GPAtlas🧭 When to suspect
Familial adenomatous polyposis (FAP) is an inherited cancer-predisposition syndrome caused by a pathogenic variant in the APC gene (adenomatous polyposis coli, chromosome 5q21). It is autosomal dominant – each child and sibling of an affected person has a 1-in-2 (50%) chance of inheriting it – but around 20–25% of cases arise de novo, so a negative family history does not exclude it. Prevalence is roughly 1 in 7,000–30,000.
The hallmark is the development of hundreds to thousands of adenomatous polyps throughout the large bowel, typically from the teenage years. Left untreated the lifetime risk of colorectal cancer (CRC) approaches 100%, usually before the late 40s – which is why these patients are managed with lifelong surveillance and prophylactic surgery, not watchful waiting.
The GP's task is rarely to make the diagnosis single-handedly. It is to recognise the pattern, refer appropriately, and ensure at-risk relatives are offered testing. Suspect FAP – and consider referral for constitutional (germline) APC testing – in the following situations.
| When to suspect FAP / consider referral for genetic testing |
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• A known APC pathogenic variant in the family – offer predictive testing to first-degree relatives. |
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• Colorectal, endometrial or small-bowel cancer diagnosed before 40, or colorectal cancer with ≥ 5 adenomatous polyps. |
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• ≥ 10 adenomatous polyps before age 60 (or ≥ 5 before 40), or ≥ 20 adenomatous polyps at any age. |
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• A strong family history of bowel cancer at a young age, or of FAP / multiple bowel polyps. |
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• FAP-associated extra-intestinal features – multiple jaw/skull osteomas, multiple dental anomalies, a desmoid tumour, hepatoblastoma, cribriform-morular thyroid cancer, or multifocal retinal pigmentation (CHRPE). |
Source: BSG hereditary CRC guideline · Genomics Education Programme · NHS Genomic Medicine Service
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