Mitotic index and benefit of adjuvant anthracycline-based chemotherapy in patients with early breast cancer.

PURPOSE: We have evaluated whether the mitotic index could predict the benefit of adjuvant anthracycline-based chemotherapy in patients with early breast cancer who are eligible for adjuvant chemotherapy according to Saint Gallen guidelines. PATIENTS AND METHODS: A total of 937 patients from a single institution were included in two randomized trials that compared adjuvant anthracycline-based chemotherapy with no chemotherapy. These patients account for 83% of the overall population included in these trials. The first trial included premenopausal patients with node-negative disease, and the second one included postmenopausal patients, regardless of lymph node status. The treatment benefit was assessed according to the number of mitoses per field (x400). RESULTS: The mitotic index was assessable in 888 patients (94%). All the patients presented as either node-positive or an average-risk breast cancer according to 2003 Saint Gallen consensus conference guidelines. The 5-year overall survival rates were 91% and 87% for patients treated or not with adjuvant chemotherapy (P = .09). In patients with low/medium mitotic index (< three mitoses/field; n = 450), the 5-year overall survival rate was 95% for patients treated or not with adjuvant chemotherapy (P = .56). In patients with high mitotic index (>/= three mitoses/field; n = 438), the 5-year overall survival rates were 86% and 79% for patients treated or not treated with adjuvant chemotherapy, respectively (P = .02). CONCLUSION: A high mitotic index is associated with the efficacy of adjuvant anthracycline-based chemotherapy in patients eligible for adjuvant chemotherapy in daily practice.