Mitotic rate

The mitotic rate describes how many cells in a tumour are actively dividing. When cells divide, they create new cells. This is a normal process in healthy tissue, but in a tumour, a high number of dividing cells can be a sign that the tumour is growing quickly. In a pathology report, the mitotic rate helps doctors understand how aggressive a tumour may be.

How do pathologists determine the mitotic rate?

Pathologists examine the tumour under a microscope to count the number of dividing cells. These dividing cells are called mitotic figures, and they have a distinct appearance that sets them apart from non-dividing cells. The mitotic rate is usually reported as the number of mitotic figures seen in a specific area of tissue, often described as mitoses per square millimetre (mm²) or mitoses per high-power field (HPF).

To make the count as accurate as possible, the pathologist searches for the area of the tumour with the most mitotic activity and counts the number of mitotic figures in one or more high-power fields. The exact method can vary depending on the type of tumour being examined.

What is the significance of a high mitotic rate?

A high mitotic rate means that more cells in the tumour are dividing. This often suggests that the tumour is growing quickly and may behave more aggressively. In some types of cancer, such as melanoma or sarcoma, the mitotic rate is used to help determine the tumour grade. A high mitotic rate may also increase the chance that cancer cells have spread to other parts of the body or will return after treatment.

However, the mitotic rate is just one factor that doctors consider when planning treatment. It is often looked at alongside other features like tumour size, depth of invasion, and whether the tumour has spread to nearby lymph nodes or organs.