Some breast cancers detected by screening mammography are cured and would have led to death had they not been detected early.

Other breast cancers detected by screening can be treated just as effectively if diagnosed later, may not have needed treatment at all, or may be so advanced that treatment does not prevent death from breast cancer. 

Balancing the benefits of screening mammography with the harms of unnecessary treatment is important when considering screening mammography 

“I am so thankful that my breast cancer was detected early by screening. Because the cancer was small, I only needed to have the cancer tissue removed and didn’t need to have my whole breast removed.  I am cancer-free fifteen years later. Mammography saved my life.”

Compelling testimonials from women who feel their lives were saved through screening programs are common. But, are these women right that their lives were saved by screening?

What kind of breast cancers does regular screening mammography need to detect in order to decrease deaths from breast cancer?

The Canadian Task Force on Preventive Health Care reported a 15% relative decrease in death from breast cancer associated with screening. In order to decrease the risk of death from breast cancer, screening programs must detect cancers with the following characteristics: a) the cancer is aggressive enough to cause death before the woman dies from another cause, b) the cancer will be cured by treatment, and c) the cancer would have caused death if it was detected later.

The 15% relative decrease in the risk of death from breast cancer means that some breast cancers detected by screening programs meet these criteria. However, others do not. Some cancers detected by screening would never have led to death because they are slowly growing or regress spontaneously, some cannot be cured even when detected “early”, and some would still be cured when they are diagnosed a few months to years later.

Early diagnosis means less aggressive treatment per patient, but more diagnostic investigation and treatment overall

Women who undergo screening mammography have more breast cancers diagnosed than women who do not get screened. These cancers are, on average, smaller and less likely to have spread beyond the breast than cancers that are detected without screening. This means that screening programs lead to more women undergoing treatment for breast cancer, but these women undergo treatment that is, on average, less aggressive than those whose breast cancers were not detected by screening mammography.

Proponents of screening point to the less intense treatment required by women whose breast cancers are detected through screening. A recent statement signed by 41 researchers argues that “Early detection, in combination with appropriate treatment, significantly lowers breast cancer mortality and improves the life quality of patients with the disease.”

Opponents of screening point out that more women who are screened are treated for breast cancer and therefore suffer the side-effects of treatment. A study of nearly 600,000 women participating in randomized trials of screening mammography for breast cancer concluded that “it is thus not clear whether screening does more harm than good”.

Steven Narod, director of the Familial Breast Cancer Research Unit at Women’s College Hospital says that “in the next ten years the debate on screening will concentrate on overdiagnosis” and suggests that the scientific community needs to focus on whether it is appropriate to treat some of these small, less invasive breast cancers. A recent study suggested that many screening-detected breast cancers spontaneously regress and disappear. Narod notes that this area lacks research and “it would be useful if we as a scientific community looked at questions of why some breast cancers disappear on their own, without intervention” and what the implications are for cancer screening and prevention.

Slowly growing cancers are more likely to be first detected by screening mammography than rapidly growing ones

Breast cancers do not all behave in the same way – some grow slowly and are unlikely to spread beyond the breast, while others spread almost immediately.

Screening programs for women at average risk of breast cancer are more likely to detect slowly growing cancers than more aggressive, fast growing cancers. This is because an aggressive cancer is more likely to have spread beyond the breast in the time between screening visits, while slowly growing cancers are too small to be detected by any means other than mammography and tend to not have spread beyond the breasts.

The Canadian Task Force on Preventive Health Care notes that between about 450 and 2100 women need to be screened for 11 years to prevent 1 death from breast cancer, depending on their age. Screening programs are more likely to detect slowly growing tumours than rapidly growing ones; which is one explanation for why so many women need to be screened to prevent one death from breast cancer.

How to interpret patients’ testimonials of lives saved by screening

Because of the limitations of screening, it is unlikely that a woman who is alive and breast cancer free 15 years after it was diagnosed with screening mammography had her life saved by routine screening – a recent study estimated that this likelihood is less than 20%. At the same time, its also impossible to know whether a woman who did not get screened and died of breast cancer could have been ‘saved’.

Almost all women with a breast cancer diagnosed by screening mammography will choose to have their cancer treated without knowing whether they will be cured by the treatment or if they are undergoing unnecessary treatment.

In the end, a woman’s choice of whether or not to opt for regular screening is based on many different reasons, including balancing the risks of dying from breast cancer against the harms of over-diagnosis. This is the topic healthydebate.ca will cover in our final story about breast cancer screening next week.