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A woman undergoes a mammogram.

The Ontario government has decided to replace nearly a quarter of the mammography machines in the province because a new study has revealed that they are 20 per cent less effective in detecting breast cancers than other devices.

The decision leaves women to learn from their clinic what kind of technology was used in their most recent mammogram. And it leaves other provinces to make their own judgments about the machines in question – a relatively new form of digital technology called computed radiography, or CR. Cancer Care Ontario officials have shared the findings of their study with their counterparts in other provinces.

Alberta, where fewer than 3 per cent of mammography machines use computed radiography, is studying the use of CR technology and may phase it out, officials there said.

A mix of machines is in use. The CR devices display the image on a computer screen. Two other types of machines employ traditional screen-film mammography, which produces black-and-white X-ray images, and another form of digital technology called direct radiography, or DR.

Scientists have long thought that all three machines were equally effective at helping radiologists find tumours. But the landmark study by researchers at Cancer Care Ontario, to be published Tuesday, is the first in the world to indicate that they are not equivalent – and that cancers are essentially going undetected with the CR machines.

"I thought it was very important to take action to correct this," Ontario Health Minister Deb Matthews said Monday in an interview. The study provides "very clear evidence on what we need to do," she added. "We are moving very quickly to replace the mammogram technology that isn't providing as high a quality as it could be."

Ontario would be the first province in Canada to essentially junk the CR machines. They will be replaced with DR machines at an estimated cost of $25-million.

The study, which compared the performance of the machines, is based on an assessment of 816,000 mammograms carried out on 688,000 women aged 50 to 74. The images were collected between 2008 and 2009 as part of Ontario's breast screening program.

The analysis revealed that screen-film mammography and DR identified cancers at roughly the same rate: For every 1,000 women screened, tumours would be found in about 4.8 or 4.9 of them. However, the detection rate using CR was significantly less – just 3.4 cancers per 1,000 mammograms. That indicates CR is 20 per cent less effective at finding tumours than screen-film mammography and DR. In real terms, that means at least one cancer would be missed out of every 1,000 women screened using CR digital technology.

"I was surprised. I didn't expect to find this," said the study's lead author, Anna Chiarelli, a senior scientist at CCO. "Previous studies did not find this difference, although most of them didn't actually separate out the two types of technology – they looked at digital mammography all together."

Dr. Chiarelli and her research colleagues completed their analysis a few months ago and have been waiting for their study to be accepted for publication in Radiology, a peer-reviewed journal. In the meantime, health officials have been working behind the scenes on a plan to deal with the shortcomings of the CR equipment.

"We made a recommendation to the Ministry of Health that the CR machines be replaced with DR machines," Linda Rabeneck, vice-president of prevention and cancer control at CCO, said in an interview Friday.

Out of the 316 mammography machines used for screening in Ontario, 76 – or about a quarter – use CR technology, 188 are DR devices and 52 are film-based.

If a woman's tumour were missed by a CR machine, it would likely be discovered on a subsequent mammogram or by a physical examination between screenings. But the tumour would have had more time to grow, and potentially spread, before the start of medical treatment. "Whether that would mean she would have a worse outcome is very difficult to know," said Dr. Rabeneck, who noted that tumours discovered at a later stage may require different forms of therapy from those caught earlier.

Women can find out the technology that was used for their most recent mammogram by contacting the clinic where they were last screened. A list of all Ontario clinics that offer breast mammography is available at Women can also contact Cancer Care Ontario at 1-800-668-9304.

"I would really hope that there is no loss of confidence in breast screening," Ms. Matthews said. "It is really important that women get screened."

In Saskatchewan, mammography – both at clinics and the mobile unit that visits remote communities – is done exclusively through DR, the provincial government said. Other technology may be used during diagnosis, but as a first step it relies on DR.

In British Columbia, the B.C. Cancer Agency's screening program has only one location – Victoria – that uses CR. In 18 sites it uses DR, and another 22 rely on film. Lesley Pritchard, a spokeswoman with the Provincial Health Services Authority, noted there has been no difference in the cancer detection rate at the Victoria clinic compared with B.C. centres that use film and DR.


Digital mammography machines at a glance

  • Direct Radiology, or DR, machines produces an immediate image. “DR is like a digital camera,” said Dr. Muradali, Radiologist-in-Chief of the Ontario Breast Screening Program of Cancer Care Ontario. “You take a picture and it pops up on a screen.” So images can be produced quickly and they don’t require a lot of storage space like big bulky X-rays on film. What’s more, DR machines use less radiation than film-based equipment, making them potentially safer for the public.
  • Computed Radiography, or CR, involves a two-step process. The X-ray of the breast is captured within a cassette on an image plate. The cassette is then put into another machine to generate the digital image. Like DR, CR machines require less radiation. But “CR images are more grainy and tend to be less sharp,” than DR and film, said Dr. Muradali. In fact, that characteristic may help explain why fewer cancers are detected using CR technology. A radiologist looking at a CR image may miss a small turmour that would be more apparent on DR image or on film. “If we want to get better images with CR we would have to increase the dose [of radiation] to the extent that it is equivalent to film screen.” The additional radiation would eliminate one of the key advantages of going digital, he added.

Paul Taylor