The American College of Radiology (ACR) has developed a standard way of describing mammogram findings. In this system, the results are sorted into categories numbered 0 through 6. This system is called the Breast Imaging Reporting and Data System (BI-RADS). Having a standard way of reporting mammogram results lets doctors use the same words and terms and ensures better follow up of suspicious findings. Here’s a brief review of what the categories mean:

X-ray assessment is incomplete

Category 0: Additional imaging evaluation and/or comparison to prior mammograms is needed.

This means a possible abnormality may not be clearly seen or defined and more tests are needed, such as the use of spot compression (applying compression to a smaller area when doing the mammogram), magnified views, special mammogram views, or ultrasound.

This also suggests that the mammogram should be compared with older ones to see if there have been changes in the area over time.

X-ray assessment is complete

Category 1: Negative

There’s no significant abnormality to report. The breasts look the same (they are symmetrical) with no masses (lumps), distorted structures, or suspicious calcifications. In this case, negative means nothing bad was found.

Category 2: Benign (non-cancerous) finding

This is also a negative mammogram result (there’s no sign of cancer), but the reporting doctor chooses to describe a finding known to be benign, such as benign calcifications, lymph nodes in the breast, or calcified fibroadenomas. This ensures that others who look at the mammogram will not misinterpret the benign finding as suspicious. This finding is recorded in the mammogram report to help when comparing to future mammograms.

Category 3: Probably benign finding – Follow-up in a short time frame is suggested

The findings in this category have a very good chance (greater than 98%) of being benign (not cancer). The findings are not expected to change over time. But since it’s not proven benign, it’s helpful to see if an area of concern does change over time.

Follow-up with repeat imaging is usually done in 6 months and regularly thereafter until the finding is known to be stable (usually at least 2 years). This approach helps avoid unnecessary biopsies, but if the area does change over time, it allows for early diagnosis.

Category 4: Suspicious abnormality – Biopsy should be considered

Findings do not definitely look like cancer but could be cancer. The radiologist is concerned enough to recommend a biopsy. The findings in this category can have a wide range of suspicion levels. For this reason, some doctors may divide this category further:

• finding with a low suspicion of being cancer
• finding with an intermediate suspicion of being cancer
• finding of moderate concern of being cancer, but not as high as Category 5

Not all doctors use these subcategories.

Category 5: Highly suggestive of malignancy – Appropriate action should be taken

The findings look like cancer and have a high chance (at least 95%) of being cancer. Biopsy is very strongly recommended.

Category 6: Known biopsy-proven malignancy – Appropriate action should be taken

This category is only used for findings on a mammogram that have already been shown to be cancer by a previous biopsy. Mammograms may be used in this way to see how well the cancer is responding to treatment.

BI-RADS reporting for breast density

Mammogram reports can also include an assessment of breast density. BI-RADS classifies breast density into 4 groups:

BI-RADS 1: The breast is almost entirely fat

This means that fibrous and glandular tissue makes up less than 25% of the breast

BI-RADS 2: There are scattered fibroglandular densities

Fibrous and glandular tissue makes up from 25 to 50% of the breast.

BI-RADS 3: The breast tissue is heterogeneously dense

The breast has more areas of fibrous and glandular tissue (from 51 to 75%) that are found throughout the breast. This can make it hard to see small masses (cysts or tumors).

BI-RADS 4: The breast tissue is extremely dense

The breast is made up of more than 75% fibrous and glandular tissue. This can lead to missing some cancers.

In some states, the summary of the mammogram report that is sent to patients (sometimes called the lay summary) must contain information about breast density. This information may be worded in lay language instead of the BIRADS categories. Women whose mammograms show BI-RADS 3 or 4 for breast density may be told that they have “dense breasts.”

Limitations of mammograms

As is the case with most medical tests, mammography has limitations.

Although breast cancer screening is the best way we have now to find cancer early, finding cancer early does not always reduce a woman’s chance of dying from breast cancer. Even though mammograms can detect breast cancers too small to be felt, treating a small tumor does not always mean it can be cured. A fast-growing or aggressive cancer may have already spread before it’s found.

The value of a screening mammogram also depends on a woman’s overall health status. Detecting breast cancer early may not help prolong the life of a woman who has other kinds of serious or life-threatening health problem such as congestive heart failure, end-stage renal disease, or chronic obstructive pulmonary (lung) disease. ACS screening guidelines emphasize that women with serious health problems or short life expectancies should discuss with their doctors whether to continue having mammograms. Our guidelines also stress that age alone should not be the reason to stop having regular mammograms.

False-negative results

A false-negative mammogram appears normal even though breast cancer is present. Overall, screening mammograms miss about 1 in 5 breast cancers.

The main cause of false-negative results is high breast density. False negatives occur more often among younger women than among older women because younger women are more likely to have dense breasts. Breasts usually become less dense as women age. False-negative results can delay treatment and promote a false sense of security for the woman.

False-positive results

A false-positive mammogram looks abnormal but no cancer is actually present. Abnormal mammograms require extra testing (diagnostic mammograms, ultrasound, and sometimes biopsy) to find out if cancer is present.

False-positive results are more common in women who are younger, have dense breasts, have had breast biopsies, have breast cancer in the family, or are taking estrogen. *With annual screening, over a 10-year period the odds that a woman will have a false-positive finding are greater than 50%. The odds of a false-positive finding are highest for the first mammogram, and are lower on subsequent mammograms. Women who have prior films available for comparison reduce the odds of a false-positive finding by 50%.

False-positive mammograms can cause temporary anxiety. The extra tests needed to be sure cancer isn’t there cost time and money and also cause physical discomfort. Still, most studies of attitudes towards false positives have shown that women accept false positive findings as part of the process of finding breast cancer early.

Overdiagnosis and overtreatment

While screening mammograms can find invasive breast cancer and ductal carcinoma in situ (DCIS, cancer cells in the lining of breast ducts) that need to be treated, it’s also possible that some invasive cancers and DCIS detected on mammography will not keep growing. This means that some tumors are not life-threatening, and never would have been detected if a woman had not gotten a mammogram. Since doctors can’t tell these cancers from those that will grow and spread, our only hint that overdiagnosis may exist is through statistical analysis that compares the number of cancers found by mammography over long periods of time with the numbers of cancers that would have been expected without screening. Overdiagnosis is a concern because an overdiagnosed cancer still needs to be treated. This means that some women are treated unnecessarily. These cases would be considered overtreatment, which exposed the women to the adverse effects of cancer therapy. Because doctors often cannot be sure which cancers and cases of DCIS will become life-threatening, they are all treated. Although there is a wide range of estimates of the percentage of breast cancers that might be overdiagnosed by mammography, the most credible estimates range from 0-10%.

Radiation exposure

Mammograms require very small doses of radiation. The risk of harm from this radiation is extremely low, but in theory, repeated x-rays might have the potential to cause cancer. Still, the benefits of mammography outweigh any possible harm from the radiation exposure.

Women should always let their health care providers and x-ray technologists know if there is any chance that they are pregnant, because radiation can harm a growing fetus.