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What is a mammogram?

A mammogram is an x-ray picture of the breast.

Mammograms can be used to check for breast cancer in women who have no signs or symptoms of the disease. This type of mammogram is called a screening mammogram. Screening mammograms usually involve two or more x-ray pictures, or images, of each breast. The x-ray images often make it possible to detect tumors that cannot be felt. Screening mammograms can also find microcalcifications (tiny deposits of calcium) that sometimes indicate the presence of breast cancer.

Mammograms can also be used to check for breast cancer after a lump or other sign or symptom of the disease has been found. This type of mammogram is called a diagnostic mammogram. Besides a lump, signs of breast cancer can include breast pain, thickening of the skin of the breast, nipple discharge, or a change in breast size or shape; however, these signs may also be signs of benign conditions. A diagnostic mammogram can also be used to evaluate changes found during a screening mammogram or to view breast tissue when it is difficult to obtain a screening mammogram because of special circumstances, such as the presence of breast implants.

How are screening and diagnostic mammograms different?

The same machines are used for both types of mammograms. However, diagnostic mammography takes longer to perform than screening mammography and the total dose of radiation is higher because more x-ray images are needed to obtain views of the breast from several angles. The technologist may magnify a suspicious area to produce a detailed picture that can help the doctor make an accurate diagnosis.

What are the benefits and potential harms of screening mammograms?

Early detection of breast cancer with screening mammography means that treatment can be started earlier in the course of the disease, possibly before it has spread. Randomized clinical trials and other studies show that screening mammography can help reduce deaths from breast cancer among women ages 40 to 74 years at average risk of breast cancer, with the evidence of benefit being strongest for women ages 50 to 69 years (1, 2). However, no studies to date have shown a benefit from regular screening mammography in women under age 40 or from baseline screening mammograms (mammograms used for comparison) taken before age 40.

The benefits of screening mammography need to be balanced against its harms, which include:

False-positive results. False-positive results occur when radiologists see an abnormality (that is, a potential “positive”) on a mammogram but no cancer is actually present. All abnormal mammograms should be followed up with additional testing (diagnostic mammograms, ultrasound, and/or biopsy) to determine whether cancer is present.

False-positive mammogram results can lead to anxiety and other forms of psychological distress in affected women. The additional testing required to rule out cancer can also be costly and time consuming and can cause physical discomfort.

False-positive results are more common for younger women, women with dense breasts, women who have had previous breast biopsies, women with a family history of breast cancer, and women who are taking estrogen (for example, menopausal hormone therapy). The chance of having a false-positive result increases with the number of mammograms a woman has. More than 50% of women screened annually for 10 years in the United States will experience a false-positive result, and many of these women will have a biopsy.

Overdiagnosis and overtreatment. Screening mammograms can find cancers and cases of ductal carcinoma in situ (DCIS, a noninvasive tumor in which abnormal cells that may become cancerous build up in the lining of breast ducts) that need to be treated. However, they can also find cases of DCIS and small cancers that would never cause symptoms or threaten a woman’s life. This phenomenon is called "overdiagnosis." Treatment of overdiagnosed cancers and overdiagnosed cases of DCIS is not needed and results in "overtreatment."

Because doctors cannot easily distinguish cancers and cases of DCIS that need to be treated from those that do not, they are all treated.

False-negative results. In cancer screening, a negative result means no abnormality is present. False-negative results occur when mammograms appear normal even though breast cancer is present. Overall, screening mammograms miss about 20% of breast cancers that are present at the time of screening. False-negative results can lead to delays in treatment and a false sense of security for affected women.

One cause of false-negative results is high breast density. Breasts contain both dense tissue (i.e., glandular tissue and connective tissue, together known as fibroglandular tissue) and fatty tissue. Fatty tissue appears dark on a mammogram, whereas fibroglandular tissue appears as white areas. Because fibroglandular tissue and tumors have similar density, tumors can be harder to detect in women with denser breasts.

False-negative results occur more often among younger women than among older women because younger women are more likely to have dense breasts. As a woman ages, her breasts usually become more fatty, and false-negative results become less likely.

Some breast cancers grow so quickly that they appear within months of a normal (negative) screening mammogram. This situation does not represent a false-negative result, because the negative result of the screening was correct. But it means that a negative result can give a false sense of security. Some of the cancers missed by screening mammograms can be detected by clinical breast exams (physical exams of the breast done by a health care provider).

Finding breast cancer early may not reduce a woman’s chance of dying from the disease. Even though mammograms can detect malignant tumors that cannot be felt, treating a small tumor does not always mean that the woman will not die from the cancer. A fast-growing or aggressive cancer may have already spread to other parts of the body before it is detected. Instead, women with such tumors live a longer period of time knowing that they likely have a potentially fatal disease.

In addition, finding breast cancer early may not help prolong the life of a woman who is suffering from other, more life-threatening health conditions.

Radiation exposure. Mammograms require very small doses of radiation. The risk of harm from this radiation exposure is low, but repeated x-rays have the potential to cause cancer. Although the potential benefits of mammography nearly always outweigh the potential harm from the radiation exposure, women should talk with their health care providers about the need for each x-ray. In addition, they should always let their health care provider and the x-ray technologist know if there is any possibility that they are pregnant, because radiation can harm a growing fetus.

Where can I find current recommendations for screening mammography?

Many organizations and professional societies, including the United States Preventive Services Task Force (which is convened by the Agency for Healthcare Research and Quality, a federal agency), have developed guidelines for mammography screening. All recommend that women talk with their doctor about the benefits and harms of mammography, when to start screening, and how often to be screened.

Although NCI does not issue guidelines for cancer screening, it conducts and facilitates basic, clinical, and translational research that informs standard clinical practice and medical decision making that other organizations may use to develop guidelines.

What is the best method of screening for breast cancer?

Regular high-quality screening mammograms and clinical breast exams are the most sensitive ways to screen for breast cancer.

Regular breast self-exam, or BSE—that is, checking one’s own breasts for lumps or other unusual changes—is not specifically recommended for breast cancer screening. In clinical trials, BSE alone was not found to help reduce the number of deaths from breast cancer.

However, many women choose to examine their own breasts. Women who do so should remember that breast changes can occur because of pregnancy, aging, or menopause; during menstrual cycles; or when taking birth control pills or other hormones. It is normal for breasts to feel a little lumpy and uneven. Also, it is common for breasts to be swollen and tender right before or during a menstrual period. Whenever a woman notices any unusual changes in her breasts, she should contact her health care provider.

What is the Breast Imaging Reporting and Database System (BI-RADS®)?

The American College of Radiology (ACR) has established a uniform way for radiologists to describe mammogram findings. The system, called BI-RADS, includes seven standardized categories, or levels. Each BI-RADS category has a follow-up plan associated with it to help radiologists and other physicians appropriately manage a patient’s care.

Breast Imaging Reporting and Database System (BI-RADS)

Category Assessment Follow-up
0 Need additional imaging evaluation Additional imaging needed before a category can be assigned
1 Negative Continue regular screening mammograms
2 Benign (noncancerous) finding Continue regular screening mammograms
3 Probably benign Receive a 6-month follow-up mammogram
4 Suspicious abnormality May require biopsy
5 Highly suggestive of malignancy (cancer) Requires biopsy
6 Known biopsy-proven malignancy (cancer) Biopsy confirms presence of cancer before treatment begins

BI-RADS also includes four categories of breast density that may be reported. The radiologist who reads the mammogram chooses the category that best describes the level of breast density seen on the mammogram film. The categories, from the least amount of breast density to the highest, are as follows:

  • The breasts are almost entirely fatty
  • There are scattered areas of dense glandular tissue and fibrous connective tissue (together known as fibroglandular density)
  • The breasts are heterogeneously dense, which means they have more of these areas of fibroglandular density. This may make it hard to see small masses in the breast tissue on a mammogram.
  • The breasts are extremely dense, which makes it hard to see tumors in the breast tissue on a mammogram.

Many states in the U.S. have enacted laws requiring mammography providers to tell women if they have dense breasts (i.e., breasts that are heterogeneously or extremely dense on a mammogram) and to inform them of risks associated with having dense breasts. In addition to making mammograms harder to read, dense breasts are a risk factor for breast cancer.

Where can women get high-quality mammograms?

Women can get high-quality mammograms in breast clinics, hospital radiology departments, mobile vans, private radiology offices, and doctors’ offices.

The Mammography Quality Standards Act (MQSA) is a federal law that requires mammography facilities across the nation to meet uniform quality standards. Under the law, all mammography facilities must: 1) be accredited by an FDA-approved accreditation body; 2) be certified by the FDA, or an agency of a state that has been approved by the FDA, as meeting the standards; 3) undergo an annual MQSA inspection; and 4) prominently display the certificate issued by the agency. More information about MQSA is available from the FDA.

Women can ask their doctors or staff at a local mammography facility about FDA certification before making an appointment. Women should look for the MQSA certificate at the mammography facility and check its expiration date. MQSA regulations also require that mammography facilities give patients an easy-to-read report of their mammogram results.

Information about local FDA-certified mammography facilities is available through NCI’s Cancer Information Service at 1–800–4–CANCER (1–800–422–6237). Also, a searchable list of these facilities can be found on the FDA website.

How much does a mammogram cost?

Insurance plans governed by the federal Affordable Care Act must cover screening mammography as a preventive benefit every 1–2 years for women age 40 and over without requiring copayments, coinsurance, or deductibles. In addition, many states require that Medicaid and public employee health plans cover screening mammography. Women should contact their mammography facility or health insurance company for confirmation of the cost and coverage.

Medicare pays for annual screening mammograms for all female Medicare beneficiaries who are age 40 or older. Medicare will also pay for one baseline mammogram for female beneficiaries between the ages of 35 and 39. There is no deductible requirement for this benefit. Information about coverage of mammograms is available on the Medicare website or through the Medicare Hotline at 1–800–MEDICARE (1–800–633–4227). For the hearing impaired, the telephone number is 1–877–486–2048.

Women who need a diagnostic mammogram should check with their health insurance provider about coverage.

How can uninsured or low-income women obtain a free or low-cost screening mammogram?

Some state and local health programs and employers provide mammograms free or at low cost. For example, the Centers for Disease Control and Prevention (CDC) coordinates the National Breast and Cervical Cancer Early Detection Program. This program provides screening services, including clinical breast exams and mammograms, to low-income, uninsured women throughout the United States and in several U.S. territories. Contact information for local programs is available on the CDC website or by calling 1–800–CDC–INFO (1–800–232–4636).

Information about free or low-cost mammography screening programs is also available from NCI’s Cancer Information Service at 1–800–4–CANCER (1–800–422–6237) and from local hospitals, health departments, women’s centers, or other community groups.

What should women with breast implants do about screening mammograms?

Women with breast implants should continue to have mammograms. (A woman who had an implant following a mastectomy should ask her doctor whether a mammogram of the reconstructed breast is necessary.)

Implants can hide some breast tissue, making it more difficult for the radiologist to detect an abnormality on the mammogram. It is important to let the mammography facility know about breast implants when scheduling a mammogram. The technologist and radiologist must be experienced in performing mammography on women who have breast implants. 

If the technologist performing the procedure is aware that a woman has breast implants, steps can be taken to make sure that as much breast tissue as possible can be seen on the mammogram. A special technique called implant displacement views may be used.

What is digital mammography? How is it different from conventional (film) mammography?

In the United States, digital mammography has replaced conventional mammography. Digital and conventional mammography both use x-rays to produce an image of the breast; however, in conventional mammography, the image is stored directly on film, whereas, in digital mammography, an electronic image of the breast is stored as a computer file. This digital information can be enhanced, magnified, or manipulated for further evaluation more easily than information stored on film. Digital images can also be shared electronically, making virtual (remote) consultations between radiologists and breast surgeons easier.

Digital mammography can be done only in facilities that are certified to practice conventional mammography and have received FDA approval to offer digital mammography. The procedure for having a mammogram with a digital system is the same as with conventional mammography.

What is 3-D mammography (also known as tomosynthesis mammography)?

Three-dimensional (3-D) mammography, also known as digital breast tomosynthesis (DBT), is a type of digital mammography in which x-ray machines are used to take pictures of thin "slices" of the breast from different angles and computer software is used to reconstruct an image. This process is similar to how a computed tomography (CT) scanner produces images of structures inside of the body. 3-D mammography uses very low dose x-rays, but, because it is generally performed at the same time as standard two-dimensional (2-D) digital mammography, the radiation dose is higher than that of standard mammography. Newer tomosynthesis strategies allow DBT to be done alone, potentially reducing the radiation dose to a level closer to that of standard mammography.

Although many women are offered DBT, it has not yet been determined conclusively whether it is superior to 2-D mammography at identifying early cancers and avoiding false-positive results.

A large-scale randomized breast screening trial is comparing 3-D mammography with 2-D mammography. The Tomosynthesis Mammography Imaging Screening Trial (TMIST) is comparing the number of advanced cancers detected in women screened for 4 years with DBT with that detected in women screened with standard digital mammography. 

What other technologies or strategies are being developed for breast cancer screening?

NCI is supporting the development of several new technologies to detect breast tumors. This research ranges from methods being developed in research labs to those that are being studied in clinical trials. Efforts to improve conventional mammography include digital mammography, magnetic resonance imaging (MRI), positron emission tomography (PET) scanning, and diffuse optical tomography, which uses light instead of x-rays to create pictures of the breast.

The Women Informed to Screen Depending on Measures of Risk (WISDOM) study is a randomized trial that is testing a personalized approach to breast cancer screening. This 5-year study, which will involve about 100,000 women in California and the Midwest, aims to determine if risk-based screening—that is, screening at intervals that are based on each woman’s risk as determined by her genetic makeup, family history, and other risk factors—is as safe, effective, and accepted as annual screening.