Mitotic index (tumor)

The mitotic index measures how rapidly tumor cells are dividing. It is defined as the number of cells undergoing mitosis (cell division) relative to the total number of tumor cells observed. A high mitotic index indicates aggressive tumor growth, while a low value suggests slower proliferation.

In traditional pathology, the mitotic index is counted as the number of mitotic figures per 10 high-power fields (HPF). HistoAtlas computes a normalized variant: mitoses per 1,000 tumor cells, calculated as:

$$\text{Mitotic Index} = \frac{N_{\text{mitoses}}}{N_{\text{tumor cells}}} \times 1{,}000$$

Normalizing by tumor cell count rather than area reduces dependence on sample size and tumor fraction, making values comparable across slides, centers, and cancer types.

Mitotic index is a key component of several clinical grading systems. In breast cancer, it is one of three elements of the Nottingham histological grade (along with tubule formation and nuclear pleomorphism). In gastrointestinal stromal tumors (GISTs), mitotic count directly determines risk stratification. High mitotic activity is also prognostic in melanoma, soft tissue sarcomas, and neuroendocrine tumors.

Manual mitotic counting is labor-intensive and suffers from inter-observer variability. HistoAtlas uses deep learning to detect mitotic figures computationally from standard H&E-stained whole slide images across all 33 TCGA cancer types — enabling consistent, reproducible quantification at a scale no manual approach can achieve. Explore below how mitotic index associates with patient survival, molecular subtypes, and driver mutations across cancers.