A comment of the chief medical officer of the American Cancer Society that the benefits of cancer screening, especially for breast and prostate cancer, have been overstated received widespread media coverage. The basic problem is not that the cancers can be detected, but that there has been very little improvement in overall survival rate between screened and unscreened populations.
The Quandary of Interpreting the Results of Cancer Screening
Cancer screening has a clear-cut objective: to detect a deadly cancer while it is still small and can be treated. When small tumors are found, however, it does not necessarily mean that they will grow and metastasize into an untreatable cancer. Many tumors found during screening turn out to be benign upon biopsy. A study reported in the Journal of the American Medical Association found that many small tumors found in breast and prostate cancer screening turn out to stop growing on their own and even disappear.
Cancer screening has many risks, including the stress of receiving a cancer diagnosis, biopsies and other tests, and unnecessary operations and treatments.
Why has Screening for Breast and Prostate Cancers Resulted in Such a Small Improvement in Survival Rate?
Many prostate cancers may have already spread by the time screening takes place, and some breast cancers are so aggressive that they may spread between screenings. All of these cases enter into the statistics for screening effectiveness.
Cancer Incidence and Age
Studies of cancer incidence versus age show that the rate of most cancer diagnoses begins to shoot upwards only in the seventh and eighth decades of life. These facts indicate that most cancers require years, if not decades, to develop, so cancer is largely a disease of old age. Life expectancy would only be extended a few years if all forms of cancer could miraculously be cured. Cancer screening, therefore, focuses on the individual, and probably will not improve the survival rate of elderly persons.
Prostate Cancer Screening
Prostate cancer can be very successfully treated while the cancer is still confined to the prostate, but once the cancer spreads beyond the prostate, it is deadly. The prostate specific antigen (PSA) test was designed to catch the cancer at this early stage. Prostate cancer tends to be very slow growing, so many men die with prostate cancer but not because of prostate cancer. Current consensus is that the screening should be limited to men at high-risk, and men with a life expectancy of less than 10 years should not take the test. Christine Redmond has an excellent review of the prostate cancer screening test.
Breast Cancer Screening
Mammography has been heavily promoted as a means of early detection of breast cancer, largely based on early studies that showed a 30% reduction in cancer deaths in those women who were screened by mammography. However, a more recent meta-analysis (a statistical analysis performed by combining the results of many studies) showed only a slight reduction in cancer mortality, but accompanied by overdiagnosis and overtreatment of breast cancer.
Cervical Cancer Screening
The Pap smear is one of the most successful tests for detecting cervical cancer, and should be performed regularly on women beginning at age 21.
Colorectal Cancer Screening
Colonoscopy is the gold standard for detecting colorectal cancer, but it not one of the easiest procedures. The patient must fast for a day and take a strong laxative in order to totally evacuate the colon. The patient is given a sedative, and the doctor inserts a flexible lighted tube into a colon. If precancerous polyps are found, they can be immediately removed.
Flexible sigmoidoscopy is an alternative to colonoscopy, but it only examines the lower third of the colon. The fecal occult blood test is the most simple test as it examines the presence of blood in the stool. However, it is very inaccurate in detecting the presence of colorectal cancer.
Lung Cancer Screening
Since lung cancer continues to be the largest cancer killer, there is considerable interest in developing a screening method for people of high risk, such as for smokers. The CT scanner has been evaluated for this purpose, but has not been proven to save lives. The costs are exorbitant for a routine screen, and many small tumors are found that are not really dangerous. Smokers are much more likely to die of heart disease than lung cancer.
Cancer Screening Recommendations Vary According to Type of Cancer
Cervical and colorectal cancer screens have been shown to save lives and should play a part in individualized health care programs. Breast and prostate cancer screens should be limited to people of high risk, while screens for lung or ovarian cancer have not been proven to save lives.
References:
"Cancer Screening Tests" Centers for Disease Control and Prevention.
Christensen, SA. "Cancer Screening in the Elderly" Suite 101.com
Knox, R. "Study Questions Early Screening for Lung Cancer" National Public Radio 2007 March 7.
Kolata, G. "Cancers Can Vanish Without Treatment, but How?" New York Times 2009 Oct 27
"Mammography Screening Controversy" from Our Bodies, Ourselves. Boston Womens' Health Book Cooperative
Redmond, C. "Prostate Cancer Screening - the Debate Continues" Suite 101.com
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