Proposal for a modified grading system based on mitotic index and Bcl2 provides objective determination of clinical outcome for patients with breast cancer.

We hypothesized that the interaction between mitotic index (M) and Bcl2 could accurately discriminate between low- and high-grade breast cancer (BC) and provide a more objective measure of clinical outcome than histological grade, especially for patients with intermediate histological grade (G2), small size or oestrogen receptor (ER)-negative cancers. A well-characterized series of 1650 BCs with long-term follow-up was subjected to immunohistochemical analysis for Bcl2. Mitotic index (M) was assessed according to Nottingham Grading System (NGS) guidelines: M1: < 10 mitoses; M2: 10-18 mitoses; M3: > 18 mitoses. Results were validated in an independent series of patients (n = 245) uniformly treated with adjuvant anthracycline-based chemotherapy. Subsequently, BCs were classified according to the combined M/Bcl2 profile and compared with NGS. Multivariate Cox regression models using validated prognostic factors demonstrated that the subgroups defined by M/Bcl2 profile remained significantly associated with patients' outcome but also performed better than lymph node status and tumour size. Incorporation of the M/Bcl2 profile into the Nottingham Prognostic Index (NPI) reclassified twice as many patients into the excellent prognosis group, potentially improving decision-making and sparing patients unneeded systemic adjuvant therapy. Patients with M2-3/Bcl2- and M3/Bcl2+ (high risk) had a two- to three-fold increased risk of recurrence when treated with either adjuvant hormone therapy or anthracycline-based chemotherapy compared with those with M1/Bcl2 ± and M2/Bcl2+ (low risk) [HR = 3.4 (2.8-5.6); p < 0.0001 and HR = 2.3 (1.2-4.3); p = 0.0009]. In conclusion, a grading system defined by mitotic counting and Bcl2 expression accurately reclassified patients with NGS-G2, small tumour size or ER-negative cancers into two groups: low risk (NGS-G1-like) versus high risk (NGS-G3-like) of BC mortality and recurrence, improving prognosis and therapeutic planning.