Morphometric grading in breast cancer: Thresholds for mitotic counts

Three hundred sixty-four cases of invasive ductal breast cancer diagnosed d uring the years 1988 to 1991 were analyzed to determine quantitative thresh olds for mitotic activity. Mitotic counts were calculated in each sample an d expressed as standardized mitotic index (SMI) and mitotic activity index (MAI). Based on Kaplan-Meier curves, univariate and multivariate analysis o f Cox's regression, and maximum efficiencies of ROC analysis, optimal thres holds were determined on the basis of survival and recurrence of disease. I n our material, with a follow-up time of 5 years 9 months, we found two thr esholds-a lower and a higher-for both SMI (17 mitoses/mm(2) and 32 mitoses/ mm(2)) and MAI (13 mitoses/10 HPF and 35 mitoses/10 HPF). The thresholds: w ere the same in the whole material and in subgroups divided according to th e patients' age and axillary lymph node status at the rime of diagnosis, an d tumor size. The thresholds clearly separated patients with favorable, int ermediate, and unfavourable outcome of disease. In our material, the: risk of breast cancer death associated with the determined thresholds (ranging f rom 4.7 to 3.8) clearly exceeded those of menopausal status, axillary lymph node status and tumor size. The risk of breast cancer death associated wit h the determined thresholds was still emphasized in the groups of premenopa usal and axillary lymph node-negative patients, and with tumor size less th an 2 cm in diameter (risk ratios, 11.8, 6.0, and 6.7, respectively). The re sults suggest that the presented quantitative thresholds could be applied i n grading bf invasive ductal breast cancer. HUM PATHOL 29:1462-1468. Copyri ght (C) 1998 by W.B. Saunders Company.