Cancer screening not all it's cracked up to be
For years, doctors have urged patients to get screened for cancer. The implicit and explicit rationale behind screening is that early detection saves lives. That's a powerful statement that unfortunately does not often hold up to scientific scrutiny, according to a new study.
The study — published recently in the International Journal of Epidemiology — concluded that few of the screening tests currently available for cancer and other diseases actually save lives. Researchers from the Stanford University School of Medicine looked at the evidence on thirty-nine screening tests for nineteen major diseases. Included were most of the leading types of cancer, including breast, lung, prostate and colorectal cancer, as well as cancer of cervix and ovaries. The authors found that in cases in which death is a common outcome of the cancer...which is the case with all of the types of cancer I mentioned...it was highly uncommon to find that the use of screening tests like mammograms and PSA testing led to well-documented reductions in the number of deaths due to that type of cancer. In other words, the widespread use of the test to screen for that particular cancer did not reduce the chance of dying from that type of cancer, and did not reduce the chance of dying from any other cause as well.
The study did uncover some cancer screening successes. In a handful of clinical trials, the use of specific screening tests apparently increased survival. For instance, one study showed that screening for cervical cancer reduced mortality, as did another study that screened for liver cancer. The paper does not say that screening never saves lives... but that screening has been overrated. The expectation of major benefits from cancer screening needs to be tempered by a good deal of caution.
There are several possible reasons why these screening tests have performed so poorly.. The screening test may not detect the cancer early enough to save a life. It may be that no matter how early the cancer is detected, there is no good early treatment that cures the cancer. In some cases, there is a treatment but the risks of the treatment are so great that they cancel out the benefits of early cancer detection. Or, early detection leads to treatment that cures the cancer but the patient is more likely to die of other causes not related to the cancer.
Fortunately, there is a much brighter future for cancer screening as there will be much better screening tests. A new one developed by the Mayo Clinic in the U-S tests DNA for colorectal cancer in stool samples. It can be performed on mailed-in samples...eliminating the need for an office visit...and it has proved highly accurate at detecting precancerous polyps and early-stage colorectal cancer. Several studies are looking at breast cancer screening. The National Cancer Institute has funded research into a blood test that analyzes an abnormal protein in the blood...one that investigators hope will detect cancer. The blood test is being tested only for ovarian cancer, but researchers hope it can be used in breast cancer. One study to detect breast cancer by looking at blood cells was 95 per cent successful in spotting cancers. The complete report is currently being reviewed for publication in a medical journal.
In the meantime, talk to your doctor and follow current cancer screening guidelines from the Canadian Cancer Society and the Canadian Task Force on Preventive Health which have been in flux for the past few years. For breast cancer, screening mammography is no longer recommended for women age 40 to 49, but continues to be recommended every two years for women age 50 to 69. For cervical cancer, Pap smears are recommended every one to three years depending on your previous test results. For colorectal cancer, current guidelines call for a test of your stool for blood once every two years with a positive test leading to further testing such as colonoscopy or double contrast barium enema. For prostate cancer, PSA testing together with a DRE may help find a dangerous cancer early when it is easier to treat. Testing could save your life, and PSA testing is useful in monitoring an already diagnosed cancer. But PSA testing is no longer recommended to screen men for prostate cancer.
One day, I suspected doctors in the future will look upon 2015 as the 'dark days' of cancer screening. But the day is coming when screening will pay off.