The truth about breast cancer screening

Young woman is having mammography examination at the hospital or private clinic with a professional female doctor.

Author’s Note: This article should not be construed as medical advice.

Decisions regarding medical screening tests should always be made in conjunction with a licensed medical professional.

The key to health is disease prevention. Thanks to the remarkable progress in modern medicine, we have seen a significant increase in human lifespan and the successful cure of previously fatal diseases. However, an unintended consequence of this success was the emergence of cancer as a significant cause of death during the twentieth century. Cancer is now the second leading cause of death in the US, after heart disease.

In a perfect world, doctors would detect cancers in their early stages and wipe them out before they threaten the patient. Our intuition tells us it is a good idea to screen healthy people for early cancers to prevent possible death or disability. Although logically sound, this intuition is flawed because of facts about screening that most people need to consider. This article explores features of cancer screening that appear to defy logic (i.e., they are counterintuitive) but which are essential to understanding the practice of preventive medicine.

Some definitions

Some readers may need to familiarize themselves with the concept I will describe below, so I must define some terms.

Screening

According to the World Health Organization, screening identifies individuals in a healthy population at relatively high risk of developing a condition or disease. The example I use in this article is mammography to detect individuals at risk for breast cancer. Note that screening is not meant to diagnose disease; it is intended only to identify individuals among a healthy population who should pursue further testing to diagnose a condition or disease. For example, women with concerning findings on mammography may be referred for further testing that may include a breast biopsy.

False positive

An essential feature of screening tests is that they must capture all patients who can potentially have a disease. This feature means that the tests must be sensitive but not specific. A highly sensitive, poorly specific test will catch everybody with the disease but will flag many more individuals who do not. The latter group is called ‘false positives,’ i.e., they screened positive but did not have the disease. These individuals must nevertheless be referred to further testing, which in the case of breast cancer may include a biopsy. However, mammography has very few false negatives; in other words, women with breast cancer are doubtful to have negative mammograms.

The number needed to treat (NNT)

The Centre for Evidence-Based Medicine at Oxford University defines the NNT as the number of patients who need to be tested or treated to prevent a terrible outcome, like cancer or death. For example, 450 patients must undergo flexible sigmoidoscopy to prevent one patient from dying of colorectal cancer. The number may seem high, but the medical community considers this a valuable screening test.

The number needed to harm

In contrast, the number needed to harm is the number of patients tested before an adverse outcome follows the screening process. For screening mammography, two screening numbers are required before a false positive. In other words, half the women screened have a false positive test and must undergo further testing. The number of women screened harmed by an unnecessary surgical procedure over ten years is five. That is over ten years of annual screening, one in five women without breast cancer will undergo an invasive test like a biopsy. It is critical to note that, according to thennt.com, screening mammography may have prevented some deaths from breast cancer, but it does not prevent all-cause death. In other words, the process of mammography appears to cause harm in the form of early death that compensates for the deaths prevented by breast cancer. I appreciate that this is a problematic and non-intuitive result, but it is essential to understand. It does not mean that women should not undergo screening mammography.

In 2024, The US Preventive Services Task Force (USPSTF) recommended biennial screening for healthy women between 40 and 74. However, the USPSTF added some essential caveats to their recommendations. They stated that there is insufficient evidence to justify screening mammography for women over 74 or women with dense breasts (which comprises nearly half of all women!).

Now, the good news

The good news is that breast cancer deaths in the US have been declining since 1975. The natural response to this news is that screening mammography must be responsible. However, it is breast cancer treatment, not mammography, that is responsible for this positive trend. Mammography is accountable for a substantial amount of overdiagnosis of breast cancer. Overdiagnosis is a form of overtreatment that leads to women being treated who would never die of cancer because they would die of old age or some other disease first.

Discussing screening mammography with a physician is crucial, especially for women with dense breasts. Dense breast tissue can obscure mammogram results, making it more challenging to detect abnormalities early. Women with dense breasts are also at a higher risk for breast cancer, so it is essential to have a thorough understanding of their unique screening needs.

A physician can provide personalized advice on the most appropriate screening methods. While mammograms are the standard, additional imaging tests such as ultrasound or MRI might be recommended for more accurate detection of dense breast tissue. Understanding these tests' benefits, risks, and limitations is vital, and a healthcare professional can offer the guidance needed to make informed decisions.

Moreover, discussing mammography allows patients to address any concerns or questions about the procedure, its frequency, and its implications for their health. A licensed medical professional can explain the importance of regular screenings, interpret results accurately, and suggest follow-up steps if necessary. This conversation is essential for ensuring patients receive the most effective and timely care tailored to their health profiles. Always consult with a licensed medical professional to navigate the complexities of breast health and screening options confidently.

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