Pathologic features of histiocytic neoplasms with CSF1R mutations. Photomicrographs of hematoxylin and eosin (H&E)–, PU.1-, or CD163-stained slides. Numbers indicate cases. Shown are the histiocytic lesions with multiple Touton giant cells in cases 1 and 3, the storiform-like histologic pattern in case 4 (with a multinucleated cell in the inlet), and the monomorphic histiocytic infiltrate dissecting through the dermal collagen bundles in case 5. In addition, the intramuscular tumor of case 7 is depicted, which was located on the back and contained large collections of foamy histiocytes. In contrast, the scalp lesion of case 7 consisted of a dense monomorphic infiltrate without lipidized histiocytes. Finally, the endotracheal tumor of case 13 was characterized by multiple Touton and Touton-like giant cells (the latter have no or limited foamy cytoplasm), in addition to many mononucleated histiocytes with predominantly round to ovoid nuclei. Histiocytes stained positive for PU.1 and CD163, as illustrated by photomicrographs of the tumors of cases 1 and 3. Histology of the abdominal tumor of case 6 with a CSF1R deletion and an MRC1::PDGFRB fusion is provided in supplemental Figure 1.