Gastrointestinal cancers accompanied by a dense lymphoid component: an overview with special reference to gastric and colonic medullary and lymphoepithelioma-like carcinomas.

This review highlights the difference between so-called medullary carcinoma and lymphoepithelioma-like carcinoma with reference to the gastrointestinal tract. The two terms have been applied synonymously in the gastrointestinal tract, with the majority of carcinomas associated with lymphocytes being termed medullary carcinomas. True medullary carcinomas are disposed in syncytial sheets and have well defined peripheral margins; the preponderance of inflammation is peri-tumoural. These tumours are also associated with microsatellite instability (MSI). Lymphoepithelioma-like carcinomas tend not to have continuous sheets of tumour but are instead made up of small clusters and aggregates of tumour that are broken up by large numbers of intra-tumoural lymphocytes. These tumours may be associated with Epstein-Barr virus (EBV). Occasional cases may be associated with epigenetic silencing of MLH-1. In conclusion, carcinomas associated with dense lymphoid stroma maybe associated with either MSI or EBV. There are some morphological features that favour either type and in the stomach MSI and EBV are mutually exclusive.