Mulberry cells in the thyroid: warthin-finkeldey-like cells in hashimoto thyroiditis-associated lymphoma.

Warthin-Finkeldey type giant cells were first described in autopsies performed on young children who died during the highly lethal measles epidemic in Palermo during the winter of 1908. The cells had 8-15 nuclei without identifiable cytoplasm within the germinal centers of lymphoid organs resembling megakaryocytes. We describe a case of Hashimoto thyroiditis with an enlarging substernal throid mass. The resection specimen contained many Warthin-Finkeldey-Like Cells (WFLC) in an extranodal marginal zone lymphoma (MALT type) with focal transformation to diffuse large B-cell lymphoma. The WFLC showed nuclear features similar to those of neighboring follicular dendritic cells (FDCs), favoring the hypothesis that these cells might be the product of fusion of FDCs. This is supported by immunostaining results and the occurrence of similar cells in follicular dendritic cell sarcomas and in "dysplastic" FDCs in hyaline vascular type Castleman disease, a possible precursor of follicular dendritic cell tumors. Diagn. Cytopathol. 2017;45:212-216.