Let us recall a figure already mentioned in this series: 90% of colorectal cancers occur in people with no identified family history. Truly hereditary forms, with a transmissible genetic anomaly, only represent a small minority of cases, but their identification completely changes the prevention strategy for the family concerned.
1. Three levels of risk, three different strategies
The Haute Autorité de Santé distinguishes three levels of risk for colorectal cancer, each corresponding to different screening methods:
Having a family history therefore does not automatically mean a “very high” risk: the majority of family situations fall under high risk, with simple anticipation of the colonoscopy rather than a complex genetic protocol.
| Risk level | Affected population | Screening modality |
|---|---|---|
| Medium risk | Over 50 years old, with no particular history | Immunological test every 2 years |
| Risk high | First-degree personal or family history of cancer or polyp | Direct colonoscopy, pace adapted to the case |
| Very high risk | Confirmed Lynch syndrome or familial adenomatous polyposis | Coloscopy closer from 20-25 years old, specialized monitoring |
2. Lynch syndrome: the most common hereditary form
Lynch syndrome, also called HNPCC, is caused by a mutation in a gene involved in DNA repair. It represents approximately 3% of colorectal cancers, but it is the most common hereditary form. People carrying the mutation have a cumulative risk of developing colorectal cancer estimated between 30 and 50% at age 70, compared to around 5% in the general population.
Lynch syndrome is mentioned when: colorectal cancer is diagnosed before the age of 50, or when several members of the same family are affected by colorectal cancer or an associated cancer (endometrial, ovarian, small intestine) at an early age.
This syndrome also exposes women to an increased risk of endometrial cancer, which justifies specific gynecological monitoring in addition to digestive monitoring.
3. Familial adenomatous polyposis: rarer, earlierFamilial adenomatous polyposis (FAP) is a rarer disease, accounting for approximately 1% of colorectal cancers. It is characterized by the development of hundreds or even thousands of polyps in the colon during adolescence or early adulthood. Without treatment, the risk of cancerous transformation is almost certain, generally around ten years after the appearance of the first polyps.
This form is easier to spot than Lynch syndrome, because the very high number of polyps is visible from the first screening colonoscopies in relatives.
4. When to consult an oncogeneticist?
An oncogenetics consultation may be offered by the attending physician or gastroenterologist in several situations:
- ●Colorectal cancer diagnosed before age 50, in yourself or a close relative.
- ●Several cases of colorectal cancer or associated cancers (endometrium, ovary) in the same family branch, over one or two generations.
- ●Discovery of an unusually high number of polyps during a colonoscopy.
This consultation makes it possible to establish a detailed family tree, to assess the real level of risk and, if necessary, to offer a genetic test. Being a carrier of a mutation does not mean that cancer will definitely develop: it justifies close monitoring, not an inevitability.
⚠️ To remember from a medical point of view
An isolated family history, in a parent diagnosed after the age of 60, has little effect on the individual risk compared to the general population, it generally does not justify strongly anticipating screening.
Conversely, several cases in the family or an early diagnosis (before the age of 50) in a relative justify talking to your doctor treating, who will refer if necessary to a specialized consultation.
Screening adapted to hereditary forms (colonoscopy from 20-25 years of age for Lynch syndrome) is decided on a case-by-case basis by the specialized team, not by standard organized screening.
