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This Phase III trial compares the recurrence-free interval (RFI) among patients with early-stage, low risk HER2+ breast cancer who undergo breast conserving surgery and receive HER2-directed therapy, and are randomized to not receive adjuvant breast radiotherapy versus those who are randomized to receive adjuvant radiotherapy per the standard of care.

The landmark trials that established breast conservation therapy (BCT) (breast-conserving surgery followed by adjuvant breast irradiation) as a suitable alternative to mastectomy were conducted in an era that predated biological subtyping of breast cancer and the use of HER2-directed therapies in patients with HER2+ cancers. These trials established adjuvant radiotherapy following breast-conserving surgery as necessary to maximize local control, yet, in the intervening years, overall outcomes have improved significantly owing to widespread adoption of screening mammography, resulting in a substantial reduction in average tumor size at diagnosis, as well as improvements in surgical techniques and, crucial for this proposal, the development of highly active systemic therapies. Before the development of HER2-targeted therapies, patients with HER2-driven localized breast cancer had among the highest rates of local recurrence. However, with improved identification of these patients and the advent of HER2-directed therapies, outcomes have improved significantly, and trials have sought to optimize treatment to reduce the morbidity of both local and systemic treatment. Among the most salient of these examples is the APT trial, a single-arm adjuvant study that enrolled 410 breast cancer patients with HER2+ tumors ≤ 3cm in size and negative axillary nodes, who received adjuvant systemic therapy with weekly paclitaxel and trastuzumab for 12 weeks (TH) followed by 9 months of trastuzumab monotherapy. In addition to demonstrating a very low incidence of distant recurrence, among those on the trial who underwent BCT (lumpectomy and radiation, n = 244), only 2 local recurrence (LR) events have been reported after 7 years of follow-up (7-year LR = 1.2%), representing among the most favorable local outcomes of any breast cancer cohort studied to date. Confirmatory results are forthcoming from the ATEMPT trial, which evaluated the antibody-drug conjugate T-DM1 (ado-trastuzumab emtansine) (n=383) vs the TH regimen from the APT trial (n = 114), thus far showing only 3 LRs in each arm with a median 3-years of follow-up. Importantly, per the current standard of care for HER2+ patients undergoing BCT, all patients presumably received adjuvant breast radiotherapy. The balance of the BCT literature, including a landmark meta-analysis by the Early Breast Cancer Trialists' Collaborative Group, suggests that adjuvant radiotherapy approximately halves the risk of local recurrence following lumpectomy across all analyzable subgroups. While the relative benefit appears constant across subgroups, the absolute benefit of adjuvant radiotherapy varies with the underlying risk. Taking the favorable results of the APT trial (1.2% 7-year LR), if one presumes that omission of radiotherapy yields a doubling or tripling of local recurrence (based on the observed RR of 0.5 - 0.66 for those receiving radiotherapy across the preponderance of historical trials), this population might have manifested a LR rate of 2.4 - 3.6% with the omission of radiotherapy. That is to say, the hypothesis is that administration of RT to APT patients undergoing BCT may have reduced the 7-year absolute risk of LR by only 1.2-2.4%. Through the identification of patients who are at low risk of LR, it may be acceptable for such patients to forego radiation. This hypothesis will be studied by evaluating omission of radiotherapy among patients with pT1N0 disease at breast-conserving surgery who receive adjuvant HER2-directed therapy (trastuzumab/paclitaxel preferred, other options per protocol), or with clinical tumors ≤ 3 cm and clinically negative axillary nodes (cN0) who achieve a pathologic complete response (pCR; ypT0N0) following preoperative (neoadjuvant) administration of HER2-directed therapy (trastuzumab/paclitaxel preferred, other options per protocol).