Multiple pieces of cranial bone with their rims partially lined by tan and friable tissue were received. Histologic examination of the friable tissue revealed sheets of round cells of moderate size admixed with numerous osteoclast-like giant cells within a background of acute and chronic inflammatory cell infiltration (Figure 3). A small amount of fibrous tissue infiltrated by the round cells was also noted but the bony part of the specimen was largely free of round cell infiltration. The round cells varied from macrophage-like cells to enlarged cells with amphophilic cytoplasm with large, moderately pleomorphic nuclei (Figure 4). Deep nuclear clefts were not uncommon and frequently gave these nuclei a coffee bean like appearance. The admixed inflammatory cells were composed of a substantial amount of polymorphonuclear leukocytes, and to a less extent, lymphocytes. The amount and nature varied from areas to areas. A small number of eosinophils were also present (Figure 5).
Immunostaining for CD1a revealed strong positive staining primarily in the round cells with atypical nuclei while giant cells and inflammatory cells were negative (Figure 6). Similar results were obtained in sections stained for S100 protein. Immunostaining for CD68, in contrast, stained the osteoclast-like giant cells and macrophages (Figure 7). Electron microscopy confirmed the presence of deeply grooved nuclei in the atypical cells (Figure 8). In addition, distinctive cytoplasmic granules were noted (Figure 9).