Background The hereditary non-polyposis colorectal cancer (HNPCC) subset of tumours can broadly be divided into tumours caused by an underlying mismatch-repair gene mutation, referred to as Lynch syndrome, and those that develop in families with similar patterns of heredity but without disease-predisposing germline mismatch repair mutations, referred to as familial colorectal cancer type X (FCCTX). Recognition of HNPCC-associated colorectal cancers is central since surveillance programmes effectively reduce morbidity and mortality. The characteristic morphological features linked to Lynch syndrome can aid in the identification of this subset, whereas the possibility to use morphological features as an indicator of FCCTX is uncertain.

Objective and methods To perform a detailed morphological evaluation of HNPCC-associated colorectal cancers and demonstrate significant differences between tumours associated with FCCTX and Lynch syndrome.

Results The morphological features associated with Lynch syndrome, that is, right-sided tumour location, poor differentiation, expansive growth pattern, tumour-infiltrating lymphocytes, peritumorous lymphocytes, Crohn-like reactions, and lack of dirty necrosis, were significantly less often observed in FCCTX tumours.

Discussion The less typical morphology in FCCTX implies that family history of cancer needs to be taken into account since these tumours cannot readily be recognised based on histopathological features.