Abstract PO1-28-10: Tumor Infiltrating Lymphocytes identify patients with stage I TNBC with excellent outcome without chemotherapy

Abstract Introduction International guidelines recommend chemotherapy for most patients with stage I triple negative breast cancer (TNBC), except for special histological subtypes. However, the absolute benefit of systemic treatment for all stage I TNBC patients is not well known and no tools are yet available to select patients with an excellent prognosis for whom the chemotherapy can be safely omitted. High levels of stromal tumor infiltrating lymphocytes (sTILs) are strongly associated with favorable prognosis in TNBC, however, data solely focusing on patients with stage I TNBC who did not receive chemotherapy are lacking. Methods Patients with pT1N0M0 TNBC diagnosed between 2005 and 2015 not treated with (neo)adjuvant chemotherapy were identified from the nationwide Netherlands Cancer Registry (NCR) and linked to the national pathology database (PALGA). Newly sectioned hematoxylin 1cm (pT1cN0). Median follow-up was 9.4 years in which 335 patients had died, of whom 107 due to breast cancer. Patients with pT1abN0 tumors had a favorable prognosis compared with patients with a pT1cN0 tumor (HR 0.47, CI 0.32- 0.72). 10-year BCSS in patients with pT1ab was 91.8% and 85.8% in patients with pT1c tumors. In the overall cohort, sTILs-levels of at least 30% were associated with better BCSS compared to sTILs 30% (HR 0.45, CI 0.26 – 0.77), with 10year-BCSS of 96% and 87%, respectively (table 1). Patients with TNBC of special histological subtype had low sTILs (median 1%). A univariate cox model showed no association between sTILs (continuous) and BCSS in TNBC of special histological subtype (p = 0.56). Next, the prognostic value of sTILs was evaluated separately for pT1ab and pT1c. High sTILs ≥50% were not associated with improved BCSS compared to low sTILs 50% in patients with pT1ab (HR 1.35, CI 0.65 – 2.82, p = 0.42). In contrast, high sTILs ≥50% showed a strong prognostic importance over low sTILs 50% (HR 0.27, CI 0.10 – 0.74) in patients with pT1C tumors. We found excellent 10year-BCSS of 95% for patients if sTILs ≥50%, which further increases to 98% with sTILs ≥75% contrasting 10year-BCSS of 83% for patients with sTILs 30%. Conclusions sTILs-levels provide important prognostic information in patients with stage I TNBC who did not receive (neo)adjuvant chemotherapy. The outcome of patients with pT1c TNBC and sTILs ≥50% was excellent, with 10year-BCSS of 95%, and 98% if sTILs 75%. This large registry study provides a foundation for clinical trials in patients with stage I TNBC with high TILs to prospectively confirm their excellent survival without chemotherapy. Citation Format: Veerle Geurts, Sara Balduzzi, Hugo Horlings, Tessa G Steenbruggen, Sabine Siesling, Sylvia Adams, Gabe Sonke, Roberto Salgado, Marleen Kok. Tumor Infiltrating Lymphocytes identify patients with stage I TNBC with excellent outcome without chemotherapy abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-28-10.