Characterisation of Familial Colorectal Cancer Type X, Lynch syndrome, and non-familial colorectal cancer

Background: Familial Colorectal Cancer Type X (FCCTX) is defined as individuals with colorectal cancer (CRC) who families meet Amsterdam Criteria-1 (AC1), but whose tumours are DNA-mismatch-repair-proficient, unlike Lynch syndrome (LS). FCCTX does not have an increased risk of extra-colonic cancers. This analysis compares epidemiologic and clinicopathologic features among FCCTX, LS, and ‘non-familial’ (non-AC1) CRC cases. Methods: From the Colon Cancer Family Registry, FCCTX ( n =173), LS ( n =303), and non-AC1 ( n =9603) CRC cases were identified. Questionnaire-based epidemiologic information and CRC pathologic features were compared across case groups using polytomous logistic regression. Results: Compared with LS, FCCTX cases were less likely to be current ( vs never) smokers; have a proximal subsite ( vs rectal) tumour; or have mucinous histology, poor differentiation, or tumour-infiltrating lymphocytes. There were no observed differences in co-morbidities or medication usage. Conclusions: FCCTX were less likely to be current tobacco users; other exposures were similar between these groups. Histopathologic differences highly suggestive of LS CRCs do not appear to be shared by FCCTX.