Mitotic Index

The mitotic index is a measurement of how many cells in a tumor sample are actively dividing at the moment the tissue was taken. Pathologists count cells caught in mitosis under the microscope and report the number per a defined area or number of cells examined. A higher mitotic index generally signals a more aggressive, faster-growing tumor.

Origin

The term comes from the Greek mitos meaning "thread," referring to the thread-like appearance of chromosomes during cell division, a process first described by Walther Flemming in the 1880s. "Index" reflects its use as a numerical measure. Mitotic counting has been a standard part of tumor grading in surgical pathology since the early 20th century.

Significance in Asbestos Context

For patients with mesothelioma, the mitotic index is one of several features a pathologist uses to gauge how aggressive the tumor is and helps shape decisions about chemotherapy, surgery, and clinical trial eligibility. It is a key component of formal grading systems for epithelioid mesothelioma, where it is combined with nuclear features to assign a low, intermediate, or high grade, which in turn influences prognosis estimates and treatment urgency. In medical records and pathology reports relevant to a legal claim, a high mitotic index supports the documented severity of disease and can be relevant to damages calculations and the medical narrative experts present.

Example Sentence

The oncologist explained that the pathology report showed a high mitotic index, which was one reason she recommended starting chemotherapy without delay.

Related Terms

Ki-67, Nuclear Grade, Epithelioid Mesothelioma, Histologic Grade, Pathology Report, Prognosis

Notes

• Ask your oncologist or pathologist where your mitotic count falls and whether a formal grade (low, intermediate, or high) has been assigned to your mesothelioma.
• Request a complete copy of the pathology report, including any addenda and immunohistochemistry results. These documents are important both for second opinions and for any legal claim.
• If your tumor is graded as high or has a high mitotic index, ask specifically about clinical trial eligibility and whether referral to a mesothelioma specialty center is appropriate.
• Mitotic index can vary between biopsy samples; if treatment decisions hinge on it, consider asking whether a second pathology review at a high-volume mesothelioma center is warranted.