Proliferation accurately identifies the high-risk patients among small, low-grade, lymph node-negative invasive breast cancers

Background: The proliferation factor mitotic activity index (MAI) is the strongest prognosticator in lymph node-negative invasive breast cancer patients under age 71. The question remains, whether this also holds for 'favourable prognosis' subgroups. Patients and methods: The study was a multicentre prospective analysis of the MAI for recurrence-free survival and overall cancer-related survival of grade, MAI, and other prognosticators in 853 long-term follow-up, T1-3N0M0 breast cancer patients under 71 years. Results: In all tumours together (N = 853), in grade 3 (n = 269), in tumours <1 cm all grades (n = 84), 1-2 cm, grades 1 + 2 (n = 300), and 2-3 cm, grades 1 + 2 (n = 124), the MAI is prognostically superior. Other features [grade, estrogen receptor (ER), diameter, and age] did not enhance its prognostic value except in grades 1 + 2 tumours 2-3 cm diameter with MAI <10, where ER has an additional prognostic value. Conclusions: In women <71 years with T1-3N0M0 small or low-grade invasive breast cancer usually not receiving systemic treatment, MAI ≥10 accurately identifies those at high risk. These high-risk patients should be considered for adjuvant systemic therapy.