The answer to this frequently asked question is a long-winded “kind-a”…  Let me try to explain…

“Virtual Colonoscopy” is also known as “CT-colonography” because it is done using a CT (Computed Tomographic or CAT) scanner.  With a “virtual colonoscopy” a patient does a standard bowel preparation like colonoscopy, then lays on a CT scanner while air is inflated through a tube put in their rectum, and the CT machine takes radiographic (xray like) pictures. 

 [click for Graphic]

These images are then computer-reconstructed to provide a single-color picture of the colon. 

[Click for Graphic]

Throughout this discussion, I will follow the convention of referring to Virtual Colonoscopy as CT-colonography and abbreviate it as “CTC.” 

When I refer to simply “colonoscopy,” I am referring to standard (also known as “optical,” by contrast) colonoscopy.

WHY IS A BOWEL PREP NECESSARY FOR THIS PROCEDURE IF IT’S A “VIRTUAL”?

Remaining stool in the colon can look like a polyp on a CTC study, so the colon must be very well-cleaned out.  Unlike colonoscopy, there is no ability to wash the inside of the colon with water or suction up remaining material.  If stool is read mistakenly as a polyp, this is considered a “false-positive” (it says a polyp is there, when in reality, nothing is there).

HOW MUCH RADIATION IS THE PATIENT EXPOSED TO WITH CTC?

A single CTC exam is estimated to result in 0.14% risk of cancer from radiation.  The issue has more to do with the cumulative effect of radiation that one might receive if getting CTC every few years for either screening or to follow a known polyp through this technique.  The likelihood of radiation-induced cancers would likely be increased, but to an unknown degree.

IS VIRTUAL COLONOSCOPY BETTER AT DETECTING POLPYS?

How good a test is at doing what it is supposed to do is called the “sensitivity” of the test. 

A recent well-conducted study by the American College of Radiology Imaging Network (ACRIN) was published 9/18/08 in the New England Journal of Medicine (NEJM) and is considered to be reflective of how CTC would do “in the real world.” 

This study involved slightly over 2500 participants.  It essentially pitted experienced radiologists against hospital staff gastroenterologists.  Table 4 from this study tells it best:

What the above table shows is that the ability of CTC to detect  polyps 10 mm or greater in size was 84% (a miss rate of 16% compared to colonoscopy).  The ability to detect polyps 6 mm in size or greater was 70% (a miss rate of 30% compared to colonoscopy).  CTC is not considered a reliable test to detect polyps 5 mm or under, so they are not reported even if seen.

HOW COULD CTC MISS SO MANY THINGS?

I think there could be multiple explanations for this.  CTC is a 3-D static picture only of the colon – there is no ability to wash, clean or manipulate the finding.  This means that residual stool could be covering over a polyp and CTC has no ability to wash it off.   Some polyps are “flat”, which means that they don’t have a bumped shape to be seen on a 3-D image of the colon (more on these flat polyps in a future blog).  We can see them on colonscopy because we can tell that the texture of the tissue is different – CTC doesn’t detect texture. 

CTC also can’t see in multi-color, it is all one-shade.  Color can be very helpful for identifying when something “isn’t right.”  To illustrate this point, this is a case of early colon cancer in which the main findings are the roughed up texture and red discoloration of the tissue.   In a 3-D view in CTC, this would probably appear like any other fold and would likely be missed. 

OK, SO IT MISSES SOME POLYPS.  IS VIRTUAL COLONOSCOPY ACCURATE WHEN IT FINDS A POLYP?

This has to do with the “specificity” of the CTC test - - in other words, when it says a polyp is there, is it really there?  When it says there is one there, and there isn’t, this is called a “false positive.”   Approximately 5-10% of patients with suspected polyps 6 mm or greater in size on CTC have a “false positive”.  When a colonoscopy is then done, the reasons for this false-alarm  can include things like an inverted appendix, inverted diverticulum, remaining stool or a bump from an underlying  vein.

HOW OFTEN WOULD I NEED A CTC?

Because of the above accuracy issues (sensitivity & specificity), even if the CTC is normal, it is currently recommended to have a repeat CTC every 5 years.  This is in contrast to guidelines which would give the same person 10 years before their next colonoscopy.  Some groups suggest “following” a polyp by CTC until it gets to a point where then a colonoscopy is recommended, so this would mean even more frequent CTC’s, such as every 1-2 years.

WILL MY INSURANCE COVER CTC?

Probably not – but you can check with them. 

The problem is that this is currently NOT a cost-effective strategy for the whole of society.  A decision-analysis study out of Canada concluded that CTC would cost $2.27 million dollars extra per 100,000 patients screened, compared to colonoscopy (reference).  I recognize the different healthcare systems and economics of the two countries, however the cost to society of CTC vs. colonoscopy probably doesn’t differ greatly.

If 25% of 50 year olds and 50% of 70 year olds have pre-cancerous polyps (nevermind other polyps seen on CTC which are not pre-cancerous) plus 5-10% “false positive” cases, this would end up being a pretty high number of people who get a CTC and then need a follow-up colonoscopy…certainly more than the 20% that proponents of CTC toss around.  This means essentially TWO procedures to accomplish the one important thing, namely removing the polyps. 

In part because of this lack of cost-effectiveness and in part because the evidence of medical benefit for CTC is yet to be fully proven/embraced, most insurance companies only permit it in unusual situations where colonoscopy is not an option first.   

The U.S. Preventative Task Force (USPSTF) is probably the most-unbiased group out there who reviews issues like this, and they recently gave CTC a thumbs-down.  This was then followed by Medicare doing the same.  Usually what Medicare does first, commercial insurance companies then follow…

WHAT ABOUT HAVING A CTC AND IF SOMETHING IS FOUND HAVING A COLONOSCOPY ON THE SAME DAY?

This is the ideal way of utilizing CTC and this is the way it is done at all the research institutions that are studying CTC (otherwise, there is NO way they’d get 2500 participants!).  In reality, this is pretty tough to organize.  It would involve the patient getting the CTC study, a radiologist being ready/able to read the study promptly, and then a gastroenterologist also on stand-by waiting to do the colonoscopy if the CTC study found something. 

Because of the real-world issues of this approach, most patients who need a colonoscopy after CTC have to re-prep their bowels a 2^nd time because of the time involved between the study, the reading of study, and arranging for colonoscopy.

The other practical problem of this approach is that right now most of the patients who are referred for CTC are those in whom a doctor is hoping to avoid doing a colonoscopy all-together!  This includes people with very serious medical conditions that might not handle the sedation well or patients on blood-thinning medications that can’t be stopped.  So performing an immediate colonoscopy on these patients is medically not feasible/safe.  In the first instance, yet another doctor (an anesthesiologist) will need to be coordinated to provide safe sedation and airway management.  In the later example, blood thinners will need to be held for several days before polyps can be removed and other medical needs will need to be accounted and planned for.  This is not something to be rushed.

CAN CTC BE USED TO INVESTIGATE OTHER THINGS LIKE ABDOMINAL PAIN, BLOOD IN STOOL OR OTHER GASTROINTESTINAL BLEEDING?

No, not well.  For instance, ulcerative colitis or Crohn’s Disease (both considered Inflammatory Bowel Diseases) could cause all of the above and would likely not be recognized by CTC, but would be identified by colonoscopy.

ENOUGH ABOUT INSURANCE COMPANIES, IS CTC PREFERRED BY PATIENTS THOUGH?  WOULD I LIKE IT BETTER?

Most studies suggest a similar degree of patient satisfaction for CTC or colonoscopy.  This is not too surprising.  Both studies require a bowel prep, so this part is unavoidable.  The main discomfort of CTC relates to the gas (air or carbon dioxide) used to inflate the colon.  Patients are not sedated for CTC, which allows patients to recover faster, but can make for a more uncomfortable test due to distention and cramping. 

Honestly, anything up one’s backside is potentially embarrassing and having one’s colon inflated with air can be uncomfortable.  Many patients prefer to be sedated to sleep through the whole thing (or at least the insertion) and wake up when it’s over.  This is part of why patients who have had both a colonoscopy and the shorter flexible sigmoidoscopy usually report preferring  the colonoscopy…        

WHAT IS THE TAKE-HOME MESSAGE?

CTC is an exciting technology that is emerging and one to keep your eye on.  For now, it is neither good enough at finding polyps, nor good enough at being right when it claims polyps are found, to be recommended routinely.  It is currently not cost-effective for society, and is generally not covered by insurance plans.  There are a select few patient in whom this is the best available method of screening for colon cancer or polyps, but for the vast majority, colonoscopy is a better choice and is still considered the “gold standard” test.

This entry was posted on Thursday, March 12th, 2009 at 12:01 am and is filed under Colon Cancer Awareness. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.