What is FAP?
FAP is an inherited condition, where polyps develop in the intestinal tract, but primarily it affects the large intestine, including the colon and rectum. It is due to a genetic mutation that interferes with the normal function of APC, an important gene that controls how quickly cells grow. The gene mutation is most often inherited from a parent, but 30 percent of FAP patients develop the mutation during conception and do not have a parent with FAP.
Those who inherit the gene mutation develop numerous adenomatous polyps, which are precancerous growths, in the lining of the colon and rectum. If undetected and untreated, one or more of the polyps will eventually become cancerous. Fortunately, with early diagnosis and treatment, FAP patients can avoid cancer and lead normal, healthy lives.
How common is FAP?
The reported incidence of familial adenomatous polyposis varies from 1 in 7,000 to 1 in 22,000 individuals.
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.
Diagnosis of FAP
- Colonoscopy
- Genetic testing of patient and 1st-degree relatives
- Offspring screened for hepatoblastoma
Diagnosis is made by finding > 100 polyps on colonoscopy. Diagnosed patients should have genetic testing to identify the specific mutation, which should then be sought in 1st-degree relatives. If genetic testing is unavailable, relatives should be screened with annual sigmoidoscopy beginning at age 12, reducing frequency with each decade. If no polyps are evident by age 50, screening frequency is then the same as for average-risk patients.
Children of parents with FAP should be screened for hepatoblastoma from birth to age 5 yr with annual serum α-fetoprotein levels and possibly liver ultrasound.
How is FAP inherited?
Each child of a parent with FAP has a 50 percent chance of inheriting the gene. Those who have inherited the gene for FAP will eventually develop polyps and may pass along the condition to their children. However, those who have not inherited the gene have only the “normal” risk of developing large bowel polyps and cannot pass FAP to their children.
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
Treatment of FAP
- Colectomy
- Endoscopic surveillance of remainder of GI tract
- Perhaps aspirin or coxibs
Colectomy should be done at the time of diagnosis. Total proctocolectomy, either with ileostomy or mucosal proctectomy and ileoanal pouch, eliminates the risk of cancer. If subtotal colectomy (removal of most of the colon, leaving the rectum) with ileorectal anastomosis is done, the rectal remnant must be inspected every 3 to 6 mo; new polyps must be excised or fulgurated. Aspirin or coxibs may inhibit new polyp formation. If new ones appear too rapidly or prolifically to remove, excision of the rectum and permanent ileostomy are needed.
After colectomy, patients should have upper endoscopy every 5 yr. If adenomatous polyps are seen during endoscopy, one approach is follow-up endoscopy and multiple biopsies every 6 mo for a minimum of 2 yr, with endoscopy thereafter at 3-yr intervals. Annual physical examination of the thyroid, and possibly ultrasound, also is recommended.
