremely scanty to almost absent.^2,9