What is it?
A mammogram procedure is a low-dose x-ray of the breast.
Types of mammography imaging
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 performed since the 1970s. A diagnostic mammogram is used to check for breast cancer when there is a sign or symptom of disease, or to evaluate an abnormality seen on a screening mammogram. A diagnostic mammogram is monitored by the radiologist at the time of the examination. Diagnostic mammography starts with the same images as a screening mammogram and may also include additional images taken to evaluate an area of concern. Symptoms can include a lump, nipple discharge, skin or nipple retraction, or a change in the size or shape of the breast. Symptoms can be due to breast cancer, but are more often due to benign (noncancerous) conditions. Breast pain, in particular, is very rarely due to cancer unless there is also a lump (and tender lumps are much more often benign than cancerous).
How mammography works
The breast is placed on the surface (detector) of the mammography system and is briefly squeezed (compressed) between two paddles for a few seconds while an x-ray is 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. 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 can 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 can also produce masses and calcifications and even normal tissue can appear as areas of asymmetry. Radiologists use additional mammographic views, such as magnification or spot compression views, or ultrasound to help distinguish benign from cancerous changes. A needle biopsy is usually recommended when there is even a low (> 2%) level of suspicion for cancer.
For every 1000 women screened, 2 to 7 will be found to have cancer on mammography (see Summary of Cancer Detection Rates). Screening mammography is the only breast cancer screening technology that has been evaluated in randomized controlled trials (RCTs) of mortality. In multiple RCTs performed from the 1960s to the 1990s, mammography has been proven to reduce deaths from breast cancer by 15-22% . Those trials used technology that is now obsolete. In current practice, the observed reduction in deaths from breast cancer among women participating in mammography is 40-60% [2, 3].
Mammographic 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, adding mammographic screening by age 30.
Types of mammographic technology are shown below (Figs. 1-2).
1) Digital, 2-dimensional, also known as a “Full Field Digital Mammogram” (FFDM), 2D mammogram or digital mammography, uses a dedicated electronic detector system to computerize and display the x-ray image (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 “reconstructed” by the computer to create thin slices or slabs of multiple 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. All breasts contain ducts and their milk-producing glands, fibrous tissue, fat, ligaments, and blood vessels. These structures overlap one another when viewed as a 2D image. Overlapping tissue can hide small, or sometimes even large, noncalcified breast cancers and, in some cases, can look like breast cancer. Tomosynthesis can help reduce the overlap of normal tissues and better show abnormalities within the breast.
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 for several seconds to minimize any possible motion and to reduce 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, like flipping through the pages of a book (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. Several studies [4, 5] have shown 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.
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 to the breast 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 tomosynthesis improves breast cancer detection in women with fatty, scattered fibroglandular density, and heterogeneously dense breasts, it does not significantly improve cancer detection in women with extremely dense breasts as some cancer 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 .
A very large NIH-funded multicenter trial (TMIST) is underway where women will have either 2D mammography alone (the control group), 2D in combination with tomosynthesis, or tomosynthesis with synthetic 2D. 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 resulting in call 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) on a mammogram by overlying or surrounding dense breast tissue (Fig. 6). A cancer masked on a 2D mammogram can 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 40% of cancers present in women with extremely dense breasts and 25% of cancers present in women with heterogeneously dense breasts [7-12]. The greatest benefits for improved cancer detection and recall reduction from tomosynthesis are for the baseline (first) screening examination, with benefit on subsequent exams varying by density and age but sustained over multiple years [4, 5, 13, 14]. Tomosynthesis does not improve cancer detection appreciably in women with extremely dense breasts [6, 14].
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