Mitotic Index

What Is the Mitotic Index?

Mitosis is the process of cell division or replication. This process is visible with a microscope and gives us some information on the behavior of a tumor.

The mitotic index (MI) is a scale or measure of mitosis in a tumor sample. It can be considered in two ways: the percentage of cells in a tumor sample that are actively dividing, or the ratio of cells that are dividing compared to the number of cells that are not actively dividing.¹

In veterinary medicine, most pathologists actually use a mitotic count, but report it as a mitotic index. Mitotic count is the number of mitotic figures (cells in the process of dividing to make two cells) per high-power field (HPF).

HPF refers to the visible area that can be seen through the microscope when it is magnified at 400x.^1,2 While this system isn’t quite as precise, it is simpler to understand and still gives valuable information about certain types of tumors.¹

Why Is the Mitotic Index Important?

A faster mitotic rate – or higher mitotic index – means that more cells within the tumor sample are dividing, and that it is growing faster. In general, tumors with a higher MI are more aggressive, but that is not always the case.¹

What Is the Mitotic Index Formula?

The mitotic index should be part of the pathology report when a tumor sample is sent out for histopathology.

First, your veterinarian will take a biopsy. This may be a small sample of the tumor, or sometimes it is the entire tumor.

The biopsy will be preserved and sent to a lab for evaluation. Once at the lab, the sample will be cut into very thin slices, put on slides, and stained so that a pathologist can see different structures within the tissue sample when looking at it through a microscope.

The pathologist will then look at the slides under a microscope and count the number of mitotic figures that they see across ten different high-power fields to get the mitotic count.

For example, if the pathologist sees 7 dividing cells over those ten fields, the mitotic count/mitotic index will be reported as 7. If the report says 7 per HPF, the MI is actually 70 (7 dividing cells x 10 fields = 70).

What the Mitotic Index Means for Your Dog’s Cancer

Mitotic index is just one piece of the puzzle when evaluating your dog’s cancer and making treatment decisions. It is important to remember that the mitotic index is just a snapshot in time and cannot guarantee how a tumor will behave. The rate of tumor growth can vary throughout the tumor’s “lifecycle,” and some regions of a tumor may be more active than others.

For example, large tumors tend to be most active on the edges, while the center of the tumor is largely dormant or even starting to die off. Depending on where the biopsy is taken from, the pathologist could get dramatically different results.

Because of this, larger samples are ideal because they give the pathologist more information to work with. Most pathologists will scan many slides before choosing where to make a mitotic count so that they can be sure they are looking at a section that is most representative of the tumor as a whole.

Higher Mitotic Rate Is Often Associated with More Aggressive Tumors

In general, a higher mitotic index means that the tumor is more aggressive and faster growing. This might mean that your surgeon takes wider margins when removing the mass to be sure it is all removed, or that your oncologist recommends chemotherapy or radiation after surgery to target any metastasis that might be present.

This is not true for all cancers, however. Plasma cell tumors, for example, often have a high MI and are fast growing, but surgery alone is usually curative for smaller masses because they don’t usually metastasize.

Mitotic index has proven to be a good indicator of prognosis for some cancer types; for those, we have identified specific MI’s that can help guide treatment decisions based on prognosis.

Hemangiopericytoma

Mitotic index is prognostic for hemangiopericytomas, meaning that tumors with an MI less than 9 usually have better survival times than those with an MI greater than 9.³

Mammary Tumor

Mitotic index is diagnostic for mammary tumors, meaning that it is useful for determining if a tumor is benign or malignant. One study found that malignant mammary tumors in dogs had an average MI of 36 (ranging from 7 to 65).⁵ Another study comparing benign and malignant tumors calculated the mitotic index as a ratio and found that benign tumors ranged from 0.63-0.9 while malignant tumors ranged from 1.08-4.19.⁴

Mast Cell Tumor

Mitotic index is prognostic for mast cell tumors. Dogs with an MI of 5 or lower generally have better survival times.⁶

Melanoma

Mitotic index is prognostic for melanoma. Tumors with an MI greater than 3 are usually more aggressive and have a poorer prognosis.⁷

Soft Tissue Sarcoma

Mitotic index is prognostic for soft tissue sarcoma. Tumors with an MI under 10 are usually classified as grade 1 and have a better prognosis, while tumors with an MI over 19 are generally classified as grade 3 and usually have a poorer prognosis.⁸

Splenic Non-angiomatous and Non-lymphomatous Tumors

Mitotic index is prognostic for non-angiomatous and non-lymphomatous tumors – those that are not made up of blood vessels and are not lymphoma – including tumor types such as leiomyosarcoma, chondrosarcoma, osteosarcoma, fibrosarcoma, histiocytic sarcoma, and liposarcoma.⁹ Dogs with these tumor types and an MI less than 9 typically have improved survival times over those with an MI greater than 9.^2,10

Additional Resources

Link to podcast with Dr. Britton https://dogcanceranswers.com/dog-cancer-vocab-mitotic-index-dr-brooke-britton/