THE SIGNIFICANCE OF ATYPIA AND HISTOLOGIC MALIGNANCY IN PILOCYTIC ASTROCYTOMA OF THE CEREBELLUM - A CLINICOPATHOLOGICAL AND FLOW CYTOMETRICSTUDY

Biologically malignant pilocytic astrocytomas are rare. Precise morpho logic criteria permitting their identification and clinically meaningf ul distinction from more ordinary pilocytic astrocytomas have not been developed. The histologic grading schemes applied to fibrillary astro cytomas-ones based on nuclear atypia, increased cellularity, mitotic a ctivity, endothelial proliferation, and necrosis-are thought not to co rrelate with prognosis in pilocytic astrocytomas. Nonetheless, these f eatures were systematically sought in 107 cerebellar pilocytic astrocy tomas, four of which were histologically malignant. The clinical, morp hologic, and flow cytometric features of these cases were studied. The incidence of histologic malignancy occurring spontaneously in otherwi se typical pilocytic astrocytomas was 0.9%; that occurring after radia tion was 1.8%. The four histologically malignant tumors occurred in tw o males and two females, ages 6 to 18 years. All showed increased mito tic activity (three to live mitoses per high-power microscopic field [ 250x]). Endothelial proliferation and necrosis were present in three a nd two cases each. DNA ploidy analysis showed the spontaneously occurr ing malignant-appearing tumors to be aneuploid, whereas both previousl y radiated tumors were tetraploid; 5% to 11% of cells were in S phase. The appearance of histologic malignancy in pilocytic astrocytoma is r are and less reliably correlated with prognosis than in patients with fibrillary astrocytomas. Essential features of such tumors include a h igh mitotic index and perhaps a high percentage of cells in S phase.