Most bowel cancer is what doctors call sporadic — it happens in people without a strong family history. But a meaningful proportion of cases occur in people with a relative who has also had bowel cancer, and a smaller proportion (around 5–10%) is caused by an inherited hereditary syndrome.
The closer the relative, the younger they were at diagnosis, and the more relatives affected, the higher your own risk. The good news is that this risk can be substantially reduced by earlier and more frequent screening. Australian guidelines from the NHMRC divide people into three risk categories — and the screening recommendation differs for each.
You are in this group if you have no first-degree relatives with bowel cancer, or just one first-degree relative diagnosed after age 55.
- The National Bowel Cancer Screening Program sends a free FOBT (faecal occult blood test) kit every 2 years — automatically from age 50 to 74, and on request from age 45 to 49.
- This is recommended for everyone without a family history that places them in the moderate or high-risk categories below.
- A positive FOBT triggers a colonoscopy — see What does a positive FOBT mean?
You are in this group if you have:
- One first-degree relative (parent, sibling, or child) diagnosed at age 55 or younger; OR
- Two first-degree relatives with bowel cancer at any age; OR
- One first-degree relative plus two second-degree relatives on the same side of the family with bowel cancer at any age.
Recommended screening
- Colonoscopy every 5 years, starting at age 50, OR starting 10 years younger than the youngest affected relative — whichever is earlier.
- FOBT every 2 years in between.
For example, if your sibling was diagnosed at 48, you would start colonoscopy screening at 38 — 10 years younger than their age at diagnosis.
You are in this group if you have:
- Three or more first- or second-degree relatives on the same side of the family with bowel cancer; OR
- Two or more first- or second-degree relatives on the same side with bowel cancer if there are also features suggesting a hereditary syndrome (e.g. multiple polyps, endometrial cancer, age younger than 50 at diagnosis); OR
- A known family history of Lynch syndrome, familial adenomatous polyposis (FAP), or another hereditary bowel cancer syndrome.
Recommended screening
The colonoscopy schedule depends on which sub-group above applies:
- Three or more first- or second-degree relatives with bowel cancer, without a confirmed syndrome: colonoscopy from age 40, every 5 years (or 10 years younger than the youngest affected relative, whichever is earlier).
- Two or more relatives with features that suggest a hereditary syndrome: referral to a family cancer clinic for genetic counselling and testing comes first — surveillance is then tailored to what testing finds.
- Confirmed Lynch syndrome, FAP, MUTYH-associated polyposis, or another inherited bowel cancer syndrome: colonoscopy every 1–2 years from age 25 (or earlier, depending on the specific syndrome).
Across all three sub-groups, a specialist genetics referral is recommended so surveillance can be tailored to your exact family history and any identified gene change. Coordinated management through a multidisciplinary family cancer team is often helpful.
See Familial Bowel Cancer Syndromes for more on Lynch syndrome, FAP, MUTYH-associated polyposis and related conditions.
First-degree relatives
- Parents
- Siblings
- Children
Second-degree relatives
- Grandparents
- Grandchildren
- Aunts, uncles
- Nieces, nephews
- Half-siblings
Relatives need to be on the same side of the family for their risk to be additive. Bowel cancer on your mother's side and bowel cancer on your father's side are treated independently.
- Write down what you know about your family history — who, what cancer, and what age at diagnosis. Ask older relatives if you are unsure.
- See your GP. Your GP can work out which risk category you fall into and arrange the right referral.
- If you are moderate- or high-risk: your GP can refer you to a colorectal surgeon for screening colonoscopy, or to a family cancer clinic if a hereditary syndrome is suspected.
You do not need to wait for symptoms to act. Screening colonoscopy in higher-risk groups is the most effective way to prevent bowel cancer — by detecting and removing polyps before they have the chance to become cancerous.
Most polyps do not change your screening. But if a first-degree relative had an advanced adenoma (a large or multiple polyp, or one with high-grade dysplasia) at a young age, that is treated similarly to a family history of cancer. Mention any history of advanced polyps to your GP.
One first-degree relative diagnosed after age 55 places you in the average-risk group — standard FOBT screening applies. One first-degree relative diagnosed at 55 or younger places you in the moderate-risk group, with earlier colonoscopy screening.
Not everyone with a family history needs genetic testing — it is recommended when the family pattern is suggestive of a hereditary syndrome. A family cancer clinic decides who should be tested based on the pattern: multiple affected relatives, very young ages at diagnosis, multiple cancers in one person, or characteristic patterns like Lynch syndrome.
Start by writing down what you do know, and ask older relatives to fill in gaps. Even partial information is useful — your GP can work with what you have. You can update your family history over time as more information becomes available.
- Cancer Council Australia — Clinical practice guidelines for the prevention, early detection and management of colorectal cancer (NHMRC-endorsed)
- Australian Government Department of Health — National Bowel Cancer Screening Program
- Cancer Council Australia — Bowel cancer
- AIHW — National Bowel Cancer Screening Program: monitoring report
Need a specialist opinion?
If something in this article matches what you're experiencing, the most useful next step is a proper assessment. A GP referral is required.