Summary
Using uniform systematic random sampling, we tested whether a drop in mitoses and PCNA (proliferation cell nuclear antigen) positive cells following a delay in tissue fixation of up to five hours, would be unequivocally discernible at the well perfused periphery from 11 aggressive rat adenocarcinomas. No statistically significant difference (0.05 < P < 0.10; 95% CI (− 13.3, 1.3)) was found. We attributed this finding to the high proliferative heterogeneity of rapidly growing solid tumors because: (1) vascular perfusion is heterogeneous as rapidly proliferating epithelial tumors tend to outgrow their blood supply; (2) the ratio of perfused/non-perfused tumor volumes may change as a solid tumor grows, altering to the same extent the ratio of proliferative vs. non proliferative regions in the tumor; and (3) the duration of mitoses is not constant. The consequences for accurate tumor grading are discussed.