The US Preventive Services Task Force (USPSTF) recommends that everyone aged 50-75 be screened for colon cancer with any one of three options: colonoscopy every 10 years, flexible sigmoidoscopy every 5 years, or fecal occult blood testing (FOBT) every year. Conventional colonoscopy is considered the “gold standard” since it allows for direct detection and biopsy of early cancers and removal of precancerous polyps. It involves passing a long colonoscope via the rectum through the full length of the colon and is also known as optical or visual colonoscopy. A newer and less invasive alternative, virtual colonoscopy or CT colonography, is being promoted by some as the test of choice. Others disagree. One area of controversy is that CTs frequently find “incidentalomas” that require further investigation. An article in the journal Radiology highlights this problem, describing “the clinical drama that follows screening or diagnostic tests.”
Virtual colonoscopy, or CT colonography, has some advantages:
- No sedation required.
- Avoids the discomfort and the small risk of bowel perforation that accompany colonoscope insertion.
- Takes less time.
- Visualizes right colon (not well visualized in 10% of optical colonoscopies).
- It is as effective as optical colonoscopy in finding advanced cancers, but results in far fewer polypectomies.
- May identify abnormalities outside the colon.
It also has some disadvantages:
- Radiation exposure.
- May miss smaller polyps and even some larger ones: CT colonography failed to detect a lesion measuring 10 mm or more in diameter in 10% of patients and missed 15% of advanced cancers measuring over 6 mm.
- Still requires thorough bowel prep (clear liquid diet, laxatives, large quantities of bad-tasting oral solutions), which some patients find to be the most objectionable part of colonoscopy.
- Requires insertion of rectal tube and insufflation of air.
- If a lesion is seen, optical colonoscopy will be required as follow-up. About 8% of patients will end up getting both procedures.
- May identify abnormalities outside the colon.
You will notice that identification of abnormalities outside the colon is listed as both an advantage and a disadvantage. Assessments of risk/benefit ratio differ. In a 2005 study of 500 patients published in Radiology, CT colonoscopy found “a substantial number” of clinically significant findings, including aneurysms and a renal carcinoma; but it did not establish whether finding them improved outcomes. 28% of the significant findings had already been diagnosed prior to colonoscopy; and the patient with the renal carcinoma declined treatment, since he already had metastatic colon cancer. In a 2009 study of 143 patients, 98% of patients had at least one incidental finding outside the colon. 24% of these required further evaluation, with 73 imaging studies, 30 lab studies, 44 clinical visits, 6 medical procedures, and 44 return visits over a mean period of 38 months. The cost was estimated at $248 per patient. They did not assess whether these evaluations improved eventual patient outcomes.
Writing in the journal Radiology, Dr. William Casarella, a radiologist at Emory University School of Medicine, tells the story of his own virtual colonoscopy. His colon was normal; but the CT also showed areas outside the colon, revealing a kidney lesion, a 2 cm. mass in the liver, and multiple non-calcified nodules in the bases of both lungs. These findings led to the following tests and interventions:
- A contrast-enhanced CT of the abdomen (with more radiation) showed that the kidney lesion was a benign cyst, but the liver mass was not.
- A high-resolution lung CT (more radiation) confirmed lung nodules.
- A CT-guided (more radiation) liver biopsy (more risks) showed necrotic tissue, no definitive findings.
- A PET scan (more radiation) was negative.
- Video-aided thoracoscopy
- 3 wedge resections of the right lung (requiring the lung to be collapsed), resulting in a diagnosis of histoplasmosis (benign, no treatment indicated).
The lung surgery led to:
- 5 hours in the recovery room before awakening from general anesthesia.
- Chest tube, subclavian central venous catheter, nasal oxygen catheter, epidural catheter, arterial catheter, subcutaneous heparin injections, prophylactic antibiotics, IV narcotics (each with associated risks).
- Excruciating pain requiring narcotics for 2 weeks.
- 4 weeks of disability.
- Persistent chest pain due to surgical interruption of intercostal nerves.
The total cost was over $50,000.
This is an atypical example, but it underlines the potential dangers of too much information. Sometimes ignorance is bliss. Dr. Casarella would have been better off had he not been screened for colon cancer at all. I support the USPSTF recommendations, but it is important not to over-rate the value of these tests. The public perception is “get this test: it will save your life.” The reality is more complicated. Screening tests can have false positive and false negative results. They don’t always provide clear black and white answers, and they don’t save as many lives as the public tends to think. While statistically benefiting the population, they may be hazardous to the health of some individuals. We need to keep in mind that even the best screening tests have a downside.
This article was originally published in the Science-Based Medicine Blog