The Abramson Cancer Center of the University of Pennsylvania
Ultima Vez Modificado: 17 de junio del 2002
Volume 2, Issue 3
OncoLink would like to recognize the contribution of the National Colorectal Cancer Research Alliance (NCCRA) whose initiative with Pharmacia Oncology and Pfizer has made this publication possible.
In 2001, there were an estimated 135,400 new cases of colon cancer and 56,700 deaths due to the disease in the United States. When detected early colon cancer can be cured ninety percent of the time. However, only 44% of adults over the age of 50 undergo screening tests for colon cancer. Current screening tests include fecal-occult blood testing, flexible sigmoidoscopy, and colonoscopy. The low rate of testing may be due to one of several factors: apprehension about the tests, the preparation before the test, or fear of what may be discovered. No matter the reason, the reluctance to undergo screening only worsens the situation. In the past two years, there has been widespread media coverage of potential alternatives to the traditional methods of screening for colon cancer. It seems that people see these tests as a way to potentially avoid colonoscopy or sigmoidoscopy. In order for new methods to replace traditional testing, they need to be as effective, similar in cost, and be accepted by patients. This article will address how these new modalities have performed in recent research studies.
Virtual colonoscopy, a method of viewing the colon from outside the body, employs the use of CT scan. Throughout the procedure, the patient is lying on a table that passes through a "donut" like machine that takes pictures from different angles around the body. The 2 dimensional images of the colon, are converted by a computer to three-dimensional images, and then reviewed by a trained radiologist. This is quite time consuming, requiring approximately 30 minutes per patient, which adds to the cost of the procedure. In order for these images to be accurate, patients must undergo a bowel prep similar to traditional colonoscopy, but for the virtual scan the bowel needs to be clearer. For the bowel prep, patients take laxatives and modify their diet for up to 24 hours before the procedure. Just prior to the CT scan, a tube is inserted into the rectum and air is pumped into the colon allowing it to expand. This allows better visualization of the bowel wall. Patients are also given an intravenous medication called glucagon, causing the bowel walls to relax, which also improves visualization of the colon.
Just how accurate is virtual colonoscopy? This varies greatly with the training of the radiologist who is interpreting the scan. Currently, the virtual test does not perform as well as its conventional counterpart. This may change as the procedure is refined and radiologists become better trained. Studies have found that virtual colonoscopy is not as accurate in detecting flat or smaller lesions (<5mm). Some physicians have suggested that the smaller polyps are less likely to progress to cancer, and therefore the decreased ability to detect them may not be significant, but this has not yet been proven.
How do patients compare the tests? Two studies have evaluated patient preferences by measuring discomfort, feelings of disrespect or embarrassment, and which test they would rather have. Both studies found that patients preferred conventional colonoscopy in all three categories. In both studies, patients received sedation for the conventional colonoscopy, but not for the virtual test, which is standard, but may have an affect on patient preferences. It is important to consider that virtual colonoscopy is a diagnostic procedure, and if a polyp is detected, the patient would then need to undergo conventional colonoscopy to remove the polyp. Some suggest that this be done on the same day to avoid the need for a second bowel prep, but this may be unrealistic in busy medical centers.
At this time virtual colonoscopy is not widely available and most are done as part of a clinical trial, in which patients undergo both procedures for comparison. Because it is still considered experimental, insurance companies do not currently cover this procedure. Many advances have been made in virtual colonoscopy in just a few years, and the technique will continue to improve. However, the issues of cost-effectiveness, accuracy, and patient preference will need to be addressed before this test can become a standard of care.
In DNA stool testing, scientists examine stool for genetic abnormalities that are found in colon cancer. A patient's stool sample is analyzed using polymerase chain reaction (PCR) testing to determine whether mutations in the adenomatous polyposis coli (APC) gene are present. One study, recently reported in The New England Journal of Medicine, analyzed samples from 28 patients with early stage (Dukes B2) colon cancers, 18 from patients with adenomas (cancerous polyp) of at least 1 cm, and samples from 28 patients without cancer. The test was able to detect mutations in 26 of the 46 patients with cancer (57% of the cases detected). The researchers reported that no gene abnormalities were detected in the 28 patients without cancer. DNA stool testing, with a 57% detection rate, would be unacceptable as a replacement for current screening methods, but this study paves the way for more research into this testing method.
Given the current low utilization rates of colon cancer screening in American adults, there is obviously a need for increased education and the development of less invasive, more acceptable, yet accurate methods of screening. Virtual colonoscopy and DNA stool testing are promising, and we are sure to hear more about them in the coming years, but for now, schedule your sigmoidoscopy or colonoscopy!
Dear OncoLink "Ask The Experts,"
I am a 48 yr.old white female who was diagnosed with adenocarcinoma of the colon contained within a polyp. I have a strong family history. My father died of pancreatic cancer at the age of 79. My mother was diagnosed with breast cancer at the age of 46. She was cancer free until 1988 when she was diagnosed with colon cancer. I considered genetic screening. I decided not to pursue it because (a) it was very expensive and (b) I had heard that genetic testing only screens for colon and breast cancer. I asked myself if genetic testing would change my approach to preventing cancer. My question. Would a person with my history benefit from genetic testing?
Timothy C. Hoops, MD, Clinical Assistant Professor of Medicine in the Gastroenterology Division at the University of Pennsylvania and Director of Gastroenterology at Penn Medicine at Radnor, responds:
With your history of colon cancer already, you know that you are at increased risk for future colon cancer. Genetic testing, if a familial colon cancer syndrome were present, would confirm this and possibly tell you that you have a higher risk. The recommendations at this time would be the same that is colonoscopic surveillance.
If it were found that you had a genetic mutation suggesting hereditary nonpolyposis colorectal cancer (HNPCC), you would probably decrease your surveillance intervals to two years as opposed to 3 to 5 years. However, your history suggests but does not clearly support the possibility of HNPCC unless there are other members of your family with cancer. The chance of finding a mutation decreases with the fewer criteria a family has for the inherited syndromes. The main benefit of finding a positive test in you would be to predict risk in other unaffected family members. A negative test would not rule out an inherited syndrome, as only 60 to 70% of well-defined families will have a detectable genetic mutation. Testing your cancer first might increase the chance of finding a mutation, but it still may not help. As such, unless you have a burning desire to know, and the funds to pay for it, genetic testing probably does not play an essential role in your decisions for your health maintenance.
OncoLink has joined forces with the National Colorectal Research Alliance to help their scientists study the risk factors associated with colorectal cancer and identify potential preventive and treatment therapies. You and your family may be interested in taking our survey. This confidential survey was developed by cancer experts as an interactive way to help our leading scientists study families with a history of colorectal cancer: OncoLink/National Colorectal Cancer Research Alliance prevention database.