Attenuated Familial Adenomatous Polyposis

Approved by the Lineagotica Editorial Board, 12/2014

What is attenuated familial adenomatous polyposis?

Attenuated familial adenomatous polyposis (AFAP) is a subtype of a condition known as familial adenomatous polyposis, called FAP or classic FAP. People with FAP or AFAP will have an increased number of adenomatous colon polyps during their lifetime and an increased risk of developing colorectal cancer. An adenomatous polyp is a lump filled with the cells that make mucous and line the inside of a person’s colon. Normally these cells are in flat sheets, but in FAP and AFAP they build up into polyps inside the intestinal tract. In AFAP, the total number of colon polyps is fewer than 100, with 30 being average. In FAP, there are many more polyps.

Polyps in people with AFAP tend to develop later in life than in individuals with classic FAP, although polyps may develop as early as the late teens. Colorectal cancer can develop later in people with AFAP as well, with the average age at diagnosis being around 50. Polyps and cancers of the stomach and small intestines are also seen in families with AFAP. 

AFAP is still being defined by doctors. It has not been determined if families with AFAP have the same risk for other types of cancer, in addition to colon cancer, as families with classic FAP. Risks for some of the other, noncancerous features associated with classic FAP have not been determined. For instance, people with AFAP appear to have a lesser chance of developing desmoid tumors, or congenital hypertrophy of the retinal pigment epithelium (CHRPE), than people diagnosed with FAP. A desmoid tumor is a noncancerous fibrous tumor that can grow anywhere in the body. CHRPE is an eye condition that does not affect vision, but it is a condition that an eye doctor may see during an examination with a special instrument called an ophthalmoscope.

What causes AFAP?

AFAP is a genetic condition. This means that the cancer risk can be passed from generation to generation in a family, but usually not all children of a person with AFAP will be affected. The gene associated with AFAP is APC; APC stands for adenomatous polyposis coli. A mutation (alteration) in the APC gene gives a person an increased lifetime risk of developing multiple adenomatous colon polyps, colorectal cancer, and other cancers of the digestive tract. People who are diagnosed with AFAP, and their family members, should talk with a genetic counselor or medical genetics specialist. A genetic counselor is a health professional with specialized training in medical genetics.

How is AFAP inherited?

Normally, every cell has two copies of each gene: one inherited from the mother and one inherited from the father. AFAP follows an autosomal dominant inheritance pattern, in which a mutation happens in only one copy of the gene. This means that a parent with a gene mutation may pass along a copy of their normal gene or a copy of the gene with the mutation. Therefore, a child who has a parent with a mutation has a 50% chance of inheriting that mutation. A brother, sister, or parent of a person who has a mutation also has a 50% chance of having the same mutation.

Options exist for couples interested in having a child when they know that one of them carries a gene mutation that increases the risk for this hereditary cancer syndrome. Preimplantation genetic diagnosis (PGD) is a medical procedure done in conjunction with in-vitro fertilization (IVF). It allows people who carry a specific known genetic mutation to have children who do not carry the mutation. A woman’s eggs are removed and fertilized in a laboratory. When the embryos reach a certain size, one cell is removed and is tested for the hereditary condition in question. The parents can then choose to transfer embryos which do not have the mutation. PGD has been in use for over a decade, and more recently has been used for several hereditary cancer predisposition syndromes. However, this is a complex procedure with financial, physical, and emotional factors for couples to consider before starting. For more information, talk with an assisted reproduction specialist at a fertility clinic.

How common is AFAP?

AFAP is uncommon and possibly under-diagnosed. Most colorectal cancer and colon polyps are sporadic, meaning it occurs by chance and is not related to AFAP, classic FAP, or other inherited syndromes. The number of colorectal cancer cases and the number of people with multiple adenomatous colon polyps that are related to AFAP are unknown.

How is AFAP diagnosed?

AFAP is suspected when a person has a history of more than 20, but fewer than 100, adenomatous colon polyps. People suspected of having AFAP can have a blood test to look for a mutation in the APC gene. If an APC gene mutation is found, a diagnosis of AFAP is confirmed. Other family members may be diagnosed with AFAP if they are tested and have the same gene mutation.

What are the estimated cancer risks associated with AFAP?

The cancer risks for AFAP are considered to be similar to the risks associated with classic FAP, but the overall cancer risks may be lower. As more information is learned about AFAP, more specific cancer risks may emerge.

  • Colorectal cancer                    considered to be high, but less than 100%, if not treated

Estimated digestive tract cancer risks for classic FAP. It is unknown if risks are the same in AFAP but screening is suggested due to the similarities in both syndromes.

  • Small bowel cancer (intestines)                       4% to 12%
  • Pancreatic cancer                                            2%
  • Stomach cancer                                               0.5%

What are the screening options for AFAP?

ASCO recommends the following screening for people with AFAP. It is important to discuss these options with your doctor, as each individual is different:

  • Colonoscopy every one to two years, beginning at age 18 to 20.
  • Yearly colonoscopy once a person develops polyps. Colectomy is the surgical removal of the entire colon. This may be considered if there are too many polyps to manage with regular colonoscopies. This is a major surgery and possible side effects may include the need for colostomy. Talk with your doctor about what to expect during and after this surgery.
  • Upper endoscopy (EGD) at age 25 to 30 or once colorectal polyps are detected, whichever occurs first
  • Yearly ultrasound of the thyroid may be considered starting at age 25 to 30
  • Computed tomography (CT) scan or magnetic resonance imaging (MRI) if a person has a family history of desmoid tumors or a mutation on the APC gene that has been linked with these tumors

Screening options may change over time as new technologies are developed and more is learned about AFAP and classic FAP. It is important to talk with your doctor about appropriate screening tests.

Learn more about what to expect when having common tests, procedures, and scans, and read more about these screening recommendations at /endorsements/HereditaryCRC.

Questions to ask the doctor

If you are concerned about your risk of developing cancer, talk with your doctor. Consider asking the following questions of your doctor:

  • What is my risk of developing colorectal cancer?
  • How many colon polyps have I had in total?
  • What type of colon polyps have I had? The two most common kinds are hyperplastic and adenomatous.
  • What can I do to reduce my risk of cancer?
  • What are my options for cancer screening?

If you are concerned about your family history and think AFAP may run in your family, consider asking the following questions:

  • Does my family history increase my risk of colorectal cancer?
  • Should I meet with a genetic counselor?
  • Should I consider genetic testing?

More Information

The Genetics of Cancer

Genetic Testing

What to Expect When You Meet With a Genetic Counselor

Collecting Your Family Cancer History

Sharing Genetic Test Results with Your Family

Additional Resources

Colon Cancer Alliance 

Colorectal Cancer Coalition (C3)

National Cancer Institute

American Cancer Society


To find a genetic counselor in your area, ask your doctor or visit the following websites:       

National Society of Genetic Counselors           

National Cancer Institute: Cancer Genetics Services Directory