by Dr Geoffrey Modest
An observational study in the UK initially reported that a single flexible sigmoidoscopy screening significantly reduced subsequent colorectal cancer incidence as well as colorectal cancer mortality after 11 years of follow-up. They now found similar results after 17 years of follow-up (see doi.org/10.1016/ S0140-6736(17)30396-3).
— multicenter randomized trial done from 1994-1999, randomized 170,432 men and women aged 55 to 64 to sigmoidoscopy (n= 57,098, of whom 40,621 were screened) vs control (n=112,936).
— mean age 60, 51% women.
— At screening in the intervention group: 18 were referred to surgery directly, 2131 (5%) were referred for a colonoscopy for high risk polyps, and 38,825 (95%) were discharged with low risk polyps or no polyps at all
— primary outcomes were: incidence and mortality of colorectal cancer
— after 17.1 years of follow-up
— colorectal cancer was diagnosed in 1230 individuals in the intervention group and 3253 in the control group: distal colorectal cancer was diagnosed in 529 in the interventions group (126 at the time of screening) and 1987 in the control group. Proximal colon cancers were diagnosed in 612 in the intervention group and 1255 in the control group.
— 353 individuals in the intervention group and 996 in the control group died from colorectal cancer
— intention to treat analyses:
— colorectal cancer incidence was reduced by 26%, HR 0.74 (0.70-0.80), p<0.0001
— colorectal cancer mortality was reduced by 30%, HR 0.70 (0.62-0.79), p<0.0001
— mortality related to distal colorectal cancer deaths was reduced by 46%, HR 0.54 (0.45-0.65), p<0.0001
— per protocol analyses (ie, those who actually had a sigmoidoscopy in the intervention group):
— colorectal cancer incidence was reduced by 35%, HR 0.65 (0.59-0.71)
— colorectal cancer mortality was reduced by 41%, HR 0.59 (0.49 0.70)
–mortality related to distal colorectal cancer deaths was reduced by 66%, HR 0.34 (0.26-0.46)
— sigmoidoscopy screening had no effect on all-cause mortality or on mortality from proximal colonic lesions (above the reach of the sigmoidoscope)
— the estimated number needed to screen (NNS) to prevent a single colorectal cancer diagnosis over 17 years was 98; it was 191 after the 11 year assessment
— the estimated NNS to prevent a certified death from colorectal cancer over 17 years was 220; it was 489 after 11 years
— no difference in the efficacy of sigmoidoscopic screening between men and women in terms of distal cancer, though the overall effect was less in women [women have been found to have more proximal colon cancers in several studies, including this one, one in the US and one in Norway]. The NNS for women was 165, much higher than men likely because of lower incidence of colorectal cancer in the control group in women than men and the fact that women had more proximal cancers. Also, there was no outcome difference between those 55-59yo versus 60-64yo at the time of the screening
— the 11 year data from the study showed that colorectal cancer incidence was reduced by 33%, distal colorectal cancer incidence by 50%, and colorectal cancer mortality by 43% (similar to above)
–A review of the graphs in the 17-year follow-up article shows that at about 4-5 years post-screening, the curves for colon cancer incidence cross (the intervention group picked up more cancers prior to the 5 year mark, the control group having much more afterwards, with curves then diverging). The incidence of proximal cancer was the same throughout the study. Colorectal cancer mortality diverged after about 5 years, with essentially parallel curves for the last 4 years. This latter finding suggests that follow-up after the 17 year mark would likely continue to show benefit from a single screen (the annual incidence rate ratio reductions were 74% between years 6 and 10 and 69% between years 11 and 16 post initial screen).
— for reasons that escape me, several studies have found that excision of colonoscopy-discovered right colonic lesions (i.e. beyond the reach of the sigmoidoscopy) do not affect mortality (eg see Baxter NN. Ann Intern Med 2009; 150:1). This finding, in and of itself, adds further support to sigmoidoscopy screening, which is significantly less invasive and has significantly fewer adverse consequences than colonoscopy.
— a pooled analysis of 3 other trials also found extended benefit for flexible sigmoidoscopy screening, though these trials did not find benefit in women >60 years old. The current study authors argue that the screening should be done in people younger than 60 years old anyway, where women seem to have equal benefit.
so, pretty powerful article. The current recommendations have not changed (though are more aggressive in the US than Canada or the UK: (eg see here ). But this article suggests a few things:
–though we may offer patients the recommended screening intervals (as per the recommendations), it is reassuring that the likelihood of cancer or mortality is significantly less even after a single negative sigmoidoscopy (and I have seen a few articles in the past suggesting that colonoscopy screening may also be effective for more than 10 years. eg Nishihara R. N Engl J Med 2013;369:1095, which found protection up to 15 years after a negative colonoscopy)
–it also raises the issue of the natural history of colon cancer, with most of us learning it was on average about 10 years to develop cancer in apparently normal cells (supporting the screening intervals of the recommendations). It seems that people with visually normal exams at screening are at much lower risk of development of cancer for many more years than those with adenomas or early cancers (ie, those with lesions of some sort have a different colonic substrate and are therefore more genetically or environmentally or both predisposed to cancer even after resection than those with apparently normal colons, so it is erroneous to apply the same finding of metamorphosis to cancer from them to normal individuals.)
–and it even raises tangentially a bigger issue about cancer: smokers with disturbed lungs, such as with COPD, seem to be at higher risk of lung cancer than those with radiographically normal lungs even with the same smoking history, or women with nonmalignant breast changes on biopsy are more predisposed to breast cancer,
–and perhaps even calculations of deleterious effects of radiation exposure is much worse in those with underlying abnormal tissues (which raises yet another issue: doing more mammograms in women with abnormal noncancerous breast tissue, or in smokers for lung cancer screening, might be much more risky than in women or smokers with normal tissue. So, the recommendations to increase radiologic screening in both cases may actually cause more harm in those with abnormal baseline tissues than the current models suggest. though i digress……
See here , which reviews a population-based study in Norway, similarly finding that a one time flexible sigmoidoscopy decreased deaths from colorectal cancer by 27%, with 10.9 years of follow-up.