Histologic analysis at initial presentation and clinical relapse. (A-B) Histologic analysis of the bone marrow biopsy at first diagnosis. (A) Foamy histiocytes and scattered giant cells (original magnification ×360; hematoxylin and eosin [H&E] stain,). (B) CD68⁺ histiocytes (original magnification ×360; immunoperoxidase stain). (C-F) Histologic analysis of the soft tissue biopsy at relapse. (C) Foamy histiocytes and scattered giant cells (original magnification ×200; H&E stain). (D) CD68⁺ histiocytes (original magnification ×200; immunoperoxidase stain). (F) Factor XIIIa-expressing putative tumor cell equivalents within the histiocytic infiltrate at relapse (original magnification ×200; immunoperoxidase stain). Roughly 25% of the total visible cells express factor XIIIa, which correlates with the detected frequency of KRAS^Q61H-mutant alleles of 12%. All scale bars represent 50 μm.