Faecal immunochemical tests (FIT) underpin national bowel cancer screening programs here and abroad but there is more evidence that the tests are failing to pick up right-sided lesions.

A retrospective study of more than 123,000 Italians enrolled in two-yearly bowel cancer screening since 2002 has found a negligible reduction in detection rates for neoplastic lesions in the proximal colon after the first round of FIT.

On the other hand, detection rates for distal and rectal cancers and advanced adenomas steadily decreased across six rounds of screening.

The study calculated the positive predictive value of colonoscopy for detecting advanced neoplasia in the proximal colon did not change by round of screening (11.4 to 13.2, p=0.147) but significantly decreased in the distal colon (32.3 to 14.5, p<0.001) and in the rectum (9.6 to 6.8, p<0.001).

Of the 150 cases of interval cancer diagnosed during the study period after a negative FIT, 78 were in the proximal colon compared to 26 in the distal colon and 35 in the rectum yielding proportional interval cancer rates (PICR) of 25.2%, 6.0% and 9.9% respectively.

There were similar findings with PICR following a negative colonoscopy of 28.1% in the proximal colon, 6.9% in the distal colon and 11.6% in the rectum.

The researchers said the findings show that FIT repetition has suboptimal efficacy for detecting proximal CRC, consistent with other studies.

“Overall, our results indicate that the association between sequential FIT rounds and the natural history of CRC differed according to site, which may be explained by 2 factors: the site-specific FIT accuracy and age-related right-side shifting,” the study said.

“These findings also may be the result of a relatively high right-colon prevalence of nonadenomatous neoplastic lesions, such as sessile serrated polyps, for which FIT has suboptimal accuracy.”

An editorial in the journal said the reduced effectiveness of FIT in the right colon may stem from lesions that grow more rapidly or bleed less because of molecular and phenotypic characteristics.

“A longer transit time also may lead to degradation of hemoglobin, rendering occult bleeding undetectable,” it said.

“Thus, enhancing detection of right colon lesions is now a crucial priority for the field of CRC screening. This requires new technologies or approaches, such as high-performing biomarkers that complement FIT or supplementing FIT with one-time colonoscopy to enhance detection of right-sided lesions, but these options need further study.”

Dr Cameron Bell, a management committee member for the development of Clinical Practice Guidelines for the Prevention, Early Detection and Management of Colorectal Cancer, agreed that one of the possible ways to improve detection would be to include a high quality colonoscopy in the screening program.

“But that would require a major public policy shift,” he said.

He said the study’s findings were consistent with some literature about colonoscopy being less protective of cancers on the right than on the left.

“And all of it is suspicious for the main culprit being sessile serrated adenomas. They are notoriously non-bleeders,” he told the limbic.

“The problem with the right sided lesions is that sessile serrated adenomas are known to be poorly detected by FOBT – even worse than adenomas. They are notorious for sitting around for a long time, often into the 60s and 70s and then becoming cancerous very quickly. So there is not a pre-malignant stage where they ever leak blood, so to speak.”

“The only answer for people with sessile serrated adenomas, thought to contribute to 20% of colorectal cancers which is a significant minority, is high quality colonoscopy.”