Sections of the mass showed loosely defined nests of cohesive, meningothelial cells with pale, round or oval nuclei. Whorl formation, in which the tumor cells wrapped themselves tightly around one another in an "onion skin" fashion, as well as psammoma body formation, was occasionally observed. Numerous areas of micronecrosis and occasional mitotic figures were also seen. In addition, regions of sheet-like (syncytial) growth were seen. No significant pleomorphism, or areas of increased cellularity, however, were noted. A Ki-67 (MIB-1) proliferation index for the tumor was 29% (228 positive/788 cells). Numerous sections of the cerebral surface of the tumor showed no brain invasion. The bone, however, showed diffuse expansion by the tumor, which extended entirely through the outer table of the skull in the area which corresponded grossly to bony softening.