Screen-detected breast cancers have a lower mitotic activity index

We know that screening for breast cancer leads to detection of smaller tumo urs with less lymph node metastases. Could it be possible that the decrease in mortality after screening is not only caused by this earlier stage, but also by a different mitotic activity index (MAI) of the tumours that are d etected by screening? Is MAI a prognostic factor for recurrence-free surviv al? A retrospective study was carried out of 387 patients with breast cance r, treated at the University Hospital Nijmegen between January 1992 and Sep tember 1997. Ninety patients had screen-detected breast cancer, 297 patient s had breast cancers detected outside the screening programme. The MAI, oth er prognostic factors and recurrence-free survival were determined. In non- screen-detected tumours the MAI is twice as high as in screen-detected tumo urs, even after correction for age took place. The MAI correlated well with other tumour characteristics. The MAI in itself is a prognostic factor for recurrence-free survival. Favourable outcome in screen detected breast can cer is not entirely caused by detecting cancer in early stages: quantitativ e features such as the MAI indicate a less malignant character of screen de tected breast cancer. The MAI is an independent prognostic factor for recur rence-free survival. (C) 2000 Cancer Research Campaign.