From our cameras to our televisions, we are now living in an undeniably digital age. Since the trend has been extended into breast cancer screening, investigators conducted a study to test the accuracy of digital mammography versus standard film mammography. While finding no significant difference between the two techniques when applied to the general population and for most women over 50, the study did show that digital mammograms detected more tumors in three specific groups: women under 50, women with dense breasts, and women not yet in menopause.
The Fundamentals of Mammography
As the authors of the study note, annual mammogram screenings have been shown to reduce the rate of death from breast cancer for women 40 and older. Until recently, all mammograms used the conventional film-based technique, which utilizes x-rays to produce an image of the breast on film. Digital mammography, introduced in the US in 2001, also uses x-rays but captures the image on a computer, where it can be viewed and manipulated for optimal contrast. Unlike film-based mammograms, digital mammography also provides image transmission, and easier retrieval and storage options.
One of the challenges in breast cancer screening is detecting breast cancers in dense breast tissue -- particularly important for younger women who are more likely to have denser breasts. (Older, postmenopausal women tend to have fattier, less dense breasts.) When viewed on a conventional film mammogram, dense breast tissue and tumors both appear white. While conventional mammograms do not allow the image to be manipulated, the digital technique allows the degree of contrast in the image to be increased in the densest areas of the breast, possibly revealing subtle variations between breast tissue and tumor.
One drawback to digital mammography is its higher cost. The machine itself costs three to five times more than a standard film-based unit, creating a significant cost differential for the two procedures. A number of private health insurers do not pay a higher reimbursement for the more costly digital procedure. In addition, to date, only about eight percent of all mammography machines are digital, making it more difficult to arrange a timely screening.
Digital Mammographic Imaging Screening Trial
The American College of Radiology Imaging Network put the two technologies to the test in its Digital Mammographic Imaging Screening Trial (DMIST), studying the comparative screening accuracy of both techniques.
In the two-year, National Cancer Institute-funded DMIST trial, approximately 43,000 women from the US and Canada with an average age of 55 received both digital and film mammography screenings. The results, published in the October 2005 issue of the New England Journal of Medicine [PubMed Abstract], showed that digital mammography was significantly more accurate than film mammography among women under 50, women with dense breasts, and women not yet in menopause. For these women, the cancers detected by digital mammograms -- but missed by film mammograms -- included many invasive, highly malignant tumors.
However, for women 50 and older, postmenopausal women, and women with less dense breasts -- who together make up the majority of the breast cancer screening population -- the study's results suggest that film mammography actually found more cancers than digital mammography, though the difference was not statistically significant. As a result, the advantage of digital mammography only applies to a minority of the screening population, while, for most women, either film or digital mammography is acceptable.
"We know that having a mammogram of either type at the recommended interval may reduce the likelihood of dying from this disease," says Delia Keating, MD, Medical Director of the Memorial Sloan-Kettering Guttman Diagnostic Center.
This is reaffirmed by D. David Dershaw, MD, Director of Breast Imaging at Memorial Sloan-Kettering, who adds, "It is important to remember that screening mammography done digitally or with film is proven to be life saving. It is impossible to predict in any individual if one technique will be better. When both are available, ask if digital or film might be better for you. But most importantly, all women over 40 should get a mammogram -- whether it is digital or film -- every year."
