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COLON CANCER, sigmoidospy, colonoscopy, & virtual colonoscopy

Why check for colon, and what type of test? 


Sigmoidoscopy is a simple 30-minute procedure with about 1 minute of pain resembling mild to moderate intestinal cramp.  If no polyps are found it needs to be performed only once every 5 years.  


Why?  Given that colon cancer is the 3rd lead type of cancer, that the risk factor rises steeply after the age of 50, that the formation of polyps is a reliable warning sign, that they are easily removed without an invasive operation, and that a person with polyps will lower his risk of developing colon cancer by 76 to 90%, it is a cost-effective, life extending procedure.  That is why HMOs cover the sigmoidoscopy.--JK 
Virtual colonoscopy least effective screening




Colorectal cancer (CRC) is the third leading cause of cancer in the United States. It accounts for 9% of all new cancer diagnoses (130,000 new cases annually) and 11% of all cancer deaths (over 56,000 annually). Not all individuals are of equal risk. A person with a first-degree relative with CRC has an 8% lifetime CRC risk, but with two first-degree relatives the risk increases to 17%. Various racial and geographic factors also contribute to the risk. For example, the CRC incidence for men (per 100,000 per year) is 8 for Miyagi, Japan, 10-17 for Europeans, 28 for Caucasians in New Orleans, and 34 for Hawaiians of Japanese decent.



Many lines of data have led to the widespread acceptance of the hypotheses that most cancers develop from colonic adenomasthe "adenoma-to-carcinoma" sequence. Removal of adenomas has been demonstrated to be associated with a 76 to 90% reduction in the number of expected CRC cases. If detected early, CRC is a very curable disease. The expected 5 year cancer-free survival for a Dukes-Turnbull Class "A" patient is 95%.




Fecal occult blood testing has been shown in three randomized trials to reduce CRC mortality from 15 to 33% in the general population


Virtual colonoscopy (or CT colonography) is an x-ray technique using high-speed CT scanning and complex software to obtain a two dimensional image of the colon.   Colonoscopy involves the direct visualization of the surface of the colon using high-resolution video equipment. 


Video colonoscopy is at least 100 times as sensitive at visualizing small objects. For example, it can accurately image a hair fiber while the current resolution of virtual colonoscopy often misses objects less than 10 mm in diameter. In a study published in June of 2001, participants were screened with both methods, and virtual colonoscopy missed 30 % of the polyps seen on video colonoscopy!  It cannot view the surface of the colon.  Polyps cannot beremoved during the x-ray technique, but can during a colonoscopy. The American Cancer Society does not currently advocate virtual colonoscopy as a screening technique.


The reported miss rate for polyps varies with the size of the polyp. For example, for those less than 5 mm, the miss rate can be as high as 16-27%; for 6-9 mm polyps, 12-13% will be missed; and for those greater than 10mm, the miss rate is 0-6%. Complications of colonoscopy are unusual but can include perforation (0.14 0.25%), bleeding (0.7 2.5%), and death (less than 0.02%).  However, virtual colonoscopy machines fail to detect nearly 10% of polyps measuring 10mm or less in diameter, and miss about 5% for those greater than 10 mm.


Asymptomatic individuals with no risk factors for colon cancer should undergo colonoscopy starting at age 50 years and every 10 years thereafter. For those found to have adenomas, especially adenomas 10mm or larger in size, follow-up colonoscopy should be performed every 3 years. For those with villous polyps, colonoscopy is advised every 2 years. For those found to have CRC, colonoscopy is advised one year after the initial cancer is removed, and then every 2 years until year 5, after which colonoscopy is performed every 3 years.  Approximately 25-30% of CRC is "familial" or associated with known genetic syndromes. The remainder are said to be "sporadic".  The most common genetic cause is HNPCC which arises from mutations in one of five mismatch  repair genes (MMR), with 95% being a mutation of either MLH1 or MLH2 genes.  Moreover with HNPCC there is a 40% risk of uterine cancer.  With HNPCC the average age of CRC is 44 years and 80% have CRC by 65 years.


The shorter flexible Sigmoidoscopy should be repeated every 3-5 years.  If polyps are found then the longer colonoscopy is performed. 


Colonoscopy is generally performed using a technique called conscious sedation or “ twilight sleep”. A mixture of drugs are given to achieve comfort and sedation. The colonoscope itself is a sterilized flexible tube measuring approximately inch in diameter. The procedure takes less than 30 minutes and the majority of patients, given the proper medications do not remember the test. Of course, the entire exam is performed under the direct supervision of a sub specialist physician. For virtual colonoscopy, a rubber catheter is inserted in the rectum and the colon is then inflated with  air by a technician. Maximal inflation of the colon with air, limited by patient tolerance, is necessary in order to achieve adequate visualization of the wall of the colon. No medications are given, and the procedure is not generally monitored by a sub specialist physician for quality or comfort during the exam. It is generally compared to a barium enema. 


From http://www.borland-groover.com/coloncancer2002.htm


Colon Cancer Test Disappoints
by Steven Reinberg, HealthDay Reporter | Apr 13 '04

A computer-assisted screening test for colon cancer is gaining popularity because it is minimally invasive, but a new study has found it isn't yet ready for widespread use.

The researchers discovered the accuracy of the virtual colonoscopy in detecting cancers is substantially lower than that of conventional colonoscopy. And they suggest the use of the technique should be limited until both technology and training are improved.

"The accuracy of this technique was surprising and disappointing," said lead researcher Dr. Peter B. Cotton, director of the Digestive Disease Center at the Medical University of South Carolina.

This study was different from ......



Sigmoidoscopy (Anoscopy, Proctoscopy)

Test Overview

Anoscopy, proctoscopy, and sigmoidoscopy tests allow a health professional to look inside the anus, rectum, and the lower part of the large intestine (colon) for abnormal growths (such as tumors or polyps), inflammation, bleeding, hemorrhoids, and other conditions (such as diverticulosis). See illustrations of a hemorrhoid and polyps .

The colon is 5 ft(1.5 m) to 6 ft(1.8 m) long. Sigmoidoscopy is a thorough examination of the last 2 ft(0.6 m) of the lower colon (sigmoid colon). Sigmoidoscopy is most commonly used to screen for colon cancer. About half of all colon tumors and polyps can be found using sigmoidoscopy.

These examinations are done using different viewing instruments that are inserted into the anus, rectum, or colon.

  • The anoscope is a short, rigid, hollow tube that may also contain a light source. It is used to look at the last 2 in.(5.1 cm) of the colon (anal canal). Anoscopy can usually be done at any time because it does not require preparations (enemas or laxatives) to empty the colon.
  • The proctoscope is slightly longer than the anoscope and is used to view the inside of the rectum. Proctoscopy usually requires the use of enemas or laxatives to empty the colon before the test is done.
  • The sigmoidoscope is lighted tube that may be rigid or flexible. The rigid sigmoidoscope is about 10 in.(25.4 cm) to 12 in.(30.5 cm) long and 1 in.(2.5 cm) wide. It allows the health professional to look into the rectum and a portion of the lower large intestine (colon). The flexible sigmoidoscope is about 2 ft(61 cm) long and 0.5 in.(1.3 cm) wide with a lighted lens system. This instrument allows the health professional to see around bends in the colon. A flexible sigmoidoscope allows a more complete view of the lower colon than a rigid scope and usually makes the examination more comfortable. The flexible sigmoidoscope generally has replaced the rigid sigmoidoscope.
  • A health professional can also remove small growths and collect tissue samples (biopsy) through a sigmoidoscope. Sigmoidoscopy requires the use of enemas or laxatives (or both) to empty the colon before the test is done.
  • Colonoscopy uses a longer flexible scope to examine the entire colon. For more information, see the medical test Colonoscopy.


Test Overview

Colonoscopy allows a doctor to look at the interior lining of the large intestine (rectum and colon) through a thin, flexible viewing instrument called a colonoscope. The colonoscope helps the doctor detect polyps, tumors, and areas of inflammation or bleeding. During a colonoscopy, tissue samples can be collected (for biopsy) and abnormal growths can be removed. Colonoscopy can also be used as a screening test to identify and remove precancerous and cancerous growths in the colon.

The colonoscope is a thin, flexible scope that ranges from 48 in.(121.9 cm) to 72 in.(182.9 cm) long. A small video camera is attached to the colonoscope so that photographic, electronic, or videotaped images of the large intestine can be made. In some cases, the colonoscope may use fiber optics. However, digital video technology has generally replaced fiber optics. The colonoscope can be used to view the entire colon as well as a small portion of the lower small intestine. Another test called sigmoidoscopy shows only the rectum and the lower third of the colon.


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Disclaimer:  The information, facts, and opinions provided here is not a substitute for professional advice.  It only indicates what JK believes, does, or would do.  Always consult your primary care physician for medical advice, diagnosis, and treatment.