Making sense of TILs: recommendations for morphological assessment of tumour-infiltrating lymphocytes in gastro-oesophageal carcinoma: A report on behalf of the International Immuno-Oncology Biomarker Working Group.

In the era of immune checkpoint inhibitors for cancers, the need for prognostic biomarkers to identify patients most likely to achieve a durable response has become increasingly more relevant. Tumour-infiltrating lymphocytes (TILs) have gained significant interest, as they can be evaluated using standard haematoxylin and eosin-stained slides, making it a widely accessible and cost-effective biomarker. In addition to their practicality, TILs provide prognostic insights into the interplay between the immune system and tumour cells. While the morphological assessment of TILs has been standardised in breast cancer, comprehensive guidelines for their evaluation in gastro-oesophageal carcinomas (GEC) are still lacking. This narrative review examines the current literature on the composition, clinical implications and therapeutic utility of TILs in GEC. These insights are used to propose a framework with recommendations for standardised evaluation and reporting of TILs in GEC, while also highlighting pitfalls specific to GEC pathology. These recommendations serve as a vital first step towards the widespread use and validation of TILs as a biomarker.