Methodologic aspects of a standardized evaluation of mitotic activity in tumor tissues

Abstract

Although estimation of mitotic activity is generally a recognized clinical practice in the diagnosis and grading of tumors, there are several methodologic aspects that merit special consideration. Our results, in relation to those of others, lead to the following conclusions: 1. The values attained by calculating the mitotic activity of tumor cells in paraffin-embedded tumor tissue are lower than those obtained in vivo. The results cannot always be interpreted clinically due to the complexity of some of the factors associated with it which require special attention. That the method still has clinical relevance and value in spite of the associated problems may be due to the fact that tumor tissues generally undergo long periods of hypoxia before being fixed for histomorphological analyses--a condition which by itself is a decisive factor in the reduction of mitotic activity--so that additional changes in some individual factors from case to case do not make much difference to the estimated or calculated value. 2. Mitotic activity should be calculated in areas of tumor sections where there are numerous mitotic tumor cells. Additionally, only cells with nuclei showing lysed nuclear membranes and identifiable single chromosomes at higher magnification ought to be considered in the calculation. Apoptotic and pyknotic elements should not be calculated. 3. The specification of the final magnification (e.g., x 400) is not a sufficient basis for comparison of optical conditions for the mitotic estimation, since the visual fields of standard light microscopes may differ up to 2.6-fold. A temporary streamlining of the standard may be the application of a 0.159 mm2 visual field (x 40 objective, x 10 ocular by visual field value 18). As an intermediate goal, mitosis should be calculated in percentages of the tumor cells or areas of the total tumor tissue. 4. The interspecific reproducibility of the quantified mitosis makes it reliable for clinical application. It compares well with cytometric and morphometric methods in assessment of tumor cells and is of higher clinical relevance than the conventional histomorphological tumor grading systems.