Counting mitoses: SI(ze) matters

Abstract

The assessment of proliferative activity by counting mitoses is something that most if not all histopathologists practice, as it is required for diagnosis and/or grading of many tumours. In the past, mitoses were often counted ‘per high power field’, which was convenient, but was regarded as scientifically questionable even 40 years ago, as it ignored the fact that microscope fields may differ substantially, even at the same high power (×400) magnification.1 Other features such as vessels, cell numbers or apoptoses are also counted in some circumstances, and the same issues apply.