Now comes a procedure that patients might be more
willing to have done: virtual colonoscopy: computer-assisted, ultrafast or spiral CT-scan
equipment that moves the patient continuously through a rotating X-ray at a
one-breath-holding speed.
"This is a disease that lays down a fingerprint years before it
actually becomes a malignancy, and with this technology we can find it at the early stages
when it is still treatable," Dr. Maloney said.
While once used to screen patients for colon cancer only in the largest
of medical centers - the Mayo Clinic for one - several different models of the equipment
are now moving into many large hospitals.
UMass Memorial Health Care doesn't have the spiral CT-scan equipment,
but St. Vincent Hospital at Worcester Medical Center does. However, St. Vincent has not
set up a virtual colonoscopy program, according to spokeswoman Paula Green.
However, Chadwick medical Associates just added virtual colonoscopy to
its array of CT-scan imaging techniques.
All use digital information to produce a three-dimensional picture of
the body's internal hollow structures like the colon, or the lungs, and the arteries,
screening for plaque buildups.PREPPING FOR TEST
For openers, a virtual colonoscopy patient has to go through much of
the same process to clean out the colon that he or she would have gone through when
prepping for a conventional test. From then on, everything is different.
Conventional colonoscopies rely on the doctor to find tumors and
precancerous polyps by looking at the video screen while the colonoscope - an instrument
about the width of your thumb - makes its way through the colon.
Virtual colonoscopy is much less invasive, with an air tube inserted an
inch or two into the rectum so that the colon can be inflated. The patient can control the
influx of air by squeezing a small rubber ball, and can stop immediately if he or she
feels uncomfortable, according to Dr. Maloney.
The patient lies first on his or her back to allow fluid that could
obscure a polyp to drip away, first from one side of the colon wall, then from the other.
Ms. LeBoeuf recently had a virtual colonoscopy at Chadwick. About 400
pictures of her colon and rectum were taken in all, half during the 30 seconds she was on
her back, holding her breath, and the other half during the 30 seconds she was on her
stomach, holding her breath.
And that was about it for the patient's part in the virtual
colonoscopy. Ms. LeBoeuf got up, got dressed, went to the bathroom, and then went back to
work.
"I have a Dunkin' Donuts coffee and a doughnut waiting for
me," said the 41-year-old nurse who had eaten nothing but Jell-O and fluids for 24
hours previously.
'NOT UNCOMFORTABLE'
As for the virtual colonoscopy, "you could feel the pressure
from the air, but it was not uncomfortable," she said.
In contrast, a conventional colonoscopy takes from 30 minutes to an
hour, with the patient sedated. Afterward, the patient usually needs someone else to drive
them home, and then usually sacks out for the remainder of the day.
A radiologist interprets the images from a virtual colonoscopy. Data
from the CT-scan is fed to a computer where pictures of the colon walls are reconstructed
so small growths or polyps can be found. Those can be converted into 3-D models, so a
radiologist can decide if suspicious images are polyps or air holes.
The whole procedure takes 10 to 30 minutes, and the pictures can be
saved for any future need.
The virtual colonoscopy also can, in some instances, see more than a
conventional colonoscopy.
"Everyone's worst nightmare," Dr. Maloney said, "is that
the endoscopist will put the scope in and not see the very edge of the rectum, and if
there is a malignancy there, it can be missed." Virtual colonoscopy can see both
sides of a fold in the colon, which could be missed by conventional colonoscopy, which
looks straight ahead down the colon.
REMOVING POLYPS
On the other hand, since there is no tube involved in virtual
colonoscopy, there is no way to cut out a polyp if one is located. If a polyp is found
during virtual colonoscopy, then a conventional colonoscopy must be done to remove it.
It's estimated that about 80 percent of people who have a colonoscopy,
however, will not have any polyps that need further exploration.
The other 20 percent will have polyps, and of course, how well a test
picks up those polyps is the main point of any discussion about testing for colorectal
cancer.
"Polyps as small as 2 to 3 millimeters can be detected by the
virtual colonoscopy, but the accuracy rate at those extremely small sizes begins to drop
off," according to Dr. Matthew Barish at the center for Virtual Colonoscopy,
Department of Radiology, Boston University Medical School, Boston Medical Center.
"The accuracy is good at detecting polyps of 5 to 10 millimeters
and above 1 centimeter - the size at which a polyp puts a patient at risk for colon cancer
- the accuracy rates at detecting those are in the high 90 percent, basically equal to
conventional colonoscopy," he said.
About one in 20 people will develop the disease in the course of a
lifetime. Ninety percent of cases occur in people over age 50, but your risk begins at age
40.
Only a few HMOs cover the cost of virtual colonoscopy, according to Dr.
Barish, although that may change. So far, neither Medicaid nor Medicare will cover the
cost.
HMO coverage of various colon cancer detection tests differs from one
HMO to another, but usually the cost of a fecal occult blood test is covered. Some also
will pay for a sigmoidoscopy, which examines only the last two feet of the colon. Most do
not cover a colonoscopy unless there is a family history or suspicious symptoms. Charges
vary, but a colonoscopy can cost anywhere from $750 to $1500.
NEW STOOL TEST
However, colon cancer testing may be revolutionized by a new
DNA-marker test coming out of a Maynard company.
Scientists at EXACT Sciences Inc. have developed a test that can pick
up mutated human DNA shed from the surface of colorectal tumors and excreted in a stool.
The test is noninvasive, with no modification of diet or any preparation of the patient
beforehand.
Next month, the company will launch the PreGen-26 test to detect colon
cancer patients who have an 80 percent lifetime risk of developing the disease because of
heredity.
"Our test is a not predispositional test, nor a gene test,"
said Robert Rochelle, vice president in charge of marketing. "We detect actual
disease."
In the fall, several thousand patients will be screened for colon
cancer in a clinical trial using the EXACT Sciences DNA-marker test, a fecal occult blood
test, and a colonoscopy for comparison of results.
Colonoscopies remain the gold standard for detecting colon cancer;
sigmoidoscopies are good, but since the test examines only the lower part of the colon, it
catches only half of precancerous polyps.
A fecal occult (hidden) blood sample is the most common detection test
used as part of an annual exam. But, not all cancerous polyps bleed, and when they do,
they bleed intermittently. At best, the test detects about 10 percent of cancerous polyps.
The EXACT Sciences test detects colon cancer in two early stages when
surgery can produce, respectively, a 95 percent and an 80 percent five-year-survival rate.
"Sadly, most colon cancers - 63 percent - are caught in the late
stages," said Mr. Rochelle, when the five-year-survival rate drops to 50 percent and
lower.
The EXACT Sciences test, which claims a 91 percent specificity, is
similar to Pap smears, where cervical scrapings are examined for cancerous cells. Mr.
Rochelle sees the test becoming part of an annual exam, just as Pap smears are.
Currently, only about 10 million to 12 million of the 80 million people
that the American Cancer Society recommends should be screened for colon cancer are
actually being screened. Embarrassment and fear of tests are two reasons why many people
don't get screened.
"You can have the greatest test in the world, but if no one uses
it, you can't help people," said Mr. Rochelle. |
