Abstract P1-20-02: Axillary conversion rates according to breast cancer subtype among patients with pathological complete remission (pCR) in the breast after primary systemic therapy (PST) - A transSENTINA subproject

Background: Modern chemotherapy carries a high potential of converting patients with a clinically positive axilla (cN+) prior to PST to a clinically (ycN0) or even pathologically (ypN0) negative axilla after PST. Patients achieving a pathological complete remission (pCR) in the breast after PST carry a favorable prognosis including a high probability of axillary conversion. Among patients with highly chemosensitive tumors (i.e. high pCR rate), axillary staging may be adjusted accordingly. Methods: In a retrospective study we analyzed the relation between breast pCR, nodal pCR and the intrinsic subtype in clinically node-positive patients who convert to ycN0. Arm C of the SENTINA trial included patients converting from cN+ to ycN0 through PST as part of a prospective study. Patients were categorized into luminal (HR+/HER2-), triple negative (TN; HR- / HER2-), triple positive (TP (HR+ / HER2+), and HER2 pos ER neg HER2 positive (HR- / HER2+) subgroups. Results: Overall 416 of 596 patients were included (exclusions due to missing parameters) with 167 luminal, 119 TN, 77 TP and 56 HER2 pos ER neg HER2 positive tumors. Among all patients, pathological axillary conversion occurred in 154 (80.21%) and 91 (32.97%) cases among patients with and without pCR, respectively (odds ratio (OR) 0.23, p-value • TN 56 (84.85%) and 24 (45.28%)• TP 33 (73.33%) and 12 (37.50%) • Luminal 18 (66.67%) and 34 (24.29%) • HER2 pos ER neg 33 (91.67%) and 9 (45.00%) We identified patient subgroups, among whom conversion rates were > 90%, i.e. among patients with (i) TN breast cancer and age at diagnosis 50 years (94.44%). Conclusion: Our analysis demonstrates that among patients with pCR, probabilities of axillary conversion were significantly higher among patients with pCR compared to patients without pCR. However, results differed according to biological breast cancer subtypes, with more aggressive subgroups presenting with higher probability of axillary conversion. We identified subgroups of patients among whom the rate of axillary residual disease after PST was lower compared to the false-negative rate of sentinel lymph node biopsy. Provided independent validation of our results, these subgroups may benefit from de-escalation of axillary stagingsurgery. Citation Format: Cornelia Kolberg-Liedtke, Hans-Christian Kolberg, Maja Krajewska, Ingo Bauerfeind, Tanja Fehm, Barbara Fleige, Gisela Helms, Annette Lebeau, Annette Staebler, Sibylle Loibl, Michael Untch, Thorsten Kuhn. Axillary conversion rates according to breast cancer subtype among patients with pathological complete remission (pCR) in the breast after primary systemic therapy (PST) - A transSENTINA subproject [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-20-02.