A study by Cancer Care Ontario researchers found that digital computed radiography (CR) mammography is 21 per cent less successful at uncovering breast tumours than either digital direct radiography (DR) or screen-film mammography.
"This is the first Canadian study to compare the performance of digital mammography to screen-film mammography," said Dr. Anna Chiarelli, a senior scientist at Cancer Care Ontario (CCO) who led the study.
To conduct the study, published Tuesday in the journal Radiology, researchers analyzed screens for almost 690,000 women who had mammograms as part of the Ontario Breast Screening Program from 2008 to 2009.
Based on the results and on CCO's recommendation, Ontario's Ministry of Health is spending about $25 million to phase out all CR devices and replace them with DR technology.
"The evidence was very clear that one particular technology wasn't as good as others," Health Minister Deb Matthews said Tuesday. "So that's why we are removing them and replacing them with more effective technology."
Matthews said she wants to replace the equipment as quickly as possible, and the province has already put out a request for proposals.
For the individual woman, Chiarelli said the chance of missing a cancer with CR mammography is still low.
In women 50 to 74 — the age group involved in the study — about five out of every 1,000 women screened will have a breast tumour detected, she said. With CR, the figure would be slightly lower.
"So this would be about a 20 per cent difference to about four per 1,000 — or one less cancer for every woman screened," Chiarelli said.
A woman who is concerned about what technology was used for her last mammogram and whether a tumour was missed may decide to wait for her next scheduled appointment for breast screening, said Dr. Linda Rabeneck, CCO's vice-president of prevention and cancer control.
"But if she wants to know if her last mammography was with CR or what type, she can call the clinic where she had her mammogram," Rabeneck said.
"And then if she learns that it was CR, she has a choice: she can ask to be rescreened when they replace their equipment and have new DR equipment (or) she can go to another site where they have DR technology."
To a woman having screening, mammography machines look the same and work in the same way — X-rays take images of the breast. It is the way these images are recorded that differs.
Screen-film mammography captures an image on film, which is then printed for examination. This was long considered the standard technique, but with advances in technology, digital mammography allowed breast images to be viewed and assessed on a computer screen instead.
With direct radiography, or DR, the image taken is immediately processed electronically as a digital image; with computed radiography, or CR, the image is captured within a cassette on an image plate, then processed as a digital image. An external reading device is needed to generate that digital image.
Chiarelli said CR may not provide as much accuracy in detecting breast abnormalities because the images are less sharp and are "a little bit more granular."
Ontario has a mixture of the three recording technologies: about 60 per cent are DR, 24 per cent are CR and the remainder are screen-film, said Rabeneck.
Across Canada, those figures differ depending on the province. Nova Scotia, for instance, has no computed radiography machines, while CR makes up a small proportion of mammography technologies in B.C. and Alberta.
"We know the other provinces will be looking at what we're doing in Ontario very closely, and frankly jurisdictions beyond Canada," she said.
Meanwhile Tuesday, the Ontario Association of Radiologists (OAR) said it had identified mammography equipment problems in a 2010 report, but the Ontario health ministry had ignored its concerns.
That report said the province lagged behind the rest of Canada in its adoption of digital mammography, and provided a plan to transition away from screen-film and CR mammography units.
"Had the Ministry of Health listened and acted in October 2010 on the OAR's recommendations concerning the critical need for digital mammography to provide Ontario women with the best diagnostic equipment to detect breast cancer, we would not be having this crisis today," president Dr. Mark Prieditis said in a statement.
Dr. David Jacobs, chair of the diagnostic imaging section for the Ontario Medical Association, echoed those concerns, saying radiologists have complained to the ministry for years about chronic underfunding for new diagnostic equipment, particularly for mammography.
"The significance of this failure to listen to radiology experts is that an unknown number of the 1.1 million women who annually receive a breast screen or diagnostic mammogram have had their health care compromised," Jacobs said.
Rabeneck countered that Chiarelli's study is the first in Canada to examine the outcome in patients related to the three types of mammography.
"We never had this evidence before. It's new science and it's new evidence," she said, adding that CCO bases all of its programs, including mammography screening, on "rigorous scientific evidence," and the data from the study was only recently available.
A list of all Ontario clinics that offer breast mammography is available at www.cancercare.on.ca/mammogram. Women can also contact Cancer Care Ontario at 1-800-668-9304 for more information.
—With files from Maria Babbage