Between 1 and 5 per cent of colorectal cancers are specifically inherited (familial adenomatous polyposis and lynch syndrome) and up to 25 per cent may have some inherited component. The features that suggest a genetic predisposition may include:

• early age at onset
• multiple primary cancers
• multiple polyps
• the histological type of polyp(s)
• family history of similar or related cancers.

Universal testing for mismatch repair deficiency by IHC or MSI testing is now recommended as standard practice for treatment-related and familial risk assessments.

Anyone diagnosed with cancer should have a detailed personal and family cancer history taken. People with a strong family history of colorectal cancer or high-risk familial syndromes should be referred to a familial cancer service for genetic risk assessment and possible genetic screening of affected relatives. Younger patients under 50 years should be referred to a familial cancer service. Consult relevant guidelines to determine if referral to a familial cancer service is appropriate.

Mainstream genetic testing is now available and funded through Medicare for the mismatch repair genes, APC, MUTYH and the hamartomatous polyposis genes. However, a familial cancer service assessment can assist and determine if genetic testing is appropriate. Genetic testing should be offered when there is at least a 10 per cent chance of finding a causative ‘gene error’ (pathogenic gene variant; previously called a mutation). Tumour testing for mismatch repair (MMR) protein expression or microsatellite testing is the main avenue to determining this risk for lynch syndrome. Usually testing begins with a variant search in a person who has had cancer (a diagnostic genetic test). If a pathogenic gene variant is identified, variant-specific testing is available to relatives to see if they have inherited the familial gene variant (predictive genetic testing). Depending on the personal and family history, the relevant state health system may support funding in the public sector for additional germline testing.

Pre-test counselling and informed consent is required before any genetic testing in any setting. A familial cancer service can organise this, as well as provide risk management advice, facilitate family risk notification and arrange predictive cascade genetic testing for the family.

Visit the Centre for Genetics Education website for basic information about cancer in a family.

For detailed information and referral guidelines for colorectal cancer risk assessment and consideration of genetic testing, see these resources:

• Clinical Guidelines Network ‘Introduction: high-risk familial syndromes’
• National Comprehensive Cancer Network ‘Genetic/Familial High-Risk Assessment: Colorectal’