Are we counting mitoses correctly?

The number of mitotic figures in a predefined area is essential in pathologic evaluation for most tumors. This information sometimes provides clues in differentiating neoplastic lesions from nonneoplastic ones and sometimes in defining and grading of the tumors as well as prognosticating expected lifetime of the patient. As a generally accepted concept, scanning a certain number of consecutive nonoverlapping areas that are rich in viable tumor cells is required. Invasion fronts or the periphery of the tumors is preferred for counting mitosis. The target area to be counted for mitotic activity for various tumors is standardized as the number of mitosis in an established number of high-power fields. However, suggested mitotic counts, which constitute the basis of these studies, were obtained via the old microscopes, which usually had narrower visual fields than the state-of-the-art microscopes. Because the visual fields of the present microscopes provide larger areas compared with the older ones, corrections in mitosis counting are needed to make them compatible with the criteria, which had been put forward in the original reference studies.