Locoregional recurrence following pathologic complete response in patients with T1-3 N0 breast cancer treated with neoadjuvant chemotherapy, breast conserving surgery and whole breast radiation: Is a trial of omission of radiation warranted?

e12625 Background: Pathologic complete response(pCR) after neoadjuvant chemotherapy(NAC) in patients with breast cancer(BC) is associated with decreased recurrence and improved survival. In NSABP B18 and B27, 10-yr locoregional recurrence(LRR) after pCR in patients with Stage I-III BC undergoing breast conserving surgery(BCS) and whole breast RT(WBRT) was 5.2-6.9%. However, LRR may be overestimated as Her2 therapy was not used and only some eligible patients received endocrine therapy. A retrospective study using modern protocols found a 5-yr LRR of up to 2.6%. We hypothesize that LRR in N0 patients is even lower, and de-escalation of therapy should be examined. The study objective is to assess if a prospective trial of omission of WBRT after BCS in patients with N0 BC and pCR after NAC is warranted and to assess feasibility. Methods: Patients with T1-T3 N0 invasive BC diagnosed between Dec 2011-2017 treated with NAC and BCS were identified from a hospital BC registry. Health records were retrospectively reviewed to identify patients with pCR, defined as absence of residual invasive or in-situ disease(ypT0N0). Incidents of locoregional and distant recurrence were recorded. Results: Of 89 patients with T1-3 N0 invasive BC treated with NAC and BCS, 29(32.6%) had pCR. Median follow-up was 61.1 months. Median age was 55 yrs and median tumour size was 2.4cm. Receptor status was 16(55.2%) HR-Her2-, 4(13.8%) HR-Her2+, 7(24.1%) HR+Her2+ and 2(6.9%) HR+Her2-. NAC protocols consisted of an anthracycline and/or a taxane in 27(90%) patients, and 6 patients were treated on NAC trials. All patients with Her2+ disease received Her2 targeted therapy. Adjuvant endocrine therapy was taken by 8 of 9 patients with HR+ disease. All patients received WBRT without nodal RT. RT plan was available for 26(86.7%) patients. RT dose ranged from 40-50Gy, and all but 4 received tumour bed boost. There were no local or regional recurrence events at last follow-up. One patient developed brain metastases at 15.7 months. Conclusions: Over 6 years, 29 patients were identified that would be eligible for a prospective trial evaluating omission of WBRT after pCR in N0 patients treated with NAC and BCS. At median 5-yr follow-up, there were no locoregional recurrences in our cohort, demonstrating that the absolute benefit provided by WBRT is likely small. Our results indicate a prospective trial is warranted and will require multi-institution participation to accrue.