Screening & Diagnosis
Screening for colorectal cancer can mean the difference between catching the disease in its early stages and catching it in its advanced stages, and this literally can be the difference between life and death. Precancerous polyps can be easily removed, and if colorectal cancer is caught early, it often is treatable and the chances of a full recovery are good. But cancerous polyps that have invaded the intestinal wall or even metastasized can prove fatal.
Types of Screening
There are a number of different colorectal cancer screening tests:
- Fecal occult blood tests (FOBT) are usually performed by the patient at home and then returned to the laboratory for analysis. The FOBT uses stool samples to detect hidden (occult) blood in the stool. If blood is found in the samples, a colonoscopy must be performed.
- Colonoscopy is a more extensive evaluation of the entire length of the colon. A colonoscope is a long, flexible tube with a video camera one end. The doctor snakes the colonoscope up into the colon and views images from it on a display monitor. Colonoscopies take about 30 minutes to perform. If polyps are found, they are removed in the course of the colonoscopy. The patient may be given a sedative prior to the test. In preparation for the colonoscopy, the colon and rectum must be completely empty. Certain foods and liquids must be avoided in the days preceding the test. The day before the colonoscopy, the patient is given a laxative solution to clean out the colon. Many patients find the preparations for the colonoscopy more unpleasant than the test itself.
- Sigmoidoscopy is similar to colonoscopy but only examines the rectum and the lower 2 feet of the colon. The procedure takes about 10-20 minutes, and sedation is optional. Preparation procedures are less demanding than those for colonoscopy, but sigmoidoscopy has a significant disadvantage compared to colonoscopy: about 50% of colon polyps and cancers are found in the upper colon and are therefore beyond the reach of sigmoidoscopes.
- Double-contrast barium enemas (DCBEs) use an X-ray to image the entire colon. The test takes about 30-45 minutes, and the patient is not sedated. For the test, barium sulfate is inserted into the rectum. The colon is then pumped with air to help the barium spread through the colon. If polyps are detected, a colonoscopy will probably be recommended. Preparations for DCBEs are similar to those for colonoscopy.
Experts agree on the value of screening but which tests should be used, and how often, is somewhat controversial. The American Cancer Society, the American College of Radiology, and the US Multi-Society Task Force on Colorectal Cancer have released the following guidelines for colorectal cancer screening for people over the age of 50 at average risk:
- Flexible sigmoidoscopy every 5 years, or
- Colonoscopy every 10 years, or
- DCBE every 5 years
- FOBT every year, or
- Fecal immunochemical test (FIT) every year
- Stool DNA test (sDNA). The frequency of testing has not yet been determined.
When someone has signs or symptoms of colorectal cancer, a doctor will usually begin by taking the person's complete medical history (including family history) to check for symptoms and risk factors. The doctor will also perform a physical exam, examining the abdomen for masses and enlarged organs and possibly performing a digital rectal exam.
The doctor may order blood tests done. Blood tests can evaluate red blood cell count, check for anemia, and look for specific tumor markers (chemicals released into the blood by cancer cells).
If the patient's symptoms, results of the physical exam, and blood tests warrant further investigation, the doctor will next examine the colon through sigmoidoscopy or colonoscopy, or imaging tests such as a DCBE, ultrasound, magnetic resonance imaging (MRI) scan, or positron emission tomography (PET) scan.
- Ultrasound uses high-frequency sound waves and their echoes to produce a picture of internal organs or masses on a computer screen. The test is painless and does not expose the patient to radiation.
- MRI scans provide images of soft tissues in the body using radio waves and strong magnets. A computer translates the pattern into a very detailed image of parts of the body. A contrast material is often injected into a vein before the scan to provide clearer images.
- CT scans use multiple X-ray images to produce detailed cross-sectional images of the body. As the patient lies on a table, the CT scanner take numerous X-rays as it rotates around him or her. Sometimes the patient is injected with a contrast dye the help outline structures of the body. A computer program then combines the images to form detailed cross sections, or slices, of the part of the body being studied. Unlike regular X-rays, CT scans can create detailed images of the body's soft tissues. Spiral CT is a recent advance in CT imaging. It uses a faster machine that rotates around the body continuously, collecting images more quickly and thus lessening the chance that the images will blurred as a result of breathing motion. In addition, the patient receives less radiation with spiral CT, and the cross sections are thinner, producing more detailed pictures.
- PET scans involve injecting a form of slightly radioactive glucose (known as fluorodeoxyglucose or FDG) into the blood. Cancer cells in the body grow rapidly and absorb large amounts of glucose. The body is then photographed by a special camera to show areas of high glucose concentration, allowing the physician to see what is happening in the body at the molecular level.
- PET/CT scans are a recent development. PET/CT scanning machines can perform both a PET and a CT scan at the same time. The radiologist can compare areas of higher radioactivity on the PET with the appearance of those areas on the CT.
A recent, promising innovation in diagnosing cancer is the development of the "virtual biopsy," in which polyps are examined in the colon using a probe sensitive enough to determine whether or not it is cancerous, without having to remove it.
Colorectal Cancer Screening (VIDEO)
Colon Anatomy & Function
What Is Colorectal Cancer
Signs & Symptoms
Screening & Diagnosis
A Less Invasive Look
Treating Colorectal Cancer
Detecting Cancer Recurrence
Related Health Centers:
Breast Cancer, Cancer Introduction, Colorectal Cancer, Prostate Cancer