Familial adenomatous polyposis (FAP) is an inherited condition that causes hundreds to thousands of small growths, called polyps, to develop along the lining of the colon and rectum. These polyps begin appearing in adolescence and, if left untreated, will almost inevitably progress to colorectal cancer, typically by age 40. FAP is rare, but it is one of the most well-understood genetic causes of colon cancer.
How a Single Gene Causes Thousands of Polyps
FAP is caused by a mutation in a gene called APC, which normally acts as a brake on cell growth. The APC gene produces a protein that stops cells from dividing too quickly and helps ensure chromosomes are distributed correctly when cells split. In people with FAP, the mutation produces a shortened, nonfunctional version of this protein. Without that brake, cells in the colon lining multiply unchecked, forming the characteristic carpet of polyps.
Each polyp starts out benign, but with so many of them, the odds that at least one will accumulate additional genetic damage and turn cancerous are extremely high. That’s why colorectal cancer is considered virtually certain in untreated classic FAP.
Classic FAP vs. Attenuated FAP
Not every APC mutation produces the same severity. Classic FAP typically involves 100 or more polyps, often numbering in the thousands, with onset during the teenage years. Attenuated FAP (sometimes called AFAP) is a milder form. People with AFAP tend to develop fewer polyps, sometimes only a few dozen, and the polyps appear later in life. The cancer risk is still significantly elevated compared to the general population, but the timeline is slower and management can be less aggressive.
The specific location of the mutation within the APC gene helps predict which form a person will develop. Mutations near the beginning or end of the gene tend to produce the attenuated form, while mutations in the middle often cause the classic, more severe presentation.
How FAP Is Inherited
FAP follows an autosomal dominant inheritance pattern. That means a person only needs one copy of the mutated gene to develop the condition. If one parent carries the mutation, each child has a 50% chance of inheriting it. About 25% to 30% of FAP cases arise from new, spontaneous mutations in people with no family history, so the absence of a known family connection does not rule it out.
Growths Beyond the Colon
FAP doesn’t only affect the colon. The condition can produce a range of growths elsewhere in the body, a presentation historically called Gardner syndrome. These include:
- Desmoid tumors: firm, fibrous growths that develop in connective tissue, most often in the abdomen. They’re noncancerous but can grow aggressively and press on surrounding organs.
- Osteomas: benign bone growths, frequently found in the jaw or skull.
- Extra or impacted teeth and other dental abnormalities.
- Skin cysts and soft-tissue tumors: including lipomas (fatty lumps) and fibromas.
- Retinal changes: dark spots on the retina called CHRPE, which don’t affect vision but can be an early clue to the diagnosis.
These features vary widely from person to person, even within the same family. Some people with FAP have several of these findings; others have none beyond the colon polyps.
Cancer Risks Outside the Colon
Duodenal polyps develop in more than 90% of people with FAP, and up to 10% of those individuals will develop duodenal cancer, usually after age 40. The lifetime risk of developing severe duodenal polyposis is estimated at around 35%. For this reason, upper endoscopy surveillance typically begins between ages 20 and 25, with follow-up intervals ranging from every few years (if no polyps are found) to every few months if the polyp burden is heavy.
Thyroid cancer risk is also elevated, with estimates ranging from about 1% to 12% over a lifetime. Current guidelines recommend thyroid ultrasound starting in the late teenage years, repeated every 2 to 5 years if results are normal. A family history of thyroid cancer or suspicious ultrasound features may warrant more frequent checks.
When Screening Starts
Because polyps can begin forming in childhood, screening for at-risk children (those with a parent who carries the mutation) typically starts early. Sigmoidoscopy or colonoscopy is recommended every 1 to 2 years beginning at age 10 to 12, or 10 years before the earliest cancer diagnosis in the family, whichever comes first. Genetic testing can identify the mutation before any polyps appear, allowing families to know which children need surveillance and which can follow standard screening schedules.
Surgical Options and What to Expect
Because of the near-certain cancer risk in classic FAP, preventive surgery to remove the colon is the standard approach. The timing depends on polyp severity, but it often takes place in the late teens or twenties. There are two main surgical paths.
Subtotal Colectomy With Ileorectal Anastomosis (IRA)
This is a simpler, one-step operation that removes most of the colon but leaves the rectum intact, reconnecting the small intestine directly to it. Recovery tends to be faster and bowel function is generally good. The tradeoff is that polyps can still develop in the remaining rectum. The cumulative risk of rectal cancer ranges from 13% to 25% over 15 to 25 years, meaning lifelong surveillance with regular rectal exams is essential. This option is best suited for younger patients who have few rectal polyps and a milder overall disease pattern.
Total Proctocolectomy With Ileal Pouch
This procedure removes both the colon and the rectum entirely. The surgeon creates an internal pouch from the end of the small intestine and connects it to the anus, preserving the ability to pass stool normally. It virtually eliminates the risk of rectal cancer. However, it’s a more complex operation, usually done in two stages with a temporary ostomy bag between them. Recovery takes longer, and there’s a higher rate of surgical complications. Even after this procedure, small polyps can develop in the pouch itself, with about an 18% chance of new polyp growth within 7 years, so ongoing endoscopic monitoring is still needed.
For people with attenuated FAP, surveillance with regular polyp removal during colonoscopy may be sufficient for years, delaying or sometimes avoiding major surgery depending on how the disease progresses.
Living With FAP Long-Term
FAP is a lifelong condition that requires ongoing monitoring even after surgery. Upper endoscopy for duodenal polyps, thyroid ultrasounds, and checks for desmoid tumors all continue on a schedule tailored to individual risk. For families affected by FAP, genetic counseling plays a central role. Testing can be done in childhood so that screening starts at the right time, and it can also spare family members who didn’t inherit the mutation from years of unnecessary procedures.
Advances in surveillance and the timing of preventive surgery have dramatically improved outcomes. Most people with FAP who are diagnosed early and followed closely avoid colorectal cancer entirely, a sharp contrast to the near-certain cancer risk that defined this condition before modern screening was available.