Cancer Screening
Questions from the Clinic:
How effective is cancer screening? What is the best overall way to avoid cancer?
How Effective is Cancer Screening?
A recent study on the overall effectiveness of cancer screening grabbed headlines across the globe, shocking many and calling into question the value of cancer screening in general. The headline: "Only 14% of diagnosed cancers in the U.S. are detected by screening.”(1)
One reason for the low number is because only four types of cancer – breast, cervical, colorectal, and lung – have recommended screening tests, while controversial tests (PSA) or no tests exist for many cancer types.
On top of this, the report only referred to the percentage of cancers detected; it did not provide the metric that matters the most to people. Notably, of the 14% of people that had cancer detected by screening, how often did this detection result in a cure?
This is where it gets murky. The confusion is due to the way a “success” is recorded in the statistics. Specifically, a “success” is often reported as 5-year survival – not overall survival, or the metric that matters to most people. In other words, just because a screening diagnoses a cancer doesn't mean it always translates into a life saved.
One of the main issues is something called lead-time bias. Lead-time bias can distort the ultimate value of cancer screening: total lives saved. Below is an example of lead-time bias.
In this example, the lives of the people screened have not been extended at all. Here's the rub: in the people screened, because the diagnosis date is recorded earlier, the statistics record a 5-year survival of 100%. The data for screening then looks excellent, yet in the only metric that matters, overall survival, it is entirely an illusion.(2)
Clinical trials have shown that screening saves lives for common cancer types, such as cervical, colorectal, lung, and breast cancer; however, the benefit amount is misunderstood. For example, in mammography in women aged 50 to 59, more than 1,300 women must be screened to save one life. Additionally, such calculations do not consider the potential harms of screening, such as unnecessary and invasive follow-up screening tests or anxiety caused by false-positive results.(2)
If you need clarification, you’re not alone. A questionnaire designed to test doctors' knowledge of cancer statistics showed widespread misunderstanding among physicians; 76% of those surveyed incorrectly believed that an improvement in 5-year survival demonstrates that a test saves lives.(2)
The only way to eliminate all the bias in cancer screening statistics is through a randomized trial comparing people assigned to screening with those assigned to a control group. The control group follows routine care without screening, and the study counts the total deaths in each group at a lengthy follow-up.
One such trial compared the lung cancer screening value of C.T. scans to traditional chest x-rays in smokers. The trial found a 0.4% reduction in lung cancer mortality, going from 1.7% in the chest x-ray group to 1.3% in the C.T. scan group. This was a statistically significant outcome that helped determine best practices, but in absolute numbers it reduced death from lung cancer by less than one out 200 compared to chest x-ray screening.(3)
Last year, the first randomized trial was published to determine colonoscopy's overall benefit. The trial compared a group screened by colonoscopy to a group not receiving a colonoscopy – a landmark, gold-standard study. The report's results shocked the cancer community and stirred up controversy. The results: colonoscopy only cut colon cancer risk by roughly a fifth, far below past estimates of the screening’s efficacy, yet, stunningly, it didn’t provide any significant reduction in colon cancer mortality.(4)
When all the biases are accounted for, the reported value of cancer screening may be overstated. "Everyone has been led to believe that screening is better than it actually is," said Dr. Otis Brawley, an oncology professor at Johns Hopkins University.(5)
Amidst all this confusion, how should someone objectively evaluate cancer screening? For one, it helps to understand the way the data is reported. You may see pamphlets in your doctor’s office that show 5-year survival changes from 10% to 90% if you're screened for a specific type of cancer. These statistics don’t account for lead-time bias and ignore the only value that genuinely matters – overall survival.
When evaluating any cancer screening, ask your doctor the one question that cuts through all the noise:
“What's my chance of dying from this if I'm screened or if I'm not screened?”
I am not suggesting you abandon these screening programs but rather better understand their impact. Don't settle into complacency that cancer is not a worry, given your standard screening schedule. There are no guarantees to avoid or significantly delay any disorder, but “banking” only on our established guidelines on screening to safeguard you and your family is false hope.
What is the Best Way to Prevent Cancer?
The above survey on cancer screening begs the following question: If only 14% of cancer is detected through cancer screening -- and the value of early detection is not absolute -- what can I do to prevent cancer from occurring in the first place?
The idea of “chemoprevention” (the use of certain substances to help lower a person's risk of developing cancer or keep it from coming back) has captivated cancer researchers for decades. Initially, researchers took a reductionist approach, searching for the nutritional “magic bullets” that might reduce the risk of cancer. The NIH has spent enormous amounts of time and money looking at antioxidants like beta-carotene, selenium, and vitamin E, and vitamins like folate, vitamin D, and others. When the dust settled, the results were often counterintuitive. For example, researchers initiated a randomized, double-blind, placebo-controlled primary-prevention trial in the 1980s to test the ability of beta-carotene to prevent cancer in smokers. Surprisingly, the study found an 18% increased risk of lung cancer in over 29,000 male smokers taking 20 mg of beta-carotene daily.(6) A second research effort found the same for vitamin E, once suspected to reduce the risk of prostate cancer. A 2001 study of 35,000 men demonstrated that the men who took 400 international units (I.U.) of vitamin E daily had more prostate cancers than men who took a placebo.(7)
The Effect of Overall Lifestyle on Cancer Prevention
Even though single nutrients failed to deliver the hoped-for results in cancer prevention, epidemiologists studying the effect of overall lifestyle arrived at a far different conclusion.
A 2017 American Institute of Cancer Research report examined these studies and estimated that 50% of all cancers are preventable through better diet, physical activity, weight reduction, and other modifiable risk factors.(8)
Studies like these suggest that it is not any single nutrient or phytochemical that reduces cancer risk. Rather, it is a “biological action package” – or, in other words, the totality of nutritional and lifestyle influences that combine in a “network effect” to reduce cancer risk.(9)
Cancer Prevention, Diet, and Insulin Resistance
Let’s start with diet; many Americans have a condition called insulin resistance – the most recent study shows that it affects up to 88% of Americans to some degree.(10) Insulin resistance leads to weight gain and vice versa – a vicious cycle. Two leading causes of insulin resistance are overconsumption of processed carbohydrates and lack of physical activity.
The connection between insulin resistance and cancer begins with the hormone insulin. Insulin resistance can slowly develop over decades, causing the pancreas to release more and more insulin to compensate for the cell’s increasing “resistance” to insulin’s signal – a condition called hyperinsulinemia.
The connection to cancer then cente