Mitosis is a biological process where a single cell divides into two daughter cells. This process is how the body grows, replaces old or damaged cells, and repairs tissues. During mitosis, a cell duplicates and segregates its chromosomes into two new nuclei. The mitotic rate measures how frequently cells in a specific tissue sample are undergoing this division.
The Pathologist’s Measurement
When a tissue sample is obtained, through a biopsy, it is sent to a pathology laboratory for examination. There, a pathologist prepares the tissue by slicing it into thin sections, placing them on glass slides, and applying special stains. These stains help highlight cellular structures, making dividing cells visible under a microscope. The pathologist then scans the prepared slide to identify areas exhibiting the most intense cell division, often referred to as “hot spots.”
Within these identified “hot spots,” the pathologist counts the number of cells undergoing mitosis. These identifiable mitotic figures are distinct shapes formed by the chromosomes within the nucleus as it divides. The count is then standardized and expressed as the number of mitoses per square millimeter (mitoses/mm²). This standardized measurement ensures consistency, allowing for comparative analysis of cell proliferation across different samples.
Interpreting the Results
The numerical value derived from the mitotic rate measurement provides information about the biological behavior of the tissue being examined. This rate serves as an indicator of how rapidly cells within a growth are proliferating. A higher mitotic rate suggests more aggressive cellular activity and quicker cell turnover within the tissue. Conversely, a lower mitotic rate indicates slower proliferation.
For instance, in the context of certain tumors, a mitotic rate of 0 mitoses per square millimeter in melanoma has been associated with a 10-year survival rate of approximately 93%. In contrast, a rate of 20 or more mitoses per square millimeter correlates with a lower 10-year survival rate, around 48%. This indicates that a higher count indicates a more active and more aggressive growth pattern. The mitotic rate is therefore considered an indicator of a tumor’s growth potential.
Role in Cancer Staging
The interpreted mitotic rate plays a direct role in formal cancer staging systems, classifying a patient’s diagnosis based on specific criteria. For example, in the American Joint Committee on Cancer (AJCC) staging system for cutaneous melanoma, the mitotic rate was incorporated into the seventh edition. This inclusion recognizes its significance as a factor influencing patient outcomes.
The mitotic rate helps differentiate between early-stage melanomas. A melanoma with a tumor thickness of 1.0 millimeter or less, if it exhibits at least 1 mitosis per square millimeter, is classified as T1b. This differs from T1a melanomas of similar thickness but with a mitotic rate of 0 per square millimeter and no ulceration. This distinction demonstrates how the mitotic count directly impacts the tumor’s classification, making it integral to the patient’s diagnosis.
Guiding Treatment Decisions
The information provided by the mitotic rate, particularly its influence on cancer staging, directly shapes subsequent treatment recommendations. When a higher mitotic rate contributes to a more advanced cancer stage, it can prompt a more aggressive approach to patient management. For instance, a T1b melanoma, characterized by a mitotic rate of at least 1 mitosis per square millimeter, might lead to a recommendation for a wider surgical excision to ensure complete removal of the tumor.
A higher mitotic rate can also influence the decision to perform a sentinel lymph node biopsy, a procedure to check for cancer spread to nearby lymph nodes. An elevated mitotic rate may lead to consideration of adjuvant therapies, such as immunotherapy, following initial surgery. This approach ensures that treatment plans are tailored to the tumor’s biological behavior, aiming for optimal patient outcomes.