What is familial adenomatous polyposis?
Classic familial adenomatous polyposis, called FAP or classic FAP, is a genetic condition. It is diagnosed when a person develops more than 100 adenomatous colon polyps. An adenomatous polyp is an area where normal cells that line the inside of a person’s colon begin to make mucous and form a mass on the inside of the intestinal tract. The average age for polyps to develop in people with FAP is in the mid-teens. More than 95% of people with FAP will have multiple colon polyps by age 35. If FAP is not recognized and treated, there is almost a 100% chance that a person will develop colorectal cancer.
There is also an increased chance of developing cancer in the stomach and/or small intestines. Other types of cancer found in families with FAP include hepatoblastoma, a type of liver cancer seen in young children; desmoid tumors/desmoid fibromatosis, a locally aggressive tumor that does not metastasize; papillary thyroid cancer; pancreatic, adrenal, and bile duct cancers; and a low risk of a type of brain cancer called medulloblastoma.
Not all symptoms of FAP are cancer-related. Some additional features of FAP may include:
- Osteomas, which are noncancerous bony growths, usually found on the jaw
- Extra, missing, or unerupted teeth
- Congenital, meaning present at birth, hypertrophy of the retinal pigment epithelium (CHRPE). This 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.
- Benign (noncancerous) skin changes, such as epidermoid cysts and fibromas
- Adrenal masses
What causes classic FAP?
FAP is passed from generation to generation in a family. The APC gene is linked to FAP; APC stands for adenomatous polyposis coli. A mutation (alteration) in the APC gene gives a person an increased lifetime risk of developing colorectal cancer or other cancers of the digestive tract.
How is classic FAP inherited?
Normally, every cell has two copies of each gene: one inherited from the mother and one inherited from the father. FAP follows an autosomal dominant inheritance pattern. In autosomal dominant inheritance, 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.
How common is classic FAP?
FAP is uncommon; specific estimates on how many people have FAP vary from one in 22,000 up to one in 7,000. About 30% of people with FAP do not have any family history of the condition; they have a de novo (new) mutation in the APC gene.
Most colorectal cancer is sporadic, meaning it occurs by chance, and is not related to FAP or other known inherited genetic changes. Less than 1% of all colorectal cancer is thought to be due to FAP.
How is classic FAP diagnosed?
Classic FAP is a clinical diagnosis. This means that it is typically diagnosed when the doctor finds many polyps, rather than by the results of a laboratory test. A person with more than 100 adenomatous colon polyps is considered to have FAP. People with FAP can also have a blood test to look for a mutation in the APC gene. If an APC gene mutation is found, other family members may be diagnosed with FAP if they are tested and have the same gene mutation.
What are the estimated cancer risks associated with classic FAP?
- Colorectal cancer almost 100% if not treated
- Desmoid tumor 10% to 20%
- Small bowel (intestines) 4% to 12%
- Pancreatic cancer 2%
- Papillary thyroid cancer 2%
- Hepatoblastoma (a type of liver cancer) 1.5%
- Brain or central nervous system tumor less than 1%
- Stomach cancer 0.5%
- Bile duct cancer small, but increased
- Adrenal gland cancer small, but increased
What are the screening options for classic FAP?
ASCO recommends the following screening for people with FAP. It is important to discuss these options with your doctor, as each individual is different:
- Sigmoidoscopy or colonoscopy every one to two years, starting at age 10 to 11
- Yearly colonoscopy once polyps are found until a colectomy is planned. People with classic FAP may need a colectomy, the surgical removal of the entire colon, at some point due to a high number of polyps and the high risk of colorectal cancer. 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 is linked with these tumors
Screening options may change over time as new technologies are developed and more is learned about 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 www.asco.org/endorsements/HereditaryCRC.
Questions to ask the doctor
If you are concerned about your risk of colorectal cancer or other types of 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 kind of colon polyps have I had? The two most common kinds are hyperplastic and adenomatous.
- What is my risk of developing another type of cancer?
- 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 your family may have FAP, 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?
Colon Cancer Alliance
Colorectal Cancer Coalition (C3)
Desmoid Tumor Research Foundation
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