Microscopically the left frontal mass showed a heterogeneous architecture with areas that were densely cellular alternating with areas that were hypocellular and myxoid (Figure 2). The cellular regions were composed of highly pleomorphic, plump, spindled cells arranged in fascicles, bands, and storiform patterns (Figure 3). Elsewhere, pleomorphic and multinucleated cells were more widely dispersed on a myxoid background (Figure 4). The vascular bed was composed of variably sized capillary and sinusoidal vessels. Mitotic figures were easily found throughout the tumor with an average of 4 mitotic figures / 10 HPF.
Individual cells possessed a light reticulin investment (Figure 5). Immunohistochemical staining for actin, desmin, EMA, cytokeratin, S100, GFAP, NSE, synaptophysin, and KP1 was negative. The neoplastic cells were positive for CD34 (Figure 6), vimentin, and alpha-1-antichymotrypsin. Proliferative markers (Ki67) revealed variable staining density with up to 25 % of cells labeling focally (11% average in 10 high power fields). Electron microscopy failed to reveal features of specific lineage for the neoplastic cells.