Mammography/3D Mammography (Tomosynthesis):
What is it? Mammograms are low-dose x-rays of the breast that have been used for screening since the 1980s.
How mammography works: The breast is briefly squeezed (compressed) in two different positions and x-rays of the breast are taken. The total examination takes about 10 minutes to complete. Sometimes additional images are needed to fully include all the breast tissue. The compression reduces the amount of radiation needed to penetrate the tissue and also spreads out the breast tissue to help produce excellent images. Compression also reduces motion which can blur the image and cause important findings to be missed. Cancers are seen as masses, areas of tissue asymmetry, calcifications, and/or areas of distortion. Many noncancerous conditions also produce masses and calcifications and normal tissue can appear as areas of asymmetry.
Benefits: 2-D mammograms allow detection of 2 to 7 cancers for every thousand women screened. A woman’s breast density is determined through her mammogram (rated on a four-level scale). Mammography is the recommended first step in breast cancer screening for all women aged 40 years and older except those who are pregnant. Some women at high risk may start mammographic screening by age 30. Screening mammography is the only technology that has been studied by multiple randomized controlled trials. Across those trials, mammography has been shown to reduce deaths due to breast cancer.
Types of mammography are seen below (Figs. 1-3)
Mammograms can be:
1) Film, 2-Dimensional, known as “analog” (Fig. 1)
Figure 1. Analog (Film) Mammograms from a 62-year-old female with lump felt under her right arm. The breasts are not dense, with only scattered fibroglandular density. Left-hand images show views from above (known as craniocaudal or “CC” views) and right-hand images are taken from a side angle (known as mediolateral oblique or “MLO” views) and show a dense mass in right underarm (axilla, white arrow, triangle marker). Ultrasound-guided biopsy showed this mass to be a lymph node involved with cancer spread from the breast (i.e. a metastatic lymph node). The primary cancer in the right breast itself was not initially seen on these images but can be seen in retrospect on the CC view only (short red arrow).
2) Digital, 2-Dimensional, known as “Full Field Digital Mammogram” (FFDM), which uses a dedicated electronic detector system to computerize and display the x-ray information (Fig. 2).
Figure 2. Digital CC and MLO Mammograms from the same patient as in Figure 1 again show the metastatic cancerous lymph node (arrows). Better seen is a subtle mass with associated distortion (red ovals) in the upper inner right breast. The skin and tissues near the skin are also better seen on digital mammography than on film.
3) Tomosynthesis, also referred to as “3-Dimensional mammography” (3D mammography) or “tomo”, uses a dedicated electronic detector system to obtain multiple projection images which are “synthesized” by the computer to create thin slices of the breast.
Figure 3. 3D Tomosynthesis Images. 1-mm slices from CC and MLO tomosynthesis from the same patient as in Fig. 2 (done in combination with the standard digital mammogram, i.e. during the same breast compression). The dense metastatic node (arrows) is again noted. Even better seen on tomosynthesis is the architectural distortion from the primary right breast cancer (red ovals), an invasive ductal cancer with associated ductal carcinoma in situ (DCIS).
2-D digital mammography is slightly more sensitive than film mammography in dense tissue. The vast majority of USA facilities now use digital mammography. Digital images can be stored in a computer system called a PACS (picture archive communication system). This allows the radiologist to quickly retrieve previous exams for comparison from year to year and to manipulate the images for complete viewing.
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 the amount of ionizing radiation needed to penetrate the breast tissue and also to reduce motion. The images are reconstructed as multiple thin slices which can be individually “scrolled through” to reduce tissue overlap (Figure 4). When added to standard digital mammography, tomosynthesis depicts an additional 1 to 2 cancers per thousand women screened in the first round of screening.
Courtesy of Drs. Wendie and Jeremy Berg
Figure 4. Tomosynthesis (“3D mammography”). A) For tomosynthesis, the breast is compressed as for a regular 2D mammogram and the x-ray tube moves in an arc over the breast. Multiple short-exposure “projection” images are obtained and used to create thin “slice” (often 1-mm thick) images of the breast (A, B and C bottom row), which reduces the overlap of tissues and can help show architectural distortion. B) Image of Tomosynthesis System
aRafferty EA, Durand MA, Conant EF, et al. Breast Cancer Screening Using Tomosynthesis and Digital Mammography in Dense and Nondense Breasts. JAMA. 2016;315(16):1784-6.
- Currently, tomosynthesis is generally performed in addition to a 2-D mammogram. When a 2-D mammogram and 3-D mammogram are performed together, the study results in twice the radiation dose as from a 2-D mammogram alone - and the dose is greater in thicker breasts. Some 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 may be able to be used instead of the standard 2D mammogram so that the radiation dose from tomosynthesis would be similar to a standard mammogram.
- While slightly better performance is observed with tomosynthesis (3D) than with standard (2D) digital mammography in women of all breast densities, the effectiveness of 3-D mammography in dense breasts has not been fully evaluated and some cancers will still remain hidden by dense tissue. An analysis of over 170,000 3D mammograms 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 One studyb showed there is a benefit to having tomosynthesis every year, with fewer cancers presenting as lumps in the interval between screens, though further validation of the approach is needed.
bMcDonald ES, Oustimov A, Weinstein SP, Synnestvedt MB, Schnall M, Conant EF, Effectiveness of Digital Breast Tomosynthesis Compared With Digital Mammography: Outcomes Analysis From 3 Years of Breast Cancer Screening. JAMA Oncology. 2016
- Importantly, compared to standard mammography, tomosynthesis reduces the need for recall, typically referred to as a “diagnostic mammogram”, for additional testing (such as additional mammographic views) to evaluate areas of overlapping normal tissue. It can also reduce the number of examinations for women recalled from screening. When tomosynthesis images show a mass, the spot compression or spot magnification views which are otherwise commonly performed can be skipped, and the woman can usually have just ultrasound (see Figure 6A and 6B.)
Figure 5. Analog, Digital & Tomosynthesis 3D Mammography (same patient as Figures 1-3)
Figure 6A. Screening 2D Mammogram 3D Tomosynthesis. Standard 2D MLO digital mammogram (left) and MLO tomosynthesis 1-mm slice (right) from this 48-year-old woman with heterogeneously dense breasts shows very subtle possible distortion (arrow) in the upper right breast on standard mammogram. On tomosynthesis, the distortion is better seen, as is the underlying irregular mass (red circle).
Figure 6B. Ultrasound was performed directed to the mass seen on tomosynthesis and shows an irregular hypoechoic (dark gray) mass (marked by calipers), compatible with cancer. US-guided core needle biopsy showed grade 2-3 invasive ductal cancer with associated DCIS.
Considerations: All mammograms utilize x-ray technology and dense tissue “blocks” x-rays. This means tumors can be hidden by overlying dense tissue. The effect of breast density on a mammogram: A cancer masked on a 2D mammogram could still be masked on a 3D mammogram unless it is surrounded by fatty tissue. Standard mammography has been shown to miss about 50% of cancers present in women with dense breasts. The miss rate of tomosynthesis has not yet been fully established, but remains an issue in dense breasts. Further study is needed on the benefit of having tomosynthesis each year.