The companion UK Special Interest Group in Gastrointestinal and Abdominal Radiology (SIGGAR) trials compared computed tomographic (CT) colonography with barium enema and colonoscopy in patients with symptoms suggestive of colon cancer. As recently reported by the SIGGAR investigators in The Lancet,^1,2 the findings of the two trials indicate that CT colonography is more sensitive than barium enema and suggest that the test provides a similarly sensitive and less invasive alternative to colonoscopy, albeit with a much higher rate of referral for additional colonic investigation. Optimal use of CT colonography will require development of protocols for best practice and guidelines on patient referral.

CT Colonography vs Barium Enema Trial

In the comparison with barium enema,¹ 3,804 patients aged 55 years or older were randomly assigned to CT colonography (n = 1,277) or barium enema (n = 2,527). Patients were well balanced for sex (62% and 61% female), age (median 69 years; 55–64, 65–74, and 75–84 years for 33%, 39%, and 26% and for 33%, 39%, and 25%), and symptoms.

Symptoms included change in bowel habits in 76% of both groups (including “looser, more frequent” in 42% and 40%), rectal bleeding in 30% of both groups, abdominal pain in 32% of both groups, anemia in 12% and 13%, and weight loss in 14% and 13%. The primary outcome measure was detection of colorectal cancer or large (≥ 10 mm) polyps. Patients underwent flexible sigmoidoscopy prior to the randomized procedure at some study hospitals.

Detection Rates

The detection rate of colorectal cancer or large polyps was significantly higher in the CT colonography group (7.3% vs 5.6%, relative risk [RR] = 1.31, P = .0390), with the difference largely reflecting greater detection of large polyps (3.6% vs 2.2%, P = .0098); there was no significant difference in detection of cancer (3.7% vs 3.4%, P = .66).

Analysis of detection rates including only those patients who actually underwent their randomized procedure (n = 2,300 in barium enema group and n = 1,206 in CT colonography group) and excluding those in whom cancer or large polyps had been detected by prior flexible sigmoidoscopy (0.5% of CT colonography group and 0.6% of barium enema group) showed that the detection rate was still significantly higher in the CT colonography group (7.0% vs 5.2%, P = .023).

The rate of additional colonic investigation was significantly higher after CT colonography (23.5% vs 18.3%, P = .0003), including higher rates for investigation of suspected cancer/large polyps (11.0% vs 7.5%, P = .0005) and for suspected smaller polyps (7.2% vs 2.3%, P < .0001). Among patients having additional investigation, cancer or a large polyp was found in similar proportions in the two groups (29% and 28%). Fewer CT colonography patients underwent additional colonic investigation due to inadequate examination or clinical uncertainty (5.2% vs 8.5%, P = .0005).

During 3-year follow up, colorectal cancer was diagnosed in 3 patients in the CT colonography group and 12 patients in the barium enema group who were not diagnosed on initial CT colonography or barium enema. Miss rates were thus 7% (3 of the total of 45 cancers detected) in the CT colonography group and 14% (12 of 85 cancers detected) in the barium enema group, with the difference not being statistically significant.

Extracolonic findings led to additional investigation in 87 (7.5%) of the CT colonography patients who were not diagnosed with colorectal cancer. Extracolonic malignancy was found in 13 of these patients, and extracolonic diagnoses that explained at least one of the patients’ presenting symptoms were made in 31.

Four patients in the barium enema group (cardiac arrest, abdominal pain, rectal bleeding, and collapse) and one in the CT colonography group (free gas in the abdomen during the procedure) had unplanned hospital admissions that were considered possibly procedure-related within 30 days after the procedure.

CT Colonography More Sensitive, Preferred

The investigators concluded: Results of our study show that [CT colonography] is more sensitive than [barium enema] for detection of colorectal cancer or large polyps, and we have previously reported that [CT colonography] is preferred by patients. The higher sensitivity of [CT colonography] for small polyps and its ability to detect extracolonic lesions offer equivocal benefits, since these incur additional costs and patients might be referred for investigation of findings that are clinically unimportant. However, this risk can be managed if more widespread use of [CT colonography] is accompanied by protocols for best practice, including guidelines on patient referral for both radiologists and referring clinicians. Training and quality assurance for radiologists are also needed if the capabilities of [CT colonography] are to be fully realized. With these provisos, our results suggest that [CT colonography] should now replace [barium enema] as the preferred radiological test for patients with symptoms suggestive of colorectal cancer.¹

CT Colonography vs Colonoscopy Trial

In the second trial,² 1,580 patients aged 55 years or older were randomly assigned to receive CT colonography (n = 533) or colonoscopy (n = 1,047). The primary outcome measure was the proportion of patients with additional colonic investigations.

Patients were well balanced for sex (55% and 55% female), age (median 68 years; 55–64, 65–74, and 75–84 years for 41%, 35%, and 21% and for 37%, 36%, and 24%), and symptoms. Symptoms included change in bowel habits in 72% and 74% (including “looser, more frequent” in 40% and 39%), rectal bleeding in 45% and 41%, abdominal pain in 23% and 22%, anemia in 11% and 13%, and weight loss in 15% of both groups.

Additional Investigation

Patients in the CT colonography group were more than three times as likely to have additional colonic investigations (30.0% vs 8.2%, RR = 3.65, P < .0001). Nearly half of the referrals in the CT colonography group were for small polyps or clinical uncertainty. Overall, additional investigations were performed for suspected cancer/large polyps in 15.6% of the CT colonography group and 1.1% of the colonoscopy group, smaller suspected polyps in 9.2% and 0.1%, and clinical uncertainty in 5.3% and 7.0%. Cancer or large polyps were found in 34% of CT colonography patients and 17% of colonoscopy patients referred for additional colonic investigation.

Referral rates differed significantly by sex (P = .0002): men were more than six times as likely to have additional colonic examination after CT colonography compared with colonoscopy, with women being twice as likely to have additional investigation. There was no difference in referral rates according to age.

There was also no difference in rates of detection of colorectal cancer or large polyps between the CT colonography group and colonoscopy group (10.7% vs 11.4%, RR = 0.94, P = .69). Analysis of detection rates including only patients who actually underwent their randomized procedure (n = 967 in colonoscopy group and n = 503 in CT colonography group) and excluding those in whom cancer or large polyps had been detected by prior flexible sigmoidoscopy (0.4% of CT colonography group) showed similar results (10.7% vs 12.0%, P = .47). During 3-year follow-up, 1 additional cancer was found in the CT colonography group, yielding a miss rate of 3.4% (1 of the total of 29 cancers detected). No cancers were missed in the colonoscopy group (total of 55 detected).

With regard to other colorectal finings, significantly more patients in the CT colonography group were diagnosed with diverticulosis (54% vs 35%, P < .0001), whereas significantly more in the colonoscopy group were diagnosed with colitis (1% vs 3%, P = .0022) or anal pathology (2% vs 7%, P = .0002). Extracolonic findings led to additional investigation in 48 (10.1%) of the CT colonography patients who were not diagnosed with colorectal cancer. Extracolonic malignancy was found in 9 of these patients and extracolonic diagnoses that explained at least one of the patients’ presenting symptoms were made in 17.

Three patients in the colonoscopy group (abdominal pain, rectal bleeding, and diarrhea and vomiting) had unplanned hospital admissions that were considered possibly procedure-related within 30 days after the procedure. A patient in the CT colonography group who underwent colonoscopy 22 days after CT colonography for removal of lesions was admitted immediately after colonoscopy for suspected perforation and discharged the following day.

These investigators concluded:

[I]n our pragmatic trial of symptomatic patients, [CT colonography] was associated with a high referral rate for additional tests. These referrals have the potential to increase anxiety and overall cost, and—in patients referred for colonoscopy—mitigate the advantage of avoiding an endoscopic examination. For most patients, however, [CT colonography] offers a similarly sensitive, less invasive alternative to colonoscopy. With wider implementation, there will be a need for protocols to improve specificity, along with attention to referral criteria and an emphasis on radiologist training and assessment. With these in place, our results suggest that [CT colonography] should be considered as an alternative first-line investigation for patients with symptoms suggestive of colorectal cancer.² ■

Disclosure: The studies were funded by National Institute for Health Research (NIHR) Health Technology Assessment Program, NIHR Biomedical Research Centres, Cancer Research UK, Engineering and Physical Sciences Research Council Multidisciplinary Assessment of Technology Centre for Healthcare, and NIHR Collaborations for Leadership in Applied Health Research and Care.

References

1. Halligan S, Wooldrage K, Dadswell E, et al: Computed tomographic colonography versus barium enema for diagnosis of colorectal cancer or large polyps in symptomatic patients (SIGGAR): A multicentre randomized trial. Lancet. February 13, 2013 (early release online).

2. Atkin W, Dadswell E, Wooldrage K, et al: Computed tomographic colonography versus colonoscopy for investigation of patients with symptoms suggestive of colorectal cancer (SIGGAR): A multicentre randomized trial. Lancet February 13, 2013 (early release online).