Do levels of cell proliferation predict outcomes in breast cancer?

This paper studied whether the mitotic activity index (levels of mitotic activity) could be used to predict the outcomes in lymph node-negative invasive breast cancer patients (breast cancer that has spread to surrounding tissues but not the lymph nodes). 

Mitotic activity index is a measurement of the number of cells that are proliferating. It is suggested that levels of cell proliferation would be a good indicator on whether the cancer is likely to spread further. It may also predict whether adjuvant systemic therapy (treatment given after surgery) is beneficial. Studies have shown that mitotic activity index is a strong factor in predicting the outcomes in breast cancer that has not spread to the lymph nodes. 

The records of 678 patients aged 55 or less whose breast cancer had not spread to the lymph nodes were studied. Each patient underwent breast-conserving therapies (surgery to remove part of the breast). Of these patients, 71.0% had low mitotic activity while 21.4% had high mitotic activity. Patients were followed for 15 years.

After 15 years, 86% of women with low mitotic activity had survived with no return of breast cancer. Meanwhile, 90.3% of women with high mitotic activity had survived with no return of breast cancer.

After 15 years, 88.3% of women with low mitotic activity had survived with no spread of cancer to other parts of the body. 73.4% of women with high mitotic activity had survived with no spread of cancer to other parts of the body.

After 15 years, 90.7% of women with low mitotic activity and 75.3% of women with high mitotic activity had not died due to breast cancer (breast cancer-specific survival).

The authors concluded that mitotic activity index was predictive of cancer spread and breast cancer survival in women aged 55 years and less with breast cancer that has spread to the surrounding breast tissue but not lymph nodes.