Annual Mammograms vs. Annual Ultrasounds

Though mammograms can miss tumors, they offer more information

If you've heard that mammograms can sometimes miss tumors, or that ultrasounds can play a role in diagnosis, you may be wondering: Why don't we have annual ultrasounds instead of mammograms? After all, mammograms can be uncomfortable and expose you to radiation, and breast ultrasounds are better at detecting some abnormalities.

The reason annual mammograms are preferred to annual ultrasounds lies in how each technology works, their benefits and capabilities, and their limitations. All of this, however, doesn't necessarily mean ultrasound may not be a part of your screening schedule.

Limitations Ultrasound in Breast Cancer Screening
Verywell / JR Bee

Diagnosis vs. Screening

The primary difference between mammograms and breast ultrasounds is the role they were designed to play.

Mammograms are an effective screening test, meaning that they offer the most information for women who do not have any symptoms of breast problems.

Breast ultrasounds, by contrast, would not be an effective screening tool for several reasons, not least of which is the fact that they are unable to image the entire breast at once.

A breast ultrasound is generally used for diagnostic reasons, such as when a mammogram reveals suspicious density in a breast. Among its capabilities, a breast ultrasound is good at distinguishing a benign fluid-filled cyst from a solid mass. If a cyst is found, fine needle aspiration can be done under ultrasound to remove the fluid.

An ultrasound can also help define a mass that you feel manually, even if it does not show up on a mammogram.

Limitations of Ultrasound

There are several limitations of breast ultrasound that make it inappropriate for a screening test.

These include:

  • Ultrasound cannot take an image of the whole breast at once. It uses a handheld transducer that is moved around to find an abnormality. As such, it is prone to user error, particular if the technician is inexperienced.
  • Ultrasound cannot image areas deep inside the breast. Ultrasound is good at evaluating superficial lumps, but a mammogram is better able to note abnormalities deeper in breast tissue.
  • Ultrasound doesn't show microcalcifications, the minute accumulation of calcium around a tumor and the most common feature seen on a mammogram. Many early breast cancers are first suspected based on microcalcifications. For a mass to be detected on ultrasound, the calcification would have to be significant.

In the end, neither mammogram nor ultrasound is perfect. But in the case of an ultrasound, the skill level of the operator can significantly affect the accuracy of a test. The same can happen with a mammogram, but generally less so.

When Ultrasound May Be Helpful

There are times when an ultrasound may be appropriate in a cancer screening. Such is the case when you can feel a palpable lump, but the mammogram is normal. This is especially true for lumps found near the surface of the breast that mammograms sometimes miss.

In cases like these, a breast ultrasound may detect breast cancer better than a mammogram. The point here, however, is that when a lump is present, imaging is done for diagnosis, rather than screening.

There are some, however, who believe that the combined use of a mammogram and a breast ultrasound may be appropriate in certain scenarios. One example is in women with dense breast tissue.

According to a 2015 review of studies published in the American Journal of Roentgenology, the sensitivity of a mammogram drops from around 85 percent in the average woman to anywhere from 48 percent to 64 percent in women with dense breasts.

The same review cited a 2002 study in which the combined use of mammography and ultrasonography in 13,547 women with dense breasts increased the accuracy of screening from 74.7 percent to 97.3 percent.

Ultrasound vs. Fast MRI for People with Dense Breasts

That said, recent studies suggest that for women who have dense breasts, the combination of mammography and fast breast MRI (abbreviated MRI) may be more sensitive and produce fewer false positives than the combination of mammography and ultrasound. Fast breast MRI appears to be relatively comparable to conventional MRI (the best test for finding breast cancer but very expensive so limited to high risk patients), but takes only around 10 minutes to perform with a cost similar to that of mammography. Since the testing is relatively new, however, it is not currently available at every center that performs breast cancer screening.

Radiation Concerns

People often express concerns about mammograms because they expose you to radiation, something that doesn't occur with an ultrasound. Given that you may be screened annually, you might even have fears that the cumulative radiation may one day cause cancer.

That is rarely the case. Ultimately, the level of radiation exposure in a mammogram is extremely low. It is about the same as used in a dental X-ray and less than what is needed for a standard chest X-ray.

According to a 2016 study in the Annals of Internal Medicine, roughly 125 of every 100,000 women who undergo an annual mammogram will develop radiation-induced breast cancer (0.125 percent), of whom 16 (0.016 percent of the total 100,000) will die as a result.

For its part, the National Cancer Institute (NCI) insists that the benefits of mammography outweigh the potential harm from radiation exposure.

Nevertheless, you should alert your healthcare provider and X-ray technician if there is any possibility you are pregnant. While mammography is not contraindicated in pregnancy, according to the American Cancer Society (ACS), you would need to weigh the benefits and risks with your healthcare provider to make a fully informed choice.

Other Imaging Technologies

Neither mammograms nor breast ultrasounds will find all breast cancers. In women at high risk of developing cancer, other options may be needed to better identify malignancies.

One such option is breast magnetic resonance imaging (MRI), a technology that uses powerful magnetic and radio waves to generate highly detailed images, especially of soft tissues. This may be the most appropriate choice for young women with dense breasts who have significant risk factors for breast cancer.

Other tests include elastography (which measures the stiffness of breast tissue), digital mammography (which uses less radiation than conventional mammograms), and optical mammography without compression (which uses infrared light, instead of X-ray).

Breast thermography, which is able to spot temperature variations suggestive of cancer, apparently works in some women, though a 2016 study concluded that "at the present time, thermography cannot substitute for mammography for the early diagnosis of breast cancer."

These techniques continue to evolve as researchers look for better ways to find breast cancer in the earliest stages of the disease.

A Word From Verywell

Mammograms are usually used as a screening test in women who do not have any breast symptoms. If a woman has symptoms, such as a lump or nipple discharge, or an abnormality on a mammogram, a breast ultrasound would be the next logical step.

While ultrasounds are not reliable for breast cancer screening, there are options other than mammography for those at high risk or who wish to avoid radiation. In these settings, a breast MRI might be a better, albeit more expensive, screening option.

Finally, it's important to remember that none of these imaging technologies diagnose cancer; they only detect abnormalities. The only way to diagnose breast cancer is with a biopsy.

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