Abstract
Tumour-infiltrating lymphocytes (TILs) are recognized as critical components of the colorectal cancer (CRC) immune microenvironment, demonstrating significant prognostic and predictive implications. However, a clear and widely accepted definition of colorectal TILs has not yet been established, and TILs are not systematically used as biomarkers in current oncology practice. This highlights the need for standardized assessment methods and a deeper understanding of the complex role of TILs in CRC, necessitating collaborative work to achieve clinical importance. This report, on behalf of the International Immuno-Oncology Biomarker Working Group (ITWG), provides a comprehensive overview of TILs in CRC, including a TILs-assessment method. It details their multifaceted role of TILs within the tumour microenvironment, details current and emerging methods for their assessment and reviews their prognostic and predictive importance. The report also explores the relationship of TILs to other established and emerging biomarkers in CRC. Emerging trends, including AI-driven analysis for automated TIL quantification, and single-cell and spatial technologies for in-depth characterization of the tumour microenvironment (TME), are also highlighted. The report concludes with a discussion of future directions and the ongoing efforts of the ITWG to address the challenges and standardize and advance the clinical application of TIL assessment in CRC.