Tumor-Infiltrating Lymphocytes in HER2-Low Breast Cancer.

Introduction: Tumor-infiltrating lymphocytes (TIL) is a predictive and prognostic biomarker for breast cancer (BC) HER2-positive and triple negative, but its presence in HER2-low tumors is unknown. We aimed to determine TIL levels in HER2-low tumors and its correlation with other clinicopathologic features.

Materials And Methods: We retrospectively analyzed all the pathology reports of breast surgeries of a tertiary hospital in Sao Paulo, Brazil, from January 2021 to March 2022. Inclusion criteria were stage I to III invasive BC, and exclusion criteria were nonmalignancies and neoadjuvant therapy. We assumed HER2 categories according to ASCO/CAP guidelines. TILs were defined as absent (0), low (1%-10%), intermediate (11%40%) and high (≥ 41%). Ki-67 levels were categorized as low (up to 19%) and high (≥ 20%).

Results: From 272 patients, 198 met the inclusion criteria. Histological grade 3 was found in 10, 19 and 47% of HER2-0, low, and positive tumors (P < .001). HER2-positive tumors had 82.6% of high Ki-67 levels, while HER2-negative and HER2-low showed 25.8% and 31.4% (P = .005). TILs in HER2-0, low, and positive tumors were, respectively, absent in 16.1%, 17.6%, and 8.7%; low in 70.2%, 52.9% and 34.8%; intermediate in 11.3%, 25.5% and 47.8%; and high in 2.4%, 3.9% and 8.7%. There was a statistically significant difference in TILs between HER2-negative versus HER2-positive groups (P < .001), but not between HER2-negative versus HER2-low, or HER2-low versus HER2-positive.

Conclusion: TILs in HER2-low are marginally higher than HER2-negative, but significantly lower than HER2-positive levels. HER2-low tumors do not seem to significantly differ biologically from HER2-negative tumors.