The idea behind getting a yearly mammogram starting at age 40 (or 50, depending on whose guidelines you’re following) is pretty straightforward: If you can detect breast cancer earlier, you can improve your odds of survival. But a new study published in The British Medical Journal is questioning that logic: According to its findings, mammograms may not cut mortality risk. It adds even more intensity to a question that’s been hotly debated by the medical community in recent years: Are mammograms worthwhile?

Every medical test comes with both potential risks and benefits to consider--and some experts are becoming increasingly vocal about their belief that the guidelines for who should get mammograms and how often should be revisited.

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While this study has certainly added fuel to the fire, it shouldn’t cause you to steer clear of mammograms--here’s why

The New Findings

For the British Medical Journal study, Canadian researchers looked at data from the Canadian National Breast Screening Study, which recruited 89,835 women who were 40-59 at the beginning of the study, gave each of them a clinical breast exam, then assigned each participant to one of two groups: a group that would receive annual mammograms and clinical breast exams for the next four years, or a group that would receive only annual clinical breast exams for the next four years (or, in the case of the women 40-49, would just remain under the care of their regular doctors). Researchers then continued to follow up with the participants until 25 years after their original recruitment.

During the initial screening period, a total of 1,190 breast cancers were diagnosed (666 in the mammography group, and 524 in the control group). The tumors detected by mammograms did tend to be slightly smaller and were a little less likely to be node positive (meaning they had cancerous cells in them). But the mortality rate didn’t differ much between the two groups: During the 25-year follow-up period, 180 women in the mammogram group died, compared to 171 women in the control group.

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If you look at the entire study period, 3,250 women in the mammogram group and 3,133 in the control group were diagnosed with breast cancer. The number who died were, again, virtually identical: 500 women in the mammogram group, compared to 505 in the control group.

What’s more, the study authors are asserting that about one in five of the breast cancer diagnoses made as a result of mammograms were over-diagnoses. In other words, had these tumors not been detected, researchers say they never would have caused health issues or required treatment.

“Early detection could be of greater benefit in communities where most cancers that present clinically are larger and a higher proportion are node positive,” write the researchers (who could not be reached for comment for this article). “However, in technically advanced countries, our results support the views of some commentators that the rationale for screening by mammography should be urgently reassessed by policy makers.”

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But Was it Truly a Random Trial?

While the study authors say the women were randomly placed in either the mammogram group or the control group, some members of the medical community claim that, after examining the women at the onset of the study, nurses may have put women with larger cancers into the mammogram group so they would receive better care and improve their odds of survival, says Dr. Marisa Weiss, president and founder of Breastcancer.org.

“When you look at the methods section and it says 68 percent of the cancers in the mammography arm were palpable [meaning they were big enough to be detected without a mammogram], that doesn’t make sense,” says Weiss, who notes that the number should be much smaller if the women were truly placed randomly into either the experimental or control group. “They were more likely to go in the mammography arm because [the nurses] wanted them to have more comprehensive care, but it also made the mammography arm look worse because it had more cancers in it.”

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The researchers state in the study text that the nurses played no role in assigning participants to one group or the other--that the randomization was blinded:

“The examiners had no role in the randomisation that followed; this was performed by the study coordinators in each centre. Randomisation was individual and stratified by centre and five year age group. Irrespective of the findings on physical examination, women aged 40-49 were independently and blindly assigned randomly to receive mammography or no mammography.”
The American College of Radiology has openly disputed this claim and has gone as far as to state that the randomization couldn’t have been blinded:

“To be valid, randomized, controlled trials (RCT) must employ a system to ensure that the assignment of women to the screening group or the unscreened control group is random. Nothing can/should be known about participants until they have been assigned to one of these groups. The Canadian National Breast Screening Study violated these fundamental rules. Every woman first had a clinical breast examination by a trained nurse so that they knew which women had breast lumps, many of which were cancers, and which women had large lymph nodes in their armpits, many of which indicated advanced cancer. Before assigning the women to be in the group offered screening or the control women, investigators knew who had large incurable cancers.

This was a major violation of RCT protocol. It most likely resulted in the statistically significant excess of women with advanced breast cancers assigned to the screening arm compared to those assigned to the control arm. This guaranteed more deaths among the screened women than the control women.”

Other Issues

Even if this new research were based on a truly randomized trial, there are still some issues with it, says Weiss. For one thing, it looks only at survival--and not at other factors like quality of life. “The fact is that there are other things that are important to women besides, ‘Are you alive or dead?’” she says. “Most women would like to be diagnosed at an earlier stage when they can avoid chemotherapy.” While this study didn’t look at whether women were able to avoid chemotherapy or improve their quality of life in any other way by getting mammograms, it’s worth noting that women in the mammography group tended to have their cancers detected when they were smaller (even with the potential contamination, which would have inflated the average tumor size in the mammogram group).

Technology has also advanced quite a bit since the data was collected for this study. “When you’re talking about mammograms for women who are 40-49 who have dense breasts, we know that digital mammography is better than film screen, which is what they used in the test,” says Weiss. “If you’re thinking about predicting accidents on the street and you used data on car-safety standards from 25 years ago, would you ever do that? Would you ever choose to cast future guidelines based on old-fashioned technology?”

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As for the issue of over-diagnoses--and the fact that study authors say one in five of the tumors detected by mammography fall into this category--Weiss says there’s no one medical definition as to what that means exactly. “That’s an assumption that requires a judgment that’s not necessarily true,” she says. “Each study has to make its own claim about what researchers think is worth finding and what isn’t.” In other words, one in five of the masses detected my mammograms fit the definition these study authors came up with--but not all medical professionals agree on which tumors are “worth” detecting and which ones would be harmless if they were never identified.

The Future of Mammogram Guidelines

In an ideal world, only women in certain subgroups who are at a particularly high risk of getting breast cancer would receive mammograms, says Weiss. We’re moving toward that--but at this point, Weiss says that we simply don’t have enough information about who’s most at risk to advise the general female population against getting regular mammograms.

“Most people who get breast cancer don’t have a family history, don’t have a gene abnormality,” says Weiss. “There’s nothing about them that makes it obvious that they need a mammogram. … We want to get to a place where we’re recommending mammograms earlier on to the women who really need it and not recommending it to the women who don’t, but the default of ‘just talk to your doctor and decide if you need one’ I believe is irresponsible because we don’t know enough to identify who has a high risk at this point.”

And as the authors of this new research point out in their study, its findings contradict those of many other studies that have investigated the impact of getting regular mammograms. The fact is that there is conflicting evidence on the effectiveness of mammograms right now. We don’t know for sure that mammograms are beneficial for every woman.

That said, the risk associated with getting a mammogram--namely, a small amount of exposure to radiation (about the same amount you’d get by getting an X-ray at your dentist’s office)--is minimal. So at this point, Weiss encourages all women over the age of 40 to keep their yearly mammogram appointment. “It’s irresponsible to say mammography doesn’t lead to improved survival based on this study,” says Weiss. “What we’re talking about is the most common cancer to affect women and something that is treatable with early detection. … It makes sense to do what you can that’s reasonable to try to find it as early as possible so that you can live as long as possible and so that you can also avoid some of the more aggressive forms of treatment, like chemotherapy.”