Here is an interesting question to flummox your statistically savvy friends.
The PSA (prostate-specific antigen) test commonly used for prostate cancer screening has a sensitivity of 80% and a specificity of 60%, meaning that 80% of people with cancer will test positive and 60% of people without cancer will test negative. Based on his age and race, a man has a 1% probability of prostate cancer before the PSA test. What is his probability of cancer after the test if he tests positive?
Most people intuitively guess an approximate answer in the 60-80% range. However, they are completely surprised, even shocked, to learn that the correct answer (using the well-known Bayes’ Theorem) is only two percent.
This surprisingly low answer stems from the very low pretest probability of cancer (1%) and the fairly low specificity of the test, leading to a large number of “false positives,” i.e., positive test results for people without cancer. Cancer screening is very unlike a diagnostic test ordered by your doctor during a typical office visit. For example, when a doctor orders a throat culture to confirm a suspected case of strep throat, she has already established a high pretest probability of the disease based on symptoms like fever and sore throat.
The low post-test probability of cancer also explains why the U.S. Preventive Services Task Force (USPSTF) recently made an official recommendation against the use of the PSA test for prostate cancer screening. Earlier, the task force had recommended against breast self-examination, and reduced the scope of mammography screening by increasing the starting age from 40 to 50 years and decreasing the screening frequency from annual to biennial. Likewise, it had added a new upper age limit of 75 years for colon cancer screening.
The USPSTF recommendations are age-specific, but not race-specific or ethnicity-specific. As it turns out, the incidence rates of all these cancers among Asian-Americans and Indian-Americans are significantly below the average U.S. rates. Therefore, had the USPSTF segmented the population by ethnicity, it would have likely recommended against any cancer screening for Indian-Americans.
The USPSTF recommendations are not universally accepted by all the mainstream cancer associations in U.S., some of which may recommend more screening. However, some grassroots groups, such as the National Breast Cancer Coalition, recommend no mammography screening for any age group.
The Screening Controversy
Although early detection and treatment of cancer may benefit some people, it does not change the ultimate outcome in most cases. Moreover, the high number of false positives caused by frequent screening leads to anxiety, distress, invasive biopsies, and unnecessary and harmful cancer treatment for many healthy men and women.
For example, after a comprehensive review of the evidence, the USPSTF found that the only benefit of screening 1000 men with the PSA test is that at most one man avoids death from prostate cancer. However, the expected harms of such screening include urinary incontinence or erectile dysfunction for 30-40 men and serious side effects such as a heart attack or a serious blood clot for three men.
In the case of mammography screening, a rigorous study by the highly regarded Cochrane Group (2011) concluded that: “For every 2,000 women invited for screening throughout ten years, one will have her life prolonged. In addition, ten healthy women, who would not have been diagnosed if there had not been screening, will be diagnosed as breast cancer patients and will be treated unnecessarily. Furthermore, more than 200 women will experience important psychological distress for many months because of false positive findings.”
Similarly, colon cancer screening studies show that about 1,000 people need to be screened for ten years (using fecal occult blood test followed by colonoscopy) to avert one death. The potential harms of colonoscopy include perforation of the colon, major bleeding, severe abdominal pain, and cardiovascular events.
Finally, prostate cancer, and a type of breast cancer called DCIS, grow very slow and may forever remain localized. However, biopsy of such cocooned tumors may spread (or “seed”) cancer cells along the track of the biopsy needle. Moreover, in the case of mammography, there is a small but real risk that radiation exposure from repeated mammograms may itself cause breast cancer.
The incidence rates of these cancers among ethnic Indians vary widely (tenfold) with the lowest rates being found in rural India. An important implication of this huge variation is that these cancers are caused primarily by our lifestyle and environment, and therefore, are essentially preventable. Many of the cancer prevention strategies, such as a healthy organic diet, regular exercise, low exposure to environmental toxins, and a low-stress lifestyle, may also help slow down any incipient cancer.
In summary, cancer screening for asymptomatic people is likely to cause more harm than good. This is especially true for groups with low rates of cancer, such as Indians and Indian-Americans. Of course, people with any overt symptoms of cancer should see their doctor, as they would for a persistent sore throat. In the long run, an anxiety-free mind and a healthy lifestyle will yield far greater dividends than any screening ever could.
Vijay Gupta researches and writes about health issues from a consumer’s perspective.