[Thymoma with epithelial micronodules and lymphoid hyperplasia: six cases of a rare and equivocal subtype]

Thymomas with the characteristic pattern of small epithelial nodules separated by an abundant lymphoid tissue have been recently described with divergent interpretations. These thymomas are not specified in currently used classification systems. We present six such thymomas, including three that represented 1.38% of a series of 217 consecutive cases. These thymomas were totally encapsulated (Masaoka stage I, n=1) or minimally invasive (stage II, n=5). The epithelial cells of the nodules were oval and bland-appearing. In one case, these cells formed rosettes. Cysts, that were present in four cases, showed a strong linear expression of EMA and were associated with foci of glandular differentiation. The lymphoid tissue was composed of large immature (CD1a and CD99-positive) T-cell areas (with epithelial cells restricted to small foci of residual thymus) and of B-cell (CD20-positive) areas with germinal centers. Mature T-cells were also present. Furthermore, one case, associated with myasthenia gravis, had an important WHO type B2 (cortical) component. Such a combined case has not been previously reported. Our study demonstrates that so-called micronodular thymomas are rare, usually have clinical and pathological features of WHO type A (medullary) thymomas, and that the lymphoid component is hyperplastic corresponding to both immature T-cell lymphoid tissue and B-cell lymphoid hyperplasia with germinal centers.