Abstract PO3-22-05: Investigating HER2-low tumors from a locoregional perspective- a secondary analysis of a randomized radiotherapy trial

Abstract

Abstract Background: With the introduction of antibody-drug conjugates directed against HER2, a new subgroup of tumors with low HER2 expression (1+ or 2+ without amplification) has emerged. HER2 signaling has been implicated in radioresistance in mechanistic studies and is an accepted biomarker of immunogenicity. The study aimed to contrast tumors with low HER2 expression versus zero HER2 expression from a prognostic and radiotherapy-predictive locoregional perspective. Methods: HER2 status was evaluated using immunohistochemistry and SISH in the randomized radiotherapy (RT) cohort, SweBCG91RT, by a board-certified pathologist. HER2-low status was defined as HER2 IHC 1+ or 2+ without amplification, HER2-zero as 0, and HER2-high as HER2 IHC 3+ or SISH amplified. Tumor-infiltrating lymphocytes were assessed on whole-tissue sections and categorized as low (< 10%) or high (≥ 10%). The endpoint was time to ipsilateral breast tumor recurrence (IBTR) as first event. Results: Out of 964 evaluated primary tumors, 433 (44.9%) were classified as HER2-low, 467 (48.4%) as HER2-zero and 64 (6.6%) as HER2-high. TILs levels did not differ between HER2-low and HER2-zero tumors (p=0.33). The hazard ratio for high TILs was 0.79 in HER2-zero tumor (CI 95% 0.43-1.45, p=0.44) and 0.96 in HER2-low tumor (CI 95% 0.56-1.66, p=0.90), and no interaction effect from TILs was observed (p=0.63). The benefit from RT was similar among HER2-low (HR 0.42, CI 95% 0.25-0.72, p=0.001) and HER2-zero tumors (HR 0.31, CI 95% 0.17-0.56, p< 0.001). Conclusions: HER2-low tumors do not differ from HER2-zero tumors regarding the benefit from postoperative RT. Unlike high HER2 expression, low expression is not associated with TILs infiltration. HER2-low tumors likely do not represent a biologically distinct subtype from a locoregional aspect. Citation Format: Per Karlsson, Anikó Kovács, Fredrika Killander, Emma Niméus, Erik Holmberg, Axel Stenmark Tullberg. Investigating HER2-low tumors from a locoregional perspective- a secondary analysis of a randomized radiotherapy trial [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 PO3-22-05.