Tumor-infiltrating lymphocytes density and distribution in breast cancer: A two-year study on their...

Tumor-infiltrating lymphocytes (TILs) are emerging as key biomarkers in invasive breast carcinoma (IBC), reflecting the host immune response and influencing prognosis. While TILs are well-studied globally, data from the Indian subcontinent remains scarce. This study assesses stromal TILs (sTILs) and intratumoral TILs (iTILs) in an Indian cohort, investigating their associations with pathological features, clinical stage, and molecular subtypes.

A retrospective observational study of consecutive IBC patients at a single Indian cancer center was conducted over two years and included. Demographic, pathological, and immunohistochemical (IHC) data (estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 [HER2], Ki67 proliferation index) were collected. TILs were evaluated on Hematoxylin and eosin (H and E) slides using International TILs Working Group (ITILWG) guidelines: sTILs expressed as a percentage of stromal area and iTILs semi-quantitatively (0-300). Correlations and group comparisons were analyzed using Spearman's rank correlation, Kruskal-Wallis tests, and multivariable linear regression.

A total of 326 patients were included. The median sTIL percentage was 7% (interquartile range [IQR] 5-38), and the median iTIL score was 60 (IQR 20-130). Higher median sTIL and iTIL levels were observed in triple-negative and HER2-enriched tumors, high-grade cancers, and hormone receptor-negative disease. Median sTIL percentages increased with tumor grade (grade 1: 5% [IQR 5-5]; grade 3: 35% [IQR 9-60]). Similar patterns were observed for iTILs. After Holm adjustment, associations with nuclear pleomorphism, mitotic activity, tumor grade, molecular subtype, Ki-67 category, and hormone receptor status remained statistically significant. In multivariable analyses using log-transformed TILs, molecular subtype and tumor grade were independently associated with higher sTIL and iTIL levels, while stage IV disease was associated with lower TIL levels compared with stage II.

This study confirms a strong correlation between sTILs and iTILs, suggesting sTIL assessment alone may suffice for routine practice. Higher TIL levels were significantly linked to more aggressive pathological features, such as higher tumor grade and triple-negative/HER2-enriched subtypes. While TIL levels varied with clinical stage, no consistent monotonic relationship was observed. Future research should integrate other biomarkers predictive of immunotherapy response and establish clinically actionable TIL cut-off values.