By Paulette Turk, MD
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A mammogram can find breast cancer early. Compared to other imaging techniques, mammography is less likely to subject women to needless biopsies — and the only test proven in randomized, controlled studies to significantly reduce breast cancer mortality.
When breast cancer is found early, women have many more treatment options compared to when breast cancer is found at a later stage. Yet mammography has some limitations that can require further testing — especially for women with dense breast tissue.
Why are dense breasts a problem?
The breast is composed of fibroglandular tissue and fatty tissue. If a woman has more fibroglandular than fatty tissue in her breast, her breast tissue is considered dense. The reason this is important is because dense breast tissue can mask or hide breast cancer.
Half of all women are considered to have dense breast tissue, so don’t panic if you’re asked to come in for a second mammogram or other imaging test. It’s common for a radiologist to request more imaging information, especially on a baseline mammogram.
Knowing your breast density matters when you are being screened for breast cancer. To keep women informed, many states have passed a law ensuring that you get this information with your mammography results. Even if your state is not affected by this law, you can ask your doctor about your breast density. Every doctor has this information on file after a mammogram and can share it with you.
Remember that no single breast cancer screening tool detects 100 percent of all breast cancers, so radiologists may use other imaging techniques to evaluate patients. Here is a summary of the various tools radiologists use, including their strengths and limitations:
1. Mammograms: An effective screening tool
Mammography uses X-rays to detect different densities in the breast – including densities that may be tumors. For screening mammograms, the breast is compressed in two ways, from top to bottom and from side to side. Compression flattens the breast to capture an image of it. Essentially, mammograms turn a 3D object into a 2D object.
However, when the breast is compressed from top to bottom, tissue in the upper breast can overlap tissue in the lower breast. This overlapping tissue can cause the resulting image to look like a cancer. More importantly, the overlap can obscure small breast cancers. Whenever there is a question about the results, a radiologist will request further testing.
2. 3D mammograms: Helpful for dense tissue
3D mammography, also called digital breast tomosynthesis, is a relatively new technology. This type of screening is similar to mammography in that the breast is compressed, and X-rays are used to generate an image. However, during 3D mammography, X-rays are passed through the breast at many different angles, and technology uses this information to create a series of images.
The radiologist can page through the series of tomosynthesis images, as if paging through a book, to detect small breast cancers that may be obscured on traditional mammograms. This is why 3D mammograms reduce the degree of tissue overlap and allow radiologists to detect smaller breast cancers.
3. Breast MRI: Best when risk is high
Breast magnetic resonance imaging (MRI) uses intravenous contrast, or a liquid dye, which is given through an injection. The dye is used to detect differences in blood flow within the breast. Breast cancer often demonstrates different blood flow from the surrounding normal tissue, and takes up intravenous contrast differently than normal breast tissue.
However, the limitation of breast MRI is that non-cancerous breast lesions can look like breast cancer. So while MRI is extremely sensitive (meaning breast MRI detects a lot of lesions), it is not very specific (meaning its findings are not always breast cancer). To avoid unnecessary biopsies, screening MRI is used only in high-risk cases, in addition to screening mammography.
4. Breast ultrasound: Solid masses vs. cysts
Breast ultrasound uses sound or pressure waves to examine the breast tissue. It generates an image based on how well these pressure waves travel and are reflected through the tissue. Breast ultrasound allows radiologists to distinguish between solid and cystic breast masses. Rarely, breast ultrasound can also show breast cancers that are not seen on a mammogram.
However, while breast ultrasound is very sensitive, it is not very specific. So not all findings detected by breast ultrasound are breast cancer. Because of the number of unnecessary biopsies it can generate, not all hospitals offer whole breast ultrasound.
Thus, screening mammography and digital breast tomosynthesis (3D mammography) remain our best screening tools for breast cancer detection, given their combined sensitivity and specificity.