Fig. 3
a Bone marrow biopsy with patchy involvement by histiocytic sarcoma (circles) as well as small paratrabecular lymphoid aggregates (arrows). Hematoxylin and eosin, ×40. b Histiocytic sarcoma comprised large pleomorphic cells with frequent folded/irregular nuclei and abundant amounts of eosinophilic cytoplasm. Hematoxylin and eosin, ×400. c Histiocytic sarcoma (arrowheads) and adjacent small paratrabecular lymphoid aggregate (arrows) composed of small lymphocytes. Hematoxylin and eosin, ×400. d The atypical paratrabecular B cell-rich lymphoid aggregates are highlighted by CD20 immunohistochemistry (×40). The histiocytic sarcoma is positive for e CD68 (×400), f CD163 (×200), g lysozyme (×400), and h BRAFV600E (×200), as determined by immunohistochemistry. h An atypical paratrabecular lymphoid aggregate (asterisk) is negative for BRAFV600E immunohistochemistry