BACKGROUND: Familial colorectal cancer type X (FCCTX) is a phenotypically defined subset of hereditary colorectal cancer with unknown and potentially heterogeneous genetic aetiology. FCCTX has been characterized as a colorectal cancer-specific syndrome, which we herein challenge by estimating the risk for extra-colorectal cancer in the Danish FCCTX cohort.

METHODS: Through the national hereditary non-polyposis colorectal cancer (HNPCC) register, 213 families fulfilling the Amsterdam I criteria and showing retained mismatch repair (MMR) function were identified. In here, sex and age-specific incidence rate ratios (IRR) were calculated for 30 extra-colorectal cancer types in comparison with the general Danish population.

RESULTS: In total, 494 extra-colorectal cancers developed with significantly increased risks for cancers of the urinary tract, breast, stomach, pancreas, and eye tumours. The age groups at increased risks were 30-49 years for gastric cancer, 30-69 years for female breast cancer, 50-69 years for ocular melanoma and above age 70 for pancreatic cancer and urothelial cancer.

CONCLUSIONS: Danish FCCTX families show an increased risk of several extra-colorectal cancer types. This observation may indicate unidentified disease-predisposing genetic variants in this phenotypically defined subset of hereditary colorectal cancer and calls for awareness during genetic counselling and follow-up.