Fine needle aspiration cytology of histiocytic sarcoma with dendritic cell differentiation: a case of transdifferentiation from low-grade follicular lymphoma.

Histiocytic sarcoma (HS) is a rare malignant tumor, and in extraordinary circumstances it can be transdifferentiated from a low-grade B-cell lymphoma. In our report, a 62-year-old female was initially diagnosed with a low-grade follicular lymphoma, and 2 years later she presented with bilateral lung masses and lymphadenopathy. Fine needle aspiration (FNA) of the lung masses revealed solid nests of large pleomorphic epithelioid cells. By immunohistochemical studies, the tumor cells were positive for histiocytic markers (CD68 and CD163) and S100, supporting a diagnosis of HS with dendritic cell differentiation. Further fluorescence in situ hybridization (FISH) analyses detected IGH@/BCL2 rearrangement in both the previous follicular lymphoma and the current HS, which was highly suggestive of the clonal relationship between these two tumors. To the best of our knowledge, this is the first case of transformation of follicular lymphoma into HS diagnosed by FNA of lung masses.