As anybody who has been reading for a while knows, I'm of the opinion that some screening tests are oversold. I'm particularly skeptical of prostate cancer screening, because you get a whole lot of overdiagnosis -- i.e., "cancers" that never would have caused a problem if they weren't detected, but cause big problems when you get surgery, radiation and chemotherapy -- and false alarms -- PSA levels or physician's perception of a palpable abnormality that result in biopsies and associated sturm und drang but are benign. I'm also totally down with not starting mammography until 50 and maybe only every two years. As always, family history or other risk factors change the parameters big time, so educate yourself, talk to your doctor, and make your own decision.
However, these opinions are based on studying the research and understanding the specific facts about each screening modality. I do not have some generalized anti-screening ideology. And so we come to screening colonoscopy. It's more expensive than mammograms or PSA tests, but it's still more cost effective and has much less downside. So-called Ductal Carcinoma in Situ, DCIS, the non-invasive lesions often found by mammography, and "indolent" prostate cancers, often never progress to cause disease. They may even spontaneously remit. But if they are found by screening, they result in surgery and/or radiation and/or highly toxic chemotherapy.
The progression from a polyp in the colon to cancer, however, is much more predictable; and it's also pretty slow, which means the screening interval can be long. Best of all, pre-cancerous lesions can be removed during the colonoscopy itself, without invasive surgery, which means you get actual, real prevention as well as early detection out of the deal.
This is the most thorough recent review I could find. It's complicated and geeky -- read it at your peril. But the takeaway is simple. If you live in a rich country where treatment is available, screening is well worth it. Colonoscopy at age 50, and every ten years thereafter or more frequently if there is an indication, is extremely cost effective. These authors estimate $800-$2,200 per DALY (Disability Adjusted Life Year) which is a much better buy than an annual check up, even better than medication for high blood pressure. Some estimates even give the cost as zero because of the possibility of actually preventing cancer. And the population level benefit is substantial -- 18 million DALYs gained per year. However:
Adoption of screening policies between the ages of 50 and 80 will only eradicate a small portion (between 14%-24%) of the existing colorectal cancer burden, since the application of the compliancy rate to the intervention efficacy, even in the case of colonoscopy every 10 years will only result in a 24.1% reduction in incidence.
What this means, in English, is that we don't get the full benefit because lots of people don't do it. Yeah, it's a pain in the ass. But it beats the hell out of the alternative.