Traditionally, mammography created images of the breast using X rays that recorded the images on screen film like a photograph. Breast cancer screening programs also now use digital mammography that captures and stores the images electronically.
In Tuesday's issue of the journal Radiology, researchers in Ontario compared the effectiveness of two types of digital mammography — digital direct radiography (DR) and computer radiography with the traditional screen-films.
Digital direct radiography is like a digital camera image that allows the radiologist to examine and evaluate the breast images immediately. In the computed version, the image is recorded on a cassette that needs to be removed and taken to another device to be read.
"Although direct digital radiography is equivalent to screen-film mammography for breast screening among women aged 50-74 years, the cancer detection rate was lower for computed radiography," Anna Chiarelli, a senior scientist in prevention and cancer control at Cancer Care Ontario and her co-authors concluded in the study.
Chiarelli and her team took advantage of the fact that the province's breast screening program uses all three types of technology. The researchers compared cancer detection rates for 688,418 women who were screened from 2008 to 2009.
Based on the findings, Dr. Linda Rabeneck, vice president of prevention and cancer control at the agency, said Ontario's health ministry has agreed to pay the $25 million estimated cost to replace the province's 76 CR systems with more expensive DRs.
"We are very excited about these research results, it provides an opportunity for us to strengthen the quality of breast screening in the province," Rabeneck said.
Dr. Mark Prieditis, president of the Ontario Association of Radiologists, called the funding announcement a band-aid solution. He said their group identified a need to switch the province's 150 film mammography and CR mammography units in 2010.
Individual risk low
Rabeneck noted the switch is not based on safety concerns.
For an individual woman, the chance of having a cancer missed using CR is extremely low, Rabeneck said.
Normally, if 1,000 women are screened with mammography, less than five will have a cancer detected, she said. The researchers estimated about 10 fewer cancers would be detected per 10,000 women screened with CR compared with screen-film mammography.
Women in Ontario have options, Rabeneck suggested:
- Go ahead with their next mammogram as scheduled.
- Call the clinic to find out what type of equipment was used. If it was CR, she might decide to wait until the clinic has a DR replacement and get screened with it.
All provinces and territories have organized breast cancer screening programs using various types of mammography.
Of the BC Cancer Agency's 44 sites, one site in Victoria uses CR, a spokesperson said.
"The study's findings are not consistent with those of earlier studies examining CR; however, the Ontario study is larger and compared concurrent cohorts. Previous studies and evidence have not shown that CR is less effective in detecting breast cancer when compared with screen film mammography."
"As BC Cancer Agency's CR location has screening rates in line with other centres across the province there is nothing to indicate a concern about the quality of screening at that location or that it is necessary to repeat a screening mammogram for women that have visited this location," an agency spokesperson said in an email.
A spokesperson for Alberta Health Services said three per cent of the province's machines are CR.
Nova Scotia's breast screening program has no CR units.
Of New Brunswick's 22 mammography machines, three are CR units. New Brunswick is reviewing all the data to decide on the future of the technology and says in the interim that women will be accomodated at other clinics if they wish, a health ministry spokesperson said.
Since direct digital mammography offers a higher contrast resolution than film, it can detect more cancers that might be hidden in dense breast tissue, the researchers said.
The study was funded by the Canadian Institutes of Health Research and the Canadian Breast Cancer Research Alliance.