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 start appearing in the teenage years, and without treatment, colorectal cancer is virtually inevitable. FAP is rare, but it’s one of the most well-understood genetic causes of cancer, and early detection changes the outcome dramatically.
What Causes FAP
FAP is caused by a mutation in a gene called APC, short for adenomatous polyposis coli. This gene acts as a brake on cell growth. When it’s working normally, it prevents cells from multiplying out of control and forming tumors. When the gene is mutated, that brake fails, and cells in the colon lining begin growing into polyps that can eventually turn cancerous.
FAP follows an autosomal dominant inheritance pattern, meaning you only need one copy of the mutated gene (from one parent) to develop the condition. If a parent carries the mutation, each child has a 50% chance of inheriting it. However, not everyone with FAP has a family history. Roughly 11 to 30% of cases arise from new, spontaneous mutations, meaning the affected person is the first in their family to have the condition.
Classic FAP vs. Attenuated FAP
There are two forms of the condition, and they differ in severity and timing.
Classic FAP is the more aggressive form. Polyps begin appearing at an average age of 16, though they can start as early as 7. By age 35, 95% of people with classic FAP have polyps, typically numbering in the hundreds or thousands. The diagnostic threshold is 100 or more adenomatous polyps in the colon and rectum. Without surgery to remove the colon, colorectal cancer develops at a mean age of 39.
Attenuated FAP (sometimes called AFAP) is a milder version. People with this form develop fewer polyps, averaging around 30, and the polyps tend to cluster higher up in the colon rather than in the rectum. Cancer risk is still significant, with a 70% lifetime chance, but it develops later, typically between ages 50 and 55. Attenuated FAP is generally defined as fewer than 100 polyps found at age 25 or older.
Symptoms and How It’s Found
In its early stages, FAP often causes no symptoms at all. Polyps are small and numerous but don’t produce pain or obvious changes until they grow larger or become cancerous. When symptoms do appear, they can include rectal bleeding, changes in bowel habits, abdominal pain, or unexplained weight loss. By the time symptoms show up, many polyps may already be present.
Most people with a known family history are identified through genetic testing and surveillance rather than through symptoms. For those without a family history (the de novo cases), FAP is sometimes discovered during a colonoscopy performed for other reasons, or after colorectal cancer has already developed.
Problems Beyond the Colon
FAP doesn’t only affect the colon. The same genetic mutation can cause growths and abnormalities in other parts of the body, and awareness of these is an important part of managing the condition.
Desmoid tumors are one of the most significant. These are dense, fibrous growths that don’t spread to other organs but can be locally aggressive, pushing into surrounding tissue. They occur in about 10% of people with FAP, roughly 1,000 times more often than in the general population. They can grow in the abdominal wall, within the abdomen itself, or in the tissue that holds the intestines in place, sometimes complicating surgery.
Other features include bony growths called osteomas (particularly in the jaw, found in up to 80% of patients), dental abnormalities (reported in 17 to 75% of cases), and small dark spots on the retina called congenital hypertrophy of the retinal pigment epithelium. Soft tissue growths like lipomas, fibromas, and cysts also occur more frequently. None of these are cancerous, but they can help clinicians recognize the condition, especially in families where FAP hasn’t been previously diagnosed.
The upper digestive tract is another area of concern. Polyps can develop in the stomach and the duodenum (the first section of the small intestine), and duodenal polyps carry a real cancer risk. Screening with upper endoscopy is recommended starting at age 20 to 25, with repeat exams every six months to five years depending on how many polyps are found and how advanced they look.
Screening and Surveillance
For anyone with a known APC mutation or a parent with FAP, surveillance typically begins in the early to mid-teens. Regular colonoscopy allows doctors to monitor polyp development and determine the right time for surgery. Genetic testing can confirm the diagnosis in family members and is increasingly used to identify at-risk children before polyps even appear.
Upper endoscopy begins later, around age 20 to 25, because stomach and duodenal polyps tend to develop after colonic polyps. The frequency of these exams depends on the severity of what’s found. Someone with no duodenal polyps might need an upper endoscopy only every five years, while someone with a heavy polyp burden may need one every six months.
Surgical Treatment
Because polyps in classic FAP are so numerous and cancer is otherwise inevitable, surgery to remove the colon is the standard treatment. The question isn’t whether to operate, but which type of surgery to perform and when.
There are two main options. The first is colectomy with ileorectal anastomosis (IRA), which removes the colon but leaves the rectum in place, reconnecting the small intestine directly to it. This is a simpler operation and tends to produce better day-to-day bowel function: fewer trips to the bathroom, less need for anti-diarrheal medications, and better control over gas and stool. The trade-off is that the remaining rectum still contains tissue at risk for polyps and cancer, so it requires lifelong monitoring with regular exams.
The second option is proctocolectomy with ileal pouch-anal anastomosis (IPAA), which removes both the colon and rectum entirely. The surgeon creates a pouch from the end of the small intestine and connects it to the anus, preserving the ability to pass stool normally. This is typically done in two stages, with a temporary opening in the abdomen (ileostomy) that’s reversed about six weeks later. The major advantage of IPAA is that it eliminates the risk of rectal cancer, which has made it the preferred approach at many treatment centers.
Both surgeries come with real adjustments. Most people experience increased bowel frequency, especially in the first year. Bowel discomfort is common with either approach, reported in roughly 58 to 67% of patients. One study found that while IRA patients had fewer daytime bowel movements and less need for incontinence protection, IPAA patients had less fecal urgency. The choice between the two depends on the number and location of rectal polyps, the presence of desmoid tumors, the patient’s age, and individual priorities around quality of life and cancer risk.
Living With FAP
FAP is a lifelong condition, even after surgery. People who’ve had an IRA need ongoing colonoscopy of the remaining rectum, typically every six to twelve months. Those who’ve had an IPAA still need periodic examination of the ileal pouch, since polyps can occasionally develop there as well. Upper endoscopy continues on schedule regardless of which surgery was performed.
For families affected by FAP, genetic counseling plays a central role. Testing children of an affected parent can determine by blood test whether they carry the APC mutation, allowing surveillance to begin early for those who do and sparing those who don’t from unnecessary procedures. Because up to 30% of cases are de novo, a new diagnosis in someone without family history should still prompt genetic testing and consideration of screening for their children.