In primary breast cancer the mitotic activity yields similar prognostic information as the histological grade: a study with long-term follow-up.

We evaluated with long-term follow-up, the prognostic value of the mitotic activity index (MAI) and the volume corrected mitotic index (M/V-index) compared with that of the histological grade in breast cancer patients not treated with adjuvant systemic therapy. Of 739 consecutive patients living in the city of Nijmegen, the Netherlands, 477 patients with primary unilateral breast cancer were not treated with adjuvant systemic therapy and eligible for the study. In multivariate survival analyses the MAI and M/V-index showed similar hazard ratios (HRs) compared to HRs of histological grade for overall survival (OS) (HR: 1.45, 1.48, and grade II versus grade I (GII/GI) 1.34, grade III versus grade I (GIII/GI) 1.53, respectively) and for breast cancer specific survival (BCSS) (HR: 1.27, 1.57, and (GII/GI) 1.57 (GIII/GI) 2.32, respectively). Other independent prognostic variables for OS and BCSS were age at diagnosis, tumour size, and number of positive lymph nodes. In the present study with long term follow-up, we compared the prognostic value of mitotic activity with that of histological grade and found no advantage for the mitotic activity in predicting either BCSS or OS and concluded that histological grade and the mitotic activity were equally informative in predicting patient outcome. As histological grade is a well established and widely used prognosticator we do not have arguments to replace the histological grade by the mitotic indices MAI or M/V-index.