TILs in Head and Neck Cancer: Ready for Clinical Implementation and Why (Not)?

The assessment of tumor infiltrating lymphocytes (TILs) has recently emerged as a prognostic biomarker in several solid tumors. Quantification and subtyping of TILs reflects the immune response in the tumor microenvironment, contributing to either tumoral immune attack or escape and thereby affecting outcome. Despite the growing evidence of its value as prognosticator, TILs analysis has not yet found its way to daily clinical practice. The aim of this review is to evaluate whether the current knowledge on TILs in head and neck cancer justifies its clinical implementation. Therefore, we summarize the data on TILs in squamous cell cancer of the head and neck with focus on the most important subsets (T lymphocytes and more specifically CD8⁺ cytotoxic T cells and FoxP3⁺ regulatory T cells) and site-specific characteristics such as Human Papilloma Virus infection. In addition, we discuss methodological problems and pitfalls that can account for discordant findings and that may hamper inclusion of TILs assessment in routine practice of pathologists and oncologists.