Long-term outcomes of patients with HER2+ breast cancer with small-size residual disease (≤ypT1) in the absence of pathological response after trastuzumab-based neoadjuvant chemotherapy and without adjuvant T-DM1: A monocentric retrospective study.

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589 Background: The Katherine trial has shown that adjuvant T-DM1 improved invasive disease free survival (iDFS) of patients with HER2+ breast cancer who did not achieve a pathological complete response (pCR) with trastuzumab-based neoadjuvant chemotherapy (NAC). However, some subgroups may benefit less from this treatment escalation. Methods: All HER2+ breast cancer patients treated with trastuzumab-based NAC between 2006 and 2016 were retrieved from our institution’s database. Neo- or adjuvant T-DM1 was an exclusion criterion. We then selected the patients who did not achieve a pCR and analyzed the outcome (iDFS and overall survival (OS)) according to ypT, ypN and several factors analyzed in the Katherine trial. Results: Out of the 182 patients, 117 patients reached the inclusion criteria. Patient’s characteristics were similar to the trastuzumab arm of the Katherine trial. With a median follow-up of 75.4 months (29.3–149.7), 28 events (24%) occurred, among which 22 distant relapses. In univariate analysis, ≤ypT1 vs > ypT1, ypN0 vs ypN+, no capsular rupture, signs of histological response (Sataloff not D in T or N) were associated with a better iDFS. In multivariate analysis, only ypT status remained significant. Of note, patients with ≤ypT1 (ypTis, ypT0, ypTmic, ypT1) (n = 81; 69%) had an excellent outcome: 3 years (y) and 5y iDFS rates of 90% (83.6-96.8) and 88.6% (81.9-95.9) respectively. The remaining patients (n = 36; 31%) had a significantly lower 3y and 5y iDFS: 69.2% (55.6-86.2) and 59.5% (45-78.6) respectively (p = 0.0017). OS in a multivariate analysis was also improved in pts with the smaller residual and/or node negative disease (3y OS rates of 100% (100-100) vs 92.1% (85.7-99); 5y OS rates of 96% (90.7-100) vs 81.1% (71.6-91.9); p = 0.02). Conclusions: In the absence of pCR after trastuzumab-based NAC, patients with pathological response scored as ≤ypT1 (ypTis,ypT0, ypTmic, ypT1) have an excellent outcome. These patients may derive less benefit from adjuvant T-DM1.