Effects of adjuvant chemotherapy in HER2-positive or triple-negative pT1ab breast cancers: a multi-institutional retrospective study.

Previous studies have reported higher recurrence rates in T1a/b N0 breast cancers characterized by high-risk biology (HER2+ or triple-negative), but the benefits of adjuvant chemotherapy in these patients have not been established. This study was designed to determine whether recurrence risk is reduced with chemotherapy and to define a group of patients most appropriate for treatment based on retrospective data. We pooled cases from two multi-institutional databases spanning the period of 1996-2010. A propensity score model adjusted unbalanced confounders between the groups treated or untreated with adjuvant chemotherapy and, in case of HER2-positive disease, with trastuzumab. Competing risk analysis was utilized to study effects of chemotherapy on cancer recurrences in the matched populations. Among the 318 patients identified, 41 % received adjuvant chemotherapy and 54 % of HER2+ patients received it with trastuzumab. The cumulative risk of recurrence at 5 years was 7.3 %. Age less than 35 years and triple-negative status were the only significant prognostic factors. Overall, administration of chemotherapy was not associated with a significant decrease in the risk of recurrence (HR 0.93, p = 0.91). The rate of recurrence in HER2+ patients who received trastuzumab was lower but not statistically significant (HR 0.50, p = 0.63). Clinical characteristics are of limited prognostic value for stratifying risk of recurrence in very small, node-negative HER2+, or triple-negative cancers. While limited by the small number of events, our analysis does not support the increasingly prevalent practice of administering adjuvant chemotherapy in this population without more accurate prognostic and predictive factors.