Background: A significant amount of DCIS is ER-negative and/or overexpresses HER2. This study will test HER2-targeted therapy in DCIS. Among T-treated HER2+ patients (pts) with DCIS treated with a single dose of T, T is found in ductal aspirates and antibody-dependent cell-mediated cytotoxicity activity for HER2 is increased. T boosts the effectiveness of RT in breast cancer xenograft models and cell lines. T given during whole breast irradiation (WBI) may improve results for HER2+ DCIS treated with lumpectomy (Lx). A trial to examine this question will enhance the understanding of breast tumor biology, the prevention of such tumors, and could possibly extend breast-conserving surgery benefits for women with DCIS. Methods: After Lx for pure DCIS, each pt’s DCIS lesion is centrally tested for HER2 using ASCO/CAP guidelines. HER2+ pts are randomly assigned to receive 2 doses of T, 3 weeks apart during WBI or to WBI alone. Women ≥18 yrs with a margin-clear Lx for pure DCIS, with ECOG status 0/1 who are clinically or pathologically node negative are eligible. ER and/or PR status must be known before random assignment. Primary aims are to determine if T decreases ipsilateral breast cancer (IBC) recurrence, ipsilateral skin cancer recurrence, or ipsilateral DCIS. Secondary aims are to determine the benefit of T in preventing regional or distant recurrence and contralateral invasive breast cancer or DCIS. B-43 will determine if DFS, recurrence-free interval, and/or overall survival can be improved with the use of T. 2000 pts will be accrued over 7.9 yrs, with a definitive analysis of primary endpoints performed at163 IBC events (7.5 - 8 yrs after protocol initiation) with an 80% power to detect a hazard reduction of 36%, from 1.73 IBC events per 100 pt-yrs to 1.11 events per 100 pt-yrs. The 36% observed reduction in the hazard of IIBCR-SCR-DCIS on the T arm is based on a projection of 40% hazard reduction if the compliance were perfect, with a 10% noncompliance rate. As of 1-1-13, 1,127 pts have been randomized into the study. Support: PHS NCI-U10-CA-69651, -12027, and -P30-CA-14599 from the US NCI, and Genentech, Inc. Clinical trial information: NCT00769379.