Tests for breast cancer

New imaging technology has changed almost every aspect of medical care, and mammography, the main form of breast cancer screening, is no exception. Ultrasound, magnetic resonance imaging (MRI), and digital mammography are now available, either to complement the standard mammogram or, in the case of digital mammogram, possibly to replace it.

Standard mammography

The standard mammogram is a low-dose x-ray of the breast developed on film. It can find 85% to 90% of breast cancers, including lumps far too small to be felt. In women who get screened regularly, 40% of breast cancers are discovered by a mammogram alone.

A mammogram reveals the structures within the breast as a black-and-white negative image, in shades ranging from white to black. In a normal breast, the white areas are the glandular and other sorts of tissues, found mainly in the milk ducts. They appear white because they block the weak x-rays. The gray and black areas are fatty tissue, which let the x-rays pass through pretty much unimpeded. Depending on their size, shape, and location, white spots on the film indicate various kinds of abnormal tissue — including tumors, but also benign growths posing no harm.

When the breast is "dense" — has more milk ducts relative to fat — the white images created by abnormal tissue don't stand out as much, so conventional mammograms tend to be less accurate. Young women tend to have more milk ducts. That's why conventional mammography is less useful in premenopausal women than it is in postmenopausal women. That's also the reason there's a lot of interest in developing better screening tests for younger women, particularly those known to be at high risk because of family history.

Over all, the false negative rate for mammography — meaning instances when a cancer is missed — is 5% to 10%, but it's higher in women with dense (less fatty) breast tissue. The rate of false positives — abnormalities that turn out not to be cancer — is also higher in women with dense breast tissue.

The discomfort of mammography is another problem. To get a clear picture and hold it steady, the machine pulls the breast away from the body and compresses it between two glass plates. The very prospect is enough to scare many women off, and several studies have found that for a significant proportion of women, a mammogram causes moderate to severe pain.

Technicians have started putting a foam pad on the bottom plate to make the test a little less uncomfortable. Breast density and sensitivity varies somewhat during the menstrual cycle, so premenopausal women are often told to schedule their mammograms during the first two weeks of their cycle.

Digital mammography

Like conventional mammograms, digital mammograms use x-rays to create an image of the breast. But where traditional mammography records the image on film, digital mammograms are saved electronically as bits of computer code. Aside from the presence of more gadgets and computer monitors, the patient probably won't be able to tell the difference: The positioning and compression of the breast are the same. Digital mammograms use less radiation than the film version, but for some women that advantage may be partially offset by the need to take more views of the breast.

Mammography is going digital for many of the same reasons that photography is. You get the image instantaneously. Digital images are easier to store and send than film-based ones. Radiologists are beginning to use software that helps them interpret the digital images. The ability to add contrast, magnify, and manipulate the image with a few keystrokes should, theoretically, give radiologists the ability to pick out subtle — and telling — differences that would have been invisible on film.

But proving the superiority of digital mammography hasn't been easy. Small studies rated digital-versus-film a toss-up, and with digital costing up to four times more, the overall advantage went to film.

The Digital Mammographic Imaging Screening Trial (DMIST) was designed to settle the issue. The $26 million investigation, sponsored by the National Cancer Institute (NCI), included almost 50,000 American and Canadian women. The results, published in 2005, were a split decision. Digital mammography was more sensitive (missed fewer cancers) than film for women in three categories: those under age 50 (regardless of breast density), those with very dense breasts (regardless of age), and those who were pre- or perimenopausal (regardless of age). Almost two-thirds of the DMIST volunteers fell into at least one of these categories.

On the other hand, the trial showed that standard film mammography was just as good at detecting breast cancer as digital mammography among women 50 and older who had less dense breasts and who were postmenopausal.

Ultrasound

Breast ultrasonography, or ultrasound, uses high-frequency sound waves instead of x-rays to create an image of the inner structures of the breast. Ultrasound is painless and involves no radiation. A technician uses a small hand-held device called a transducer to direct sound waves through the skin. Some lubricating jelly is applied to improve conduction.

Ultrasound breast imaging has been around a long time; the FDA approved it as a follow-up test to mammography in 1977. It's especially useful in determining whether a dense area in the breast is a solid mass (and possibly cancerous) or just a benign, fluid-filled cyst. The high-frequency sound waves travel through the fluid, so on the sonogram (the image created by an ultrasound) the cysts look like dark circles.

Even without mammograms, doctors will use ultrasound to investigate noticeable breast lumps, particularly if the woman is pregnant. The risk is probably slight, but neither doctors nor their patients want to possibly expose a fetus to radiation from a mammogram, so they use ultrasound instead. Ultrasound is not affected by breast density, so it's also useful in investigating breast abnormalities in women with denser breast tissue.

Ultrasound won't replace mammography as the way to screen for breast cancer, partly because it would take too long: Ultrasound can show only a small area of the breast at a time. More importantly, ultrasound misses some of the microcalcifications — specks of calcium left by dividing cells — that signal the presence of ductal carcinoma in situ (DCIS), a form of early breast cancer that's still confined to the milk ducts. DCIS is a dilemma because research suggests that some cases won't progress and cause any harm, so detection (rare before mammography) and treatment may be unnecessary or even harmful. But there's no way to sort out those cases from the ones that will progress, and experts largely agree that early diagnosis and treatment of DCIS is wise.

The real question with ultrasound is whether it should be done routinely in conjunction with mammography — at least in some women. Some studies suggest that "double screening" with mammography and ultrasound would mean far fewer missed cancers, particularly among women with dense breast tissue. But there's still a lot of debate, especially when you factor in the added time and expense.

To help settle the question, the NCI launched a large, multicenter trial to compare combined mammography and ultrasound screening with mammography alone. Researchers have enrolled 2,800 women, but the final results won't be out for several years.

MRIs

MRIs use a powerful magnet linked to a computer to take pictures of the body. The patient lies on her stomach during the test, and the breast fits into a depression in the table that's surrounded by coils that read the magnetic signals. There's no radiation involved, but after an initial image, patients are usually given a contrast agent intravenously (through a vein) to make the pictures clearer.

Although there's been some misleading advertising for breast MRIs, no one is proposing that they be used for routine screening. Where they do have a role, though, is in the screening of women at very high risk for breast cancer, particularly in women with the BRCA1 or BRCA2 genes. These women are more likely to get breast cancer at a young age, and when they do, the tumors tend to grow faster. A Dutch study published in 2006 in the journal Breast Cancer Research and Treatment found that 22 of the 45 breast cancers detected by MRI scans were missed by regular mammography.

The bottom line

Breast cancer screening has been one-type-suits-all. It may soon get more targeted and tailor-made.

Depending on clinical trial results, a combination of regular mammography and ultrasound may be advisable for women with dense breast tissue. Women at very high risk for getting breast cancer are already getting screened with MRIs.

Digital mammography is coming on strong — and many women stand to benefit. But it's currently available at just a small percentage of screening facilities. It's also more expensive than conventional mammography, although prices are likely to drop. Still, for many women, especially those who are older, film mammography is just as good. And any mammogram is far better than no mammogram at all.

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