What is it?
A mammogram procedure is a low-dose x-ray of the breast.
Types of mammograms
A screening mammogram is performed at regular intervals to check for breast cancer in women who have no signs or symptoms of the disease. Screening mammograms have been used since the 1980s. A diagnostic mammogram is used to check for breast cancer when there is a sign or symptom of disease. A diagnostic mammogram is monitored by the radiologist at the time of the examination, but starts with the same images as a screening mammogram. Symptoms can include a lump, breast pain, nipple discharge or a change in the size or shape of the breast; however, these symptoms are often due to benign (noncancerous) conditions.
How mammography works
The breast is placed on the surface (detector) of the mammography system and briefly squeezed (compressed) while x-rays are taken. In a screening mammogram, x-rays of each breast are taken from two different positions to make sure the maximum amount of tissue is included. Sometimes additional images are needed to fully include all the breast tissue. The total examination takes about 10 minutes to complete although the compression lasts only seconds. Compression reduces the amount of radiation needed to penetrate the tissue and also spreads out the breast tissue to help produce clearer images. Compression also reduces motion which can blur the image and cause abnormalities to be missed.
What does cancer look like on a mammogram?
Cancers may be seen as masses (like a ball, but usually with an irregular shape), areas of asymmetry that resemble normal tissue, calcifications (white specks), and/or areas of architectural distortion (imagine the puckering caused by pulling a thread in a piece of fabric). Many noncancerous (benign) conditions also produce masses and calcifications and normal tissue can appear as areas of asymmetry. Additional views, such as magnification views for calcifications, or ultrasound for masses can help distinguish benign from cancerous entities, but a needle biopsy is ultimately required for diagnosis of suspicious findings.
Mammograms allow detection of 2 to 7 cancers for every thousand women screened. (see Summary of Cancer Detection Rates). Screening mammography is the only breast cancer screening technology that has been studied by multiple randomized controlled clinical trials evaluating deaths from breast cancer. Across those trials, mammography has been shown to reduce deaths due to breast cancer by 15-22% ; in actual practice, the observed reduction in deaths from breast cancer among women actually participating in mammography is 40-60% [2, 3].
Mammogram screening is the recommended first step in breast cancer screening for all women aged 40 years and older except those who are pregnant. In pregnant women, imaging is usually done only for diagnostic purposes, when symptoms are present, and ultrasound is usually the first imaging test performed. Some women at high risk may start screening with magnetic resonance imaging (MRI) by age 25 and mammographic screening by age 30.
Types of mammography are seen below (Figs. 1-2)
Mammograms can be:
1) Digital, 2-dimensional, known as a “Full Field Digital Mammogram” (FFDM), 2D mammogram or digital mammogram, uses a dedicated electronic detector system to computerize and display the x-ray information (Fig. 1).
2) Digital Breast Tomosynthesis, also referred to as “3-Dimensional mammography” (3D mammography) or tomosynthesis, uses a dedicated electronic detector system to obtain multiple projection images which are “synthesized” by the computer to create thin slices of the breast (Fig. 2). While the images are not truly 3-dimensional, individual slices can be displayed for review by the radiologist.
In both 2D digital mammography and tomosynthesis exams, the x-rays are transmitted to high-resolution computer monitors with electronic tools that allow the images to be magnified or manipulated for more detailed evaluation. Digital images are stored in a computer system called a PACS (picture archive communication system). This allows the radiologist to retrieve previous exams for comparison from year to year and to manipulate the images for complete viewing.
How does tomosynthesis work?
A digital mammogram provides a 2-dimensional picture of the breast, which is a 3-dimensional object. The breast is composed of different structures, such as milk ducts, fat, connective tissues and blood vessels. These structures appear at different levels within the breasts and can overlap one another when viewed as a 2D image. Overlapping tissue can hide small breast cancers and in some cases make normal tissue appear abnormal. Tomosynthesis can help to reduce the confusion created by overlapping tissues.
Tomosynthesis utilizes specially-equipped digital (x-ray) mammography machines and acquires images at multiple angles. Like standard mammography, tomosynthesis utilizes a paddle to compress the breast to minimize any possible motion as well as minimize the amount of radiation needed to penetrate the breast tissue. The images are reconstructed as multiple thin slices which can be individually “scrolled through” to reduce tissue overlap (Figs. 3a, 3b).
Benefits of tomosynthesis
When added to standard digital mammography, tomosynthesis depicts an additional 1 to 2 cancers per thousand women screened in the first round of screening and this benefit appears to continue every year .
Tomosynthesis is interpreted together with a 2D mammogram (Fig. 4). When a 2D mammogram and tomosynthesis are performed together (in “combination mode”), the study results in about twice the radiation dose as from a 2D mammogram alone – and the dose is greater in thicker breasts. Many centers have the computer software needed to create a “synthetic” 2D mammogram from the same images used to create the tomosynthesis slices. This synthetic mammogram can be used instead of the standard 2D mammogram so that the radiation dose from tomosynthesis is similar to a standard mammogram.
While slightly better performance is observed with tomosynthesis than with standard 2D mammography in women of all breast densities, some cancers will still remain hidden by dense tissue. An analysis of over 170,000 tomosynthesis exams compared to over 270,000 2D mammograms showed an increase in cancer detection of 1.6 per 1000 in women with heterogeneously dense breasts but no improvement in cancer detection in extremely dense breasts . Another large study of tomosynthesis showed an increase in cancer detection of more than 2.5 per 1000 women aged 40-49 with dense breasts . Several studies have shown [7, 8] there is a benefit to having tomosynthesis every year, with fewer recalls each year and improved cancer detection, though further validation of the approach is ongoing.
A very large NIH-funded multicenter trial (TMIST) is underway where women will have either 2D mammography alone (the control group), 2D plus tomosynthesis, or synthetic 2D plus tomosynthesis. The main endpoint of the study is to determine if using tomosynthesis reduces the rates of advanced cancers and interval cancers. These are surrogate measures to predict reduced rates of death from breast cancer, without having to follow large groups of women for decades. For more information, see Goals of TMIST on the cancer.gov website.
Importantly, compared to standard mammography, tomosynthesis reduces the number of false positive studies that require additional imaging or “callback” from screening to prove that no abnormality is present. This additional imaging beyond the screening study typically involves a “diagnostic mammogram” with additional mammographic views and/or ultrasound. About 95% of areas called back prove to be normal overlapping tissue or benign changes such as cysts. When tomosynthesis images show a mass, the spot compression or spot magnification views which are otherwise commonly performed can often be skipped, and the woman can usually just have ultrasound (Fig. 5).
All mammograms use x-ray technology and dense tissue absorbs more x-rays than fatty tissue. Some breast tumors are hidden (masked) by overlying or surrounding dense breast tissue (Fig. 6). A cancer masked on a 2D mammogram could still be masked on tomosynthesis unless the cancer is at least partially surrounded by fatty tissue. Standard 2D mammography has been shown to miss about 50% of cancers present in women with dense breasts [9-14]. In one large clinical study of population-based screening, the addition of tomosynthesis to 2D mammography produced significant gains in sensitivity (52.1% for 2D mammography vs 70.5% for 2D mammography plus tomosynthesis) across all breast densities .
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