Counting mitoses: SI(ze) matters!

Mitoses are often assessed by pathologists to assist the diagnosis of cancer, and to grade malignancy, informing prognosis. Historically, this has been done by expressing the number of mitoses per n high power fields (HPFs), ignoring the fact that microscope fields may differ substantially, even at the same high power (x400) magnification. Despite a requirement to define HPF size in scientific papers, many authors fail to address this issue adequately. The problem is compounded by the switch to digital pathology systems, where x400 equivalent fields are rectangular and also vary in the area displayed. The potential for error is considerable, and at times this may affect patient care. This is easily solved by the use of standardized international (SI) units. We, therefore, recommend that features such as mitoses are always counted per mm(2), with an indication of the area to be counted and the method used (usually hotspot or average) to obtain the results.

Automated summary

Mitoses in cancer diagnosis are often assessed by counting per square millimeter, but the variation in microscope fields, especially in digital pathology systems, can lead to errors. Standardized international units should be used to ensure accuracy in mitotic count.