Corpuscular thymoma: Entity or variant of organotypical thymomas WHO B2/B3?

Hassall's corpuscles are regular structures in the medulla of the normal thymus and in non-neoplastic thymic conditions, e.g. in multilocular thymic cysts. In thymomas, however, they are inconsistently found, and are believed to indicate medullary differentiation of WHO type B1-3 thymomas. We present five organotypical thymomas resembling WHO type B2 and B3 thymomas, but with an abundance of Hassall's corpuscles. We wonder whether this exceedingly rare observation might herald a distinct entity. Four tumors were asymptomatic, incidental findings and of low Masaoka stage (I or II) [20]. One patient suffered from myasthenia gravis which disappeared upon surgical removal of the thymus, while all other patients had no concomitant autoimmune disease. Two patients had a relapse-free follow-up of 12 and 2 years, respectively, upon curative surgery, and another tumor was an autopsy finding; follow-up data of two more recent cases was not yet available. The neoplastic epithelium other than Hassall's corpuscles was arranged either in a cortical type B2 pattern or in type B3 solid cords. In all examples, there was cyst formation, inflammatory reaction and repair, indicative of a long-standing condition. Immature T cells were present in all instances. “Corpuscular thymomas” morphologically resemble WHO type B2 and B3 thymomas, but appear biologically indolent and are rarely associated with myasthenia gravis. Whether they qualify for a separate entity has to be proven by larger series, including genetic studies.