Mammogram Analysis and Interpretation

Anatomy and Image Contrast

A normal breast has glandular breast elements surrounded by fat and breast stroma, which is surrounded by a honeycomb fibrous structure of thin strandlike Cooper’s ligaments. The glandular elements are composed of lactiferous ducts leading from the nipple and branch into excretory ducts, interlobular ducts, and terminal ducts that lead to acini that produce milk ( Fig. 2.2 ). The ducts are lined by epithelium composed of an outer cellular myoepithelial layer and an inner secretory cellular layer. The ducts and glandular tissue extend posteriorly in a fanlike distribution consisting of 15 to 20 lobes draining each of the lactiferous ducts, with most of the glandular tissue found in the upper outer breast near the axilla. Fatty tissue surrounds the glandular tissue. Posterior to the glandular tissue is retroglandular fat, described by Dr. Laszlo Tabar as a “no man’s land,” in which no glandular tissue should be seen. The pectoralis muscle lies behind the fat on top of the chest wall.

Mammography provides image contrast by using the differences in the x-ray attenuation among the different breast tissue types, such as fat, fibroglandular tissue, and carcinoma. With lower energy x-rays used by mammography (approximately 25–28 keV), the attenuation difference between the fibroglandular tissue and carcinoma is more pronounced than on standard radiographs, which use a 50-keV x-ray. Fatty tissue is the least dense and most translucent to x-rays and appears dark on mammography. Fibroglandular tissue, muscle, and lymph nodes are more dense and radiopaque (whiter) than fatty tissue, and are white on mammography. Cancers and fluid-filled cysts may be denser and whiter than normal surrounding fibroglandular tissue. Calcifications and metals are the brightest and whitest of all structures on mammography.

On the normal mediolateral oblique (MLO) mammogram, the pectoralis muscle is a concave white structure posterior to the retroglandular fat near the chest wall ( Fig. 2.3 ). Normal lymph nodes high in the axilla overlie the pectoralis muscle. Normal lymph nodes are sharply marginated, oval, or lobulated dense white masses with a radiolucent fatty hilum. They are commonly found in the axilla and in the upper outer quadrant of the breast along blood vessels. Lymph nodes can also occur normally but less commonly in any quadrant of the breast and are called normal intramammary lymph nodes. A normal lymph node has a typical, sharply marginated kidney bean shape; a white outer cortex; and a dark, fatty hilum on the mammogram and should be left alone. If one is uncertain about whether a mass represents an intramammary lymph node, a tomosynthesis slice or mammographic magnification views may help display the fatty hilum, or an ultrasound may show the typical hypoechoic appearance of the lymph node and the echogenic fatty hilum.

On the normal craniocaudal (CC) projection, the pectoralis muscle produces a half-moon–shaped or bandlike density near the chest wall ( Fig. 2.4 ). Fat lies anterior to the muscle, and the white glandular tissue lies anterior to the fat. In older women, most of the glandular tissue in the medial breast undergoes fatty involution, and most of the residual dense glandular tissue persists in the upper outer breast.

There should be only fatty tissue in the medial breast near the chest wall on the CC view. The only normal exception is the sternalis muscle, which is a white slip of muscle hugging the medial chest wall on the CC view that should not be mistaken for a mass ( Fig. 2.5 ). If there is a question that a density in the medial breast is a mass instead of the sternalis muscle, a tomosynthesis slice should show that it is sternalis muscle. If there is no tomosynthesis, a cleavage view (CV) mammogram or ultrasound can prove that the density is a just a sternalis muscle, is normal, and can be ignored.

Breast Density, Breast Density Notification Legislation, and Breast Density Classification Based on the Breast Imaging Reporting and Data System

Breast density is an important feature of the mammogram that describes how much of the breast volume is filled with white glandular tissue. Breast fibroglandular tissue is white on the mammogram, and fat is black on the mammogram. Women have varying ratios of glandular and fatty tissue in their breasts genetically. The definition of a dense breast is one that contains a lot of glandular breast tissue and looks mostly white on the mammogram. The opposite of a dense breast is a fatty breast, which looks mostly black on the mammogram. A dense breast does not mean the breast is hard to the touch. Breast density on the mammogram has little correlation to how hard or soft the breast feels on physical examination; that is, you cannot predict how soft a breast will feel to the touch by looking at the mammogram density. Conversely, there is no correlation of how white the mammogram will be based on how hard, lumpy, or soft the breast feels on physical examination.

The American College of Radiology’s (ACR) Breast Imaging Reporting and Data System (BI-RADS) lexicon classifies breast density on mammograms into four groups: extremely dense, heterogeneously dense, scattered areas of fibroglandular density, and almost entirely fatty ( Fig. 2.6 ; Box 2.4 ). From 2004 to 2013, extremely dense, heterogeneous dense, scattered areas of fibroglandular density, and almost entirely fatty defined, in quartiles, how dense or how white the breast looked on the mammogram. Dense was defined as >75% to 100% dense tissue by volume, heterogeneously dense was 51% to 75% dense, scattered was 26% to 75% dense, and fatty was <25% dense by volume. Dense and heterogeneously dense glandular tissue lowered the sensitivity of mammography, because breast cancer is also white on the mammogram, and the white dense normal background of glandular tissue can hide a cancer, just like a polar bear can hide in a snowstorm.

The fifth edition of BI-RADS, published in 2013, no longer defines breast density in the quartile system ( Fig. 2.7 ). Instead, dense and heterogeneously dense describe whether there is enough breast tissue on the mammogram to obscure a cancer. Thus a heterogeneously dense normal background of glandular tissue may have less than 50% dense breast tissue by volume on the mammogram but contains enough dense tissue to obscure a cancer. In the mammographic report, radiologists describe breast density so that referring doctors will know how white the breast looks and how confident the radiologist is in excluding cancer.

Breast density notification legislation in the United States began in 2009 in the state of Connecticut, where the law mandated that women with dense and heterogeneously dense breast tissue on mammograms receive a letter informing them that they have dense breast tissue, that dense breast tissue is normal, that dense breast tissue carries a higher risk of breast cancer, that dense breast tissue may hide breast cancer, and that they may wish to discuss supplemental screening tests with their doctor. BI-RADS 2013 shows that in the United States about 50% of all women have heterogeneously dense or dense breast tissue on mammograms. As of May 2016, 27 states have proposed or enacted dense breast notification federal legislation.

It is important to know that the breast is normally dense at a young age and decreases in density with time as glandular tissue normally involutes into fat. Young women have mostly glandular breasts, and their mammograms are usually dense or white on mammograms (see Fig. 2.6A ). As women age, their fibroglandular tissue involutes into fat, which is black. Therefore the natural progression of the mammogram is mostly white (dense) at a young age when the breasts are filled with glandular tissue and it becomes progressively darker (fatty) as the woman ages. The amount of remaining glandular tissue varies from woman to woman and depends on genetics, parity, and exogenous hormone replacement therapy. Some older women have surprisingly large amounts of dense white tissue on the mammogram, but generally as women age there are greater amounts of fat and less dense glandular tissue, which usually remains in the upper outer quadrants of the breast (see Fig. 2.6D ), producing a darker mammogram. Normal increases in breast density occur only in pregnant and lactating women, or in women starting exogenous hormone replacement therapy, because of the increase in fibroglandular tissue in response to female hormones. The breast density does not increase normally otherwise. Any unexplained increases in breast density should be viewed with suspicion and worked up.

Optimization of Reading Room and Image Display

If the mammograms are screen-film studies, the images are viewed on high-intensity viewboxes with the light parts of the films masked to block extraneous light. For full-field digital mammograms (FFDMs) and tomosynthesis images viewed on soft copy, the images are displayed on high-resolution bright monitors in a dark room with little to no ambient light, comparing old studies with new ones in the display protocol. Additionally, an ergonomic setup for the radiologist as described in our article on repetitive stress injury in breast radiologists will help the radiologist avoid injury ( ).

First Look at Two-Dimensional Views and Older Studies

The standard set of mammograms consists of paired MLO views and paired CC views. Normal breast tissue is usually symmetric, or “mirror image.” To evaluate for mammographic symmetry, the MLO and CC views are displayed back to back, and asymmetries are easily identified using the comparison of the right and left breasts ( Fig. 2.8 ). Look at the whitest part of the mammograms for normal fibroglandular symmetry to see if there is more white tissue on one side than on the other (an asymmetry) or if there are any abnormal spots focally whiter than background (a focal asymmetry).

Asymmetries can be normal. For example, normal asymmetric glandular tissue is defined as a larger volume of normal fibroglandular tissue in one breast than the other, but with one breast not necessarily being larger than the other; this occurs in about 3% of women. A normal asymmetry should be stable over time.

To detect changes over time, good quality older mammograms are placed above the new ones. Because subtle changes may take longer than a year to become evident, one compares new mammograms with last year’s mammogram, one more than 2 years old, and the oldest images of comparable quality.

Unexplained increases in breast density may indicate breast cancer. An unexplained generalized increase in breast density associated with skin thickening may represent breast edema, which has many etiologies, including inflammatory cancer. An unexplained new asymmetric focal density is called a “developing asymmetry” and should prompt investigation because developing asymmetries represent cancer in 15% of cases.

Comparing old studies with current studies makes it easier to see new or developing changes. A normal mammogram does not usually change from year to year after taking into account the normal involution of glandular tissue ( Fig. 2.9 ). Contrastingly, malignant lesions increase in size and can change the mammographic appearance over time. However, the changes on mammography caused by tumors can be very subtle. Some tumors may infiltrate into the breast tissue without producing an apparent contrast against fat ( Fig. 2.10 ). The doubling time of breast cancer is typically approximately 50 to 200 days, but some can grow more slowly ( Fig. 2.11 ).

Systematic Search on Each Mammographic Two-Dimensional View and Tomosynthesis

Once the radiologist judges the mammographic density and asymmetries, they search for masses, calcifications, and distortions on the mammograms. A common search pattern uses zigzags or strips of each image, like mowing a lawn with a lawnmower or searching for a lost boat at sea with a rescue helicopter. For two-dimensional (2D) digital mammography, the radiologist electronically magnifies the mammograms in quartiles, the upper half and the lower half of the 2D MLO views and the inner and outer 2D CC views ( Fig. 2.12 ).

On tomosynthesis, the radiologist first reviews the 2D or synthesized 2D mammogram initially using the zigzag/strip method, then proceeds to magnify the 2D/synthesized 2D mammogram in quartiles because it is harder to see the overall breast in tomosynthesis slices or slabs alone. Then the radiologist scrolls through tomosynthesis slices or slabs. Some radiologists synchronize the tomosynthesis slabs or slices together, back to back, similar to the 2D display, to look for symmetry and auto scroll the tomosynthesis as a movie to get an overall view of the breast. For detailed tomosynthesis analysis, similar to reading a computed tomography (CT) scan or magnetic resonance imaging (MRI), it is important for the radiologist to keep his or her eyes in one place as the movie is scrolled/played to analyze a specific area. Otherwise, the eye is moving and the images are moving, and a finding could be missed. For example, the radiologist scrolls through all the tomosynthesis slices/slabs keeping his or her eye on the upper right breast MLO throughout the series, the lower right MLO, the upper left MLO, and then the lower left MLO. The radiologist then repeats the procedure for the CC studies. The method to review the tomosynthesis is described in Fig. 2.13 . If the radiologist is reviewing the tomosynthesis on slabs, and there is a suspicious finding seen on a slab, then the findings are reviewed on the tomosynthesis slices.

Location Description Based on BI-RADS 2013

Finding locations are described by laterality (right or left breast), quadrant and clock face, depth, and distance from the nipple. The breast quadrants describe the breast as the breast was divided into four areas with the nipple at the crosshairs of quadrants and as if the patient is facing the examiner ( Fig. 2.14 ). The upper outer quadrant is the upper breast quadrant closest to the axilla, the upper inner quadrant is the upper breast near the sternum, the lower inner quadrant is the lower inner breast near the sternum, and the lower outer quadrant is the lower breast not near the sternum nor the axilla. The “clock face” description of a breast finding location imagines a clock superimposed on each breast as the woman faces the examiner (see Fig. 2.14 ). This means that the upper outer quadrant in the right breast is between the 9-o’clock and 12-o’clock positions, but the upper outer quadrant in the left breast is between the 12-o’clock and 3-o’clock positions. Although it is simple to see how clock-face lesion location can be easily mixed up (right upper outer quadrant versus left upper outer quadrant), the clock face allows the radiologist and surgeon to describe lesions that fall between quadrants, for example, a lesion located at the 12 o’clock or the 6 o’clock position of the breast. The breast depth includes the anterior, middle, and posterior third ( Fig. 2.15 ) of the breast. Distance from the nipple for a specific lesion is measured as the distance between the root of the nipple and the anterior edge of the lesion on one of the projections that provide the best visualization of the lesion.