Making sense of the evidence on breast cancer screening

The McMaster Health Forum, with support from the Labarge Optimal Aging Initiative, recently hosted a public talk to examine the latest evidence on breast cancer screening and the impact this evidence has on physicians and their interactions with patients.

Dr. Jonathan Sussman (Associate Professor, Department of Oncology McMaster University, and Radiation Oncologist, Juravinski Cancer Centre) and Dr. Cathy Risdon (Professor and Associate Chair, Academic, Department of Family Medicine, McMaster University, and Co-Director, McMaster Family Practice) delivered a compelling talk.

Watch the full video or read the summary below:

Click here for information on upcoming talks (and live-webcasts) on optimal aging issues.

Breast cancer statistics
Breast cancer is the most common cancer among women in Canada and the second leading cause of cancer deaths among women. An estimated 24,400 women were diagnosed with breast cancer in 2014 (26% of all new cancer cases in women).

Most women who are diagnosed with breast cancer don’t have a family history of it. When people are given a diagnosis, Dr. Sussman said that “…the first question is always why – why did this happen. For the most part we can’t say why, which can be obviously quite frustrating.”

What is screening?
Screening is trying to find something before you know it’s there.

The theory behind cancer screening is that if we catch cancers early, they’re going to be more curable. The hope with screening is that more cancers are going to found at a lower stage, resulting in less treatment being needed and fewer people that have their lives shortened.

It is important to note that screening does not prevent breast cancer. Screening tests also do not diagnose cancer, but simply identify when something may be a problem. Currently, the only way to confirm if something identified through screening test is cancerous is by taking a sample.

When is it reasonable to screen?
Screening is reasonable when there’s an important health problem for the general population and there’s an understanding of who might be at risk. Furthermore, there must be an accurate, reliable diagnostic test and an effective treatment option.

What are the downsides of screening?
One of the major downsides with current breast cancer screening is the high number of false positives (something that appears on a mammogram, but is not actually a cancer). These false positives subject people to unnecessary, invasive diagnostic testing. 

What is the evidence on breast cancer screening?
The Canadian National Breast Cancer Screening trial was one of the first, and biggest randomized control trials to examine the impact of mammography. The results from this trial, which followed 90,000 people for 25 years, found that there was no significant difference in breast cancer survival between the group who received a mammogram and those who didn’t.

Breast cancer mortality graph

One of the observations from this study was that individuals diagnosed with breast cancer on the screening arm seemed to live longer.

“What is felt to have happened in this study is that there were individuals on the mammography arm who were diagnosed with breast cancer who basically died when they were destined to die,” explained Dr. Sussman. “But what ended up happening is that they were found a little bit earlier to have the breast cancer.”

“If somebody has something detected with screening early on, they’re going to live for a number of years knowing they have cancer.  Whereas an individual who is not screened, who eventually develops symptoms, will be diagnosed with the cancer later, so this is a bit of an optical illusion. It suggests that people seemed to live longer after they were diagnosed, but in fact all that was happening was that you were finding the cancer earlier but that the person wasn’t going to live any longer.”

Lead time bias

What else was going on?
Over time, more people with breast cancer actually received treatment. Having access to quality treatments, evened out the outcomes more so than screening. The increased awareness of breast cancer, also meant that people were coming to their doctors with concerns sooner.

Family medicine perspectives on mammography (Ontario)
If you’re a woman over 50 in Ontario, you will start to automatically receive letters from Cancer Care Ontario. Dr. Risdon highlighted the incredible service offered by Cancer Care Ontario in raising awareness of breast cancer and related resources, but noted her concern with the very strong message equating “taking care of your health” to getting mammogram screening.

“What I’m starting to feel uncomfortable with in my practice, and in conversations with women, is that there is not another side to the discussion being conveyed in these letters,” said Dr. Risdon. “There’s definitely no acknowledgement that perhaps screening also has a downside or that there is other issues to consider for screening. There’s not really a message that screening is potentially optional or that you can make a sane choice to screen or to not screen.

Answering patient questions - is it a good idea to get a mammogram?
Dr. Risdon discussed how her office provides all clinicians with a resource binder with some decision aids to help them through some of these difficult tools. One of the resources is from the Harding Centre for Risk Literacy that visually shows the risks of breast cancer screening.

Breast Cancer Early Detection 2 - Harding Centre

“I don’t want to give the message that screening is a foolish thing to do,” said Dr. Risdon. “There are many, many reasons that women still choose to receive screening. The anecdotes are powerful. Family histories are powerful. A belief in taking action can motivate women.”

“I don’t feel that I have the ground to stand on to say ‘do not screen’ – I think that’s the wrong message. But what I do feel that I can say now is that choosing not to screen is very sane and can be very consistent with caring for your health and I would strongly support either choice depending on what a woman felt was going to be best for her.”

Some tough questions
As we consider policies around the use of mammography, some tough questions to consider include:

  • If screening does not alter all cause mortality, how much anxiety and worry do we inflict to exchange one cause of death for another?
  • All screening will cause, in some way, false positives and over diagnoses. Is it ethical to continue to financially incentivize providers for “keeping our numbers up”?  (Should we not incentivize shared decision making?)

“Not all cancers are the same,” said Dr. Sussman. “I think we have to do a better job of characterizing what is truly something that is a threat to somebody’s health versus something that develops that we’re just finding.” 

Breast cancer is complicated, but mammography is currently the best screening tool we have. New ways of screening need to be investigated.

For more information about healthy aging that you can trust, visit the McMaster Optimal Aging Portal.

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