Background: Asignificant amount of DCIS is ER negative and/or overexpresses HER2. This provides an opportunity to test molecular therapy in DCIS. In xenograft models and cell lines, T boosts RT effectiveness. In T-treated HER2+ patients, apoptosis occurs within 1 wk of single agent T use, with T found in ductal aspirates. Ample safety evidence for T exists. T given during whole breast irradiation (WBI) may improve results for Lx-resected HER2+ DCIS. A trial to examine this question will enhance the understanding of breast tumor biology and the prevention of such tumors and could possibly extend breast-conserving surgery benefits for women with DCIS. Methods: After Lx for pure DCIS, each patient’s DCIS lesion is centrally tested for HER2 by IHC analysis. HER2 2+ tumors undergo FISH analysis. HER2 3+ or FISH+ patients can be randomly assigned to 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 and clinically or pathologically node negative are eligible. Centrally tested DCIS must be HER2 +. ER and/or PR status must be known before randomization. Primary aims are to determine if T decreases ipsilateral breast cancer 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 OS can be improved with the use of T. 2000 patients will be accrued over 7.9 yrs, with a definitive analysis of primary endpoints performed at163 ipsilateral breast cancer events (7.5 - 8 yrs. after protocol initiation) with an 80% power to detect a hazard reduction of 36%, from 1.73 ipsilateral breast cancer 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 12-31-11, 763 patients have been randomized. NCT00769379 Grant support: PHS NCI-U10-CA-69651, -12027, and NCI P30-CA-14599 from the US NCI and Genentech, Inc.