Cancer screening is a critical tool to improve health. It helps to detect cancer early, before symptoms even appear, and can increase the chances of successful treatment. However, cancer screening does come with risks. One important and sometimes confusing risk is overdiagnosis.
When we think about cancer screening, we really have two goals: to detect potential cancers early before any symptoms are seen and to treat these cancers early to improve survivability. Cancer screening is very successful in achieving those goals. A recent study estimates that from 1996-2020, cancer screening saved about 12-16 million life years and that this number could have been even higher (about 15-21 million!!) if everyone followed the current cancer screening recommendations guidelines.
However, screening comes with both pros and cons. A lot of people assume that all screening is in someone’s best interest, but that is not necessarily true! Cancer screening does come with risks of harm and these need to be balanced against the potential benefits when making decisions whether or not to participate in screening.
One harm that we need to consider is overdiagnosis. Overdiagnosis is when we find a cancer that was unlikely to ever become clinically evident during someone’s lifetime if we didn’t we didn’t go looking for it. Overdiagnosis can be hard to wrap your brain around and feels counterintuitive. Essentially, some people develop cancers that won’t ever go on to be a problem. We don’t know which cancer is which, but we know this exists because we have studies that compare screened and unscreened populations and show an increase in cancer diagnoses without a corresponding decrease in advanced cases or deaths. This suggests that some detected cancers would not have progressed to cause harm during a person’s lifetime.
Let’s take a real world example. In a population of 1000 women screened for breast cancer using a mammogram every 2 years starting at age 40, we prevent 8.2 deaths from breast cancer. There will also be 14 “overdiagnosed” breast cancers. This means that 14 cancers will be found and treated with things like surgery, radiation or chemotherapy but that cancer would never have caused any problems for that person if they hadn’t been screened. If we start screening that same population at age 50, we would prevent 6.7 deaths and only have 12 “overdiagnosed” breast cancers, 2 fewer than if we start at age 40 (US Preventive Services Task Force: Breast Cancer Screening).
How about prostate cancer? Screening men ages 55 to 69 years old using a blood test called prostate-specific antigen (PSA) does offer a small potential benefit, reducing cases of metastatic prostate cancer by about 3 per 1000 men screened. However, many more men will experience harms like false-positives, overdiagnosis and overtreatment. About 20-50% of prostate cancers are overdiagnosed! Yikes! This means many men will unnecessarily experience harms of biopsies and treatments such as pain, incontinence, erectile dysfunction, and infection (US Preventive Services Task Force: Prostate Cancer Screening).
This doesn’t mean we want to forget about cancer screening all together. Far from it! It does mean that we need to balance the risks and benefits of cancer screening and use data to help us interpret test results. When considering your cancer screening, it is important to talk to your primary care clinician about your risk and cancer screening options and discuss your values and preferences to help you make the right decision for you.
