Cancers of the breast detected in screening (or “mammography”) are, on average, smaller than breast cancers that can be detected through physical examination. And experts know that women with small breast cancers have a better chance of long-term survival than women with large breast cancers.
But experts cannot be sure that this better chance of survival is because of screening, or if this increased survival is influenced by other factors.
These include factors called “lead time bias” and “over-diagnosis.” Lead time bias is when testing increases perceived survival time without affecting the course of the disease. Over-diagnosis is the clinical term for the detection of harmless cancers that will not result in symptoms during a patient’s lifetime or cause them to die.
As breast cancer screening occurs annually for some women as part of an effort to lower breast cancer mortality rates, the researchers behind this new study wanted to measure how effective annual screening is in preventing death from breast cancer.
“It is true that if you find cancer early it could be at a more treatable stage,” study author Prof. Anthony B. Miller told Medical News Today. “But there is no evidence that early detection affects the inherent biology of the cancer. Indeed it is possible that finding the cancer at an earlier stage will result in undertreatment. There is some evidence that is so.”
This was a large, long-term study across six Canadian provinces that followed 89,835 women between the ages of 40 and 59 over a period of 25 years. These women were randomly divided into two groups. The mammography group received one screening every year for 5 years, whereas women in the control group were screened only once.
Over the entire 25-year study period, a total of 3,250 women in the mammography group and 3,133 in the control group were diagnosed with cancer. In the mammography group, 500 women died, compared with 505 women in the control group. So the mortality rates in both groups were similar.
The authors say that an excess of 142 cancers were recorded in the mammography group during the 5 years of the screening period, with 106 excess cancers recorded after a period of 15 years. From this, the authors deduce that 22% of the cancers in the mammography group were over-diagnosed.
“These cancers – comprising half of those found by mammography alone – could not have affected the woman’s lifetime,” said Prof. Miller, “instead there were adverse consequences that she had to endure, living with the knowledge that she had had breast cancer, though in fact that detection did not benefit her at all.”
“So stopping mammography screening will not result in lives lost, but lives lived with greater quality of that life,” he added.
What did other studies find?
In 2013, Medical News Today reported on a UK-based study that also found breast cancer screening does not reduce deaths from the disease.
But the findings of this study do contradict findings from some other similar studies. A Swedish study attributed a 31% reduction in mortality to mammography. But the authors of the Canadian study think that the Swedish trial was flawed and the difference is down to “an initial imbalance of the compared groups, not a benefit of screening mammography.”
A large systematic review of data from 1976 to 2008 also had different findings to the Canadian study, estimating that 31% of all breast cancers were over-diagnosed. The Canadian researchers think this difference was due to that review taking a wider age range into account than their study, which looked at women between the ages of 40 and 59 only. They think that over-diagnosis is more common in people older than this, as there are more potential causes of death as individuals age.
The researchers also admit that the result of the Canadian study may not be generalizable to all countries.
Rationale for mammography should be ‘reassessed’
Although Prof. Miller and his team assert that education, early diagnosis and clinical care remain priorities in treating breast cancer, they say that mammography does not result in a reduction in breast cancer-specific mortality for women aged 40-59 beyond that of physical examination alone or usual care in the community.
The authors say that annual breast cancer screening should be urgently reassessed by policy makers, but they anticipate resistance from medical groups with “vested interests.”
Rather than annual breast screenings, the authors of the study would like to see funding diverted into better public education programs to convince women to seek skilled advice if they detect an abnormality in their breast, and professional education programs that ensure doctors are familiar with the latest advances in research.