Slow-Growing Mediastinal Grey Zone Lymphoma Originating From a Formerly Resected Multilocular Thymic Cyst

We report a unique case of mediastinal grey zone lymphoma that was considered to originate from CD20⁺CD30⁺ Hodgkin cell-like cells associated with a multilocular thymic cyst that had been completely removed 4 years previously. In the formerly resected thymus, irregular-shaped cysts were observed, and large CD20⁺CD30⁺ Hodgkin cell-like cells proliferated in close association with the proliferating thymic epithelial cells. This case suggests the important role of thymic columnar epithelial cells in the proliferation of thymic B cells and the tumorigenesis of mediastinal grey zone lymphoma. Also, it suggests that mediastinal grey zone lymphoma can proliferate slowly in the very early stage.