Abstract PS4-10-13: Clinical Feasibility and Reproducibility of Tumor Infiltrating Lymphocyte (TIL) Reporting for Early-Stage Triple Negative Breast Cancer Patients at the University of Washington

Abstract Background: Tumor Infiltrating Lymphocytes (TILs) have prognostic and predictive value in triple negative breast cancer (TNBC). Higher TILs correlate with improved rates of pathologic complete response (pCR) following neoadjuvant chemotherapy and lower risk of recurrence and death. While TILs have emerged as an important biomarker, they are not routinely clinically reported. Incorporating TILs into pathology reports for patients with early-stage TNBC has the potential to impact patient care and support ongoing research efforts. Here we report our institutional experience standardizing TIL reporting to generate feasible, reproducible TIL evaluation for diagnostic breast biopsies for early-stage TNBC. Methods: Stage I-III TNBC cases were identified by breast pathologists at the University of Washington. Clinical characteristics and survival data was collected by chart review. TIL status was independently assessed by two pathologists according to the International TILs Working Group Guidelines: a complete assessment was done using at least a 4-5 um section x200-400 per patient and TILs (including all mononucelear cells) were reported for the stromal component within the border of the invasive tumor (TILs outside the tumor border and those in areas of necrosis/crush artifact were excluded). TIL status was categorized as A (low 10%), B (intermediate, 10-39%), or C (high, 40%). For cases where readers disagreed on the category, TILs were reassessed by both readers together. Read time was collected to assess impact on pathologist workload. Results: We evaluated TIL status in 56 patients with histologically confirmed TNBC between 2021-2023. Twenty-eight percent had clinical stage I disease, 51% were stage II, and 21% were stage III. The median TIL percentage was 25 (range 1-90) for reader 1 and 20 (range 1-90) for reader 2 (p=0.060). For reader 1, 41% were category A, 27% were category B, and 32% were category C. For reader 2, 36% were category A, 32% were category B, and 32% were category C. Percent agreement in TIL category was 80% (45/56, 95% CI: 68-90%) and unweighted Cohen’s kappa was 0.70 (95% CI: 0.55-0.86). The average time to review was 51 seconds (range 19-93 seconds) for reader 1 and 25 seconds for reader 2 (range 6-127 seconds). There was no significant difference in review times between cases where both readers agreed on category (n = 45) compared to when they disagreed (n = 11) (median time: 40 vs. 41 seconds, p = 0.17, Wilcoxon rank-sum test). Conclusions: Using the International TIL Working Group Guidelines, we found TIL reporting to be highly reproducible between pathologists with 80% concordance. TIL reporting was feasible and the impact on workload was manageable with an average added time of less than 1 minute per case. Standardization of reporting is crucial for ongoing research efforts and valuable for patient care. Citation Format: L. Symonds, D. Reiter, D. S. Hippe, A. Kazerouni, S. Mittal, N. Hunter, R. Yung, W. R. Gwin III, V. Nair, N. Davidson, H. Linden, J. Specht, M. Kilgore. Clinical Feasibility and Reproducibility of Tumor Infiltrating Lymphocyte (TIL) Reporting for Early-Stage Triple Negative Breast Cancer Patients at the University of Washington abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-10-13.