ON BREAST IMAGING REPORTING AND DATA SYSTEM 2013 MAMMOGRAPHY
MassesThe ACR BI-RADS lexicon defines a breast mass as a three-dimen- sional (3D) space-occupying lesion seen on at least two mammo- graphic projections. Mass shapes are categorized as oval, round, or irregular (Figs. 4.2 and 4.3; Table 4.1). The probability of can- cer increases as the mass shape becomes more irregular.
Oval Round Irregular
FIG. 4.2 Schematic of mass shapes: oval, round, and irregular. An oval mass is elliptical and can have two or three undulations. A round mass is spherical in shape. An irregular mass usually implies a suspicious lesion.
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FIG. 4.3 Mass shapes: oval, round, and irregular. (A) Oval shape. Mammogram shows a low-density oval mass with circumscribed margins representing a simple cyst. Incidentally noted are scattered benign round/punctate calcifications in the breast tissue. (B) Round shape. Mammogram shows a high-density round mass with mostly circumscribed (arrow) and partly indistinct (double arrow) borders, representing invasive ductal cancer. (C) Ir- regular shape. Mammogram shows a palpable high-density irregular mass in the upper breast marked with a skin marker that has partly spiculated and indistinct margins (arrow) and obscured borders on its inferior aspect (double arrows), representing invasive ductal cancer.
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The ACR BI-RADS lexicon defines mass margins as circum- scribed (well defined or sharply defined), microlobulated, obscured by surrounding glandular tissue, indistinct, or spicu- lated (Fig. 4.4; see Table 4.1). As the mass margin becomes more spiculated, the probability of cancer increases. Masses with well- circumscribed borders are more likely to be benign (Fig. 4.5;
Video 4.2). Sharply marginated borders indicate no invasion of the surrounding tissue; few cancers have smooth, well-circum- scribed borders. An obscured mass has a border hidden by over- lapping adjacent fibroglandular tissue, and that border cannot be assessed (Fig. 4.6). Microlobulated masses have small border undulations, like petals on a flower, and are more worrisome for cancer than are masses with circumscribed margins (Fig. 4.7). An indistinct mass has a margin that can be seen but is fuzzy. Indis- tinct margins are worrisome for carcinoma because the fuzzy border suggests tumor infiltration of surrounding tissue (Fig. 4.8).
Finally, spiculated masses are characterized by thin lines radiat- ing from the central portion of the mass and are especially wor- risome for cancer (Fig. 4.9; Video 4.3). When caused by cancer, mass spiculations are caused by productive tumor fibrosis (des- moplastic reaction) or actual tumor infiltration. Tomosynthesis and the synthesized mammograms reconstructed from the tomo- synthesis slices are especially sensitive for detecting spiculated masses (Fig. 4.10; Video 4.4).
Mass density describes the mass whiteness compared with an equal volume of fibroglandular tissue (Figs. 4.11 and 4.12; see Table 4.1).
High-density masses are whiter than fibroglandular tissue, and low-density masses are darker than fibroglandular tissue. High- density masses are especially worrisome for cancer, because they may contain cells with a higher atomic number than normal glan- dular tissue and fat. Low-density masses and masses with density equal to that of surrounding fibroglandular tissue are less worri- some for cancer. However, low-density cancers, such as mucinous cancers, do exist and mimic breast cysts. These cancers are low density because they contain mucin, which is fluid density.
Fat-containing masses on 2D mammography are almost always benign, except for the rare liposarcoma or tumors surrounding fat on tomosynthesis. Fat-containing masses include lymph nodes, oil cysts (see Fig. 4.12D), hamartomas, and fat necrosis, all of which are benign. However, tomosynthesis may show fat in both benign and malignant masses, and the fat seen on tomosynthesis may not be evident on 2D mammography (Freer et al., 2014).
Cancers may appear to contain fat if the cancer shape is very irregular and has trapped fat in between arms of the tumor. This means that the old mammographic rule that masses containing fat are always benign does not apply to tomosynthesis. To avoid misdiagnosis of cancers that contain fat on tomosynthesis slices, radiologists analyze the mass for suspicious margins or shapes and proceed with biopsy based on the worst mass features, even if the mass contains fat (see section: Masses Containing Fat).
Asymmetry
The ACR BI-RADS mammography lexicon term asymmetry is used for mammography and not for US or MRI, although enhanc- ing findings at MRI may be called symmetric or asymmetric. On mammography, asymmetries are white areas, more in one breast than in the other, and may represent asymmetric fibroglandular tissue or masses obscured by adjacent tissue. The finding must be included in the field of view on two orthogonal projections to TABLE 4.1 American College of Radiology 2013 Breast
Imaging Reporting and Data System Mammography Lexicon Descriptors for Masses
Shape Oval Elliptical or egg-shaped (may include 2–3 undulations)
Round Spherical, ball-shaped, circular, or globular
Irregular Neither round nor oval
Margin Circumscribed At least 75% of the margin is sharply demarcated, with an abrupt transition between the lesion and surrounding tissue
Obscured 25% or more of the margin is hidden by superimposed or adjacent fibroglandular tissue Microlobulated A margin characterized by short-
cycle undulations
Indistinct No clear demarcation of the entire margin or any portion of it from the surrounding tissue
Spiculated Margin is characterized by lines radiating from the mass Density High density X-ray attenuation of the mass
is greater than the expected attenuation of an equal volume of fibroglandular breast tissue Equal density X-ray attenuation of the mass is the same as the expected attenuation of an equal volume of fibroglandular breast tissue Low density X-ray attenuation of the mass is less
than the expected attenuation of an equal volume of fibroglandular breast tissue
Fat containing Includes all masses containing fat, such as oil cyst, lipoma, or galactocele, as well as mixed- density lesions such as hamartoma From American College of Radiology: ACR BI-RADS®—Mammography. In ACR BI-RADS atlas, breast imaging reporting and data system, ed 5, Reston, VA, 2013, American College of Radiology.
Circumscribed Obscured
Microlobulated Indistinct Spiculated
> 25%
FIG. 4.4 Schematic of mass margins: circumscribed, obscured, microlobulated, indistinct, and spiculated. A circumscribed mar- gin is sharply demarcated from the surrounding tissue. Obscured, is used when more than 25% of the entire margin of a mass is hidden by overlapping or adjacent tissue; this descriptor is used usually when the remaining unhidden mass margins are circum- scribed. Margins with short-cycle undulations are described as mi- crolobulated margins. When no clear demarcation of the entire margin is seen, the margin is categorized as indistinct (historically
“ill-defined”). Spiculated margins are those with thin lines radiat- ing center of the lesion, usually indicating cancer. The probability of cancer increases as mass margin progresses from circumscribed to spiculated.
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FIG. 4.5 Mass margin: circumscribed. (A) Photographically magnified mammogram shows a mass with a cir- cumscribed margin. The most of the margin is sharply defined. Part of the margin is obscured by overlapping breast tissue, but is limited to less than 25%, making the use of the descriptor circumscribed appropriate. (B) With tomosynthesis, the entire circumscribed margin is more easily visible (see Video 4.2). (C) Ultrasonography shows that the corresponding mass is oval, hypoechoic, and has a circumscribed margin with posterior acoustic enhancement. Pathology was fibroadenoma.
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FIG. 4.6 Mass margin: obscured. (A) Photographically magnified mammogram shows a round high-density mass. The identifiable parts of the margin are circumscribed (arrowheads), but more than 25% of the margin is obscured by overlapping breast tissue. Thus the use of the descriptor obscured is appropriate for this mass.
(B) The corresponding ultrasonography shows an irregular, hypoechoic mass with circumscribed superficial and deep margins, and an angular margin (arrow) that is suspicious for malignancy. Biopsy showed invasive ductal carcinoma.
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FIG. 4.7 Mass margin: microlobulated. (A) Photographically magnified mammogram shows a high-density, round irregular mass with microlobulated margins. (B) The corresponding ultrasound shows a hypoechoic ir- regular mass with microlobulated margins. Biopsy showed invasive ductal carcinoma.
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FIG. 4.8 Mass margin: indistinct. (A) Craniocaudal mammogram shows a round mass in the medial left breast (arrow). The differential is a focal area of glandular tissue, a cyst, or a solid mass. (B) Magnification mammogram shows the mass has indistinct margins rather than circumscribed borders (arrow). This shows the importance of fine-detail views and of being suspicious of medial breast masses. A biopsy showed invasive ductal cancer.
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FIG. 4.9 Mass margin: spiculated. (A) Photographically magnified mammogram shows an irregular mass with a spiculated margin characterized by numerous fine lines radiating from the mass. There are a few associated pleomorphic calcifications along the superior margin. (B) Tomosynthesis (see Video 4.3) visualizes the radiating lines from the mass with less overlapping tissue. (C) Corresponding ultrasound shows a hypoechoic irregular mass with spiculated and indistinct margins. Biopsy showed invasive ductal carcinoma.
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FIG. 4.10 Spiculation and distortion shown better on synthesized views and tomosynthesis. (A) Craniocaudal (CC) 2D mammogram of dense tissue shows possible spiculation and flattening of the normally scalloped glan- dular tissue edge of the outer left breast. (B) Synthesized 2D CC mammogram reconstructed from tomosynthesis slices shows distortion (arrows) in the outer breast better than the conventional 2D mammogram. (C) Tomosyn- thesis slice shows a spiculated mass (arrows) in the outer left breast, causing the distortion and flattening of the adjacent tissue, better than on conventional 2D (A) or synthesized 2D mammograms (B). Biopsy showed invasive lobular cancer (see also Video 4.4).
High density Equal density Low density Fat-containing
FIG. 4.11 Schematics of mass density. Mass density is classified as high, equal, and low density based on the rela- tive mass density compared with an equal volume of normal breast tissue. A fat-containing mass contains very low density identical to fat density (arrow). The fat density may comprise the entire mass, such as an oil cyst or lipoma, or part of the mass, as in a lymph node or hamartoma.
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High density Equal density Low density Fat-containing
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FIG. 4.12 Mass density: high-density, equal-density, low-density, and fat-containing masses. (A) Mammogram shows a high-density mass (arrow) with spiculated margins, representing invasive ductal cancer. (B) Equal density.
Mammogram shows an equal-density oval mass with mostly circumscribed margins, representing invasive ductal cancer. (C) Low density. Mammogram shows a low-density mass adjacent to the pectoralis (arrow), representing a simple cyst. Note scattered benign round/punctate calcifications in the breast tissue. (D) Fat containing. Spot compression magnification view shows a fat-containing, thin-walled mass with a radiolucent center and faint calcifications along its rim (arrow) in the area of a previous biopsy, representing an oil cyst.
qualify as an asymmetry. Asymmetries themselves do not qualify as masses on mammography. The ACR lexicon divides asymme- tries into four categories: asymmetry, global asymmetry, focal asymmetry, and developing asymmetry (Table 4.2; Fig. 4.13). A focal asymmetry and developing asymmetry have a higher likeli- hood of representing true masses, including breast cancer.
The first category of asymmetries is asymmetry, which is a small area (less than one quadrant of the breast volume) of fibroglandular-density tissue seen only on one mammographic projection. The asymmetry is either invisible or looks like nor- mal fibroglandular tissue on the orthogonal view. Most one-view asymmetries represent overlapping tissues producing a “fake mass” or summation artifact (Fig. 4.14; Video 4.5).
Because some asymmetries proved to be cancer, radiolo- gists often recalled women with asymmetries from 2D screen- ing mammography. Breast tomosynthesis decreases asymmetry recalls from screening because tomosynthesis either shows the mass as a true finding by removing glandular tissue in front of and behind the mass, or proves that the mass is fake, comprised of superimposed tissue on contiguous slices (see Fig. 4.14; see Video 4.5). However, not all asymmetries that persist as asym- metries/possible masses on tomosynthesis are true masses on workup. A possible mass may represent a summation artifact even on tomosynthesis, especially if there is suboptimal compres- sion. These summation artifact asymmetries will spread out into their normal glandular components if there is sufficient compres- sion, as with spot compression tomosynthesis (Fig. 4.15; Video 4.6). This means that tomosynthesis examinations require good compression to correctly demonstrate asymmetries as normal overlapping glandular tissue.
A large study from England (Gilbert et al., 2015) demonstrated a tendency for higher specificity for 2D plus tomosynthesis com- pared with 2D alone for distortion/asymmetry because tomosyn- thesis showed either overlapping tissue or an underlying mass.
In another study (Nam KJ et al., 2015), lesions on tomosynthe- sis were seen as a more specific and localized pattern (eg, mass or focal asymmetry rather than asymmetry) than those on 2D
mammogram; cancers were more often constantly visible on tomosynthesis than on 2D mammography, and asymmetries were more often classified as focal asymmetry on tomosynthesis.
Decreased recalls from screening in the author’s own practice were often caused by tomosynthesis showing overlapping tissue that could be dismissed as benign, versus true distortion/mass that might be cancer.
Kopans et al. (1989) showed that asymmetries that represent normal, benign overlapping tissue are nonpalpable, contain no mass and no architectural distortion, and have no associated suspicious calcifications. Their study showed that asymmetries (formerly called asymmetric breast tissue) are present on mam- mograms in up to 3% of cases, and if benign, are stable on consec- utive studies (see Fig. 4.14). In reviewing 8406 2D mammograms, Kopans et al. (1989) showed that 221/8406 (3%) screens had TABLE 4.2 American College of Radiology Breast Imaging Reporting and Data System Mammography Lexicon Descriptors for Asymmetry
Asymmetry An area of fibroglandular-density tissue that is seen in one standard mammographic view and likely represents summation
Global
asymmetry An area of fibroglandular-density tissue that occupies at least one quadrant and likely represents normal asymmetry between breasts Focal
asymmetry
An area of fibroglandular-density tissue that is seen in two mammographic views but does not fulfill criteria of mass
Developing
asymmetry A focal asymmetry that is new, larger, or more conspicuous than on a previous examination, 15% are cancer (Leung and Sickles, 2007a) From American College of Radiology: ACR BI-RADS®—Mammography. In ACR BI-RADS atlas, breast imaging reporting and data system, ed 5, Reston, VA, 2013, American College of Radiology; Leung JWT, Sickles EA. Developing asymmetry identified on mammography: correlation with imaging outcomes and pathologic findings. AJR Am J Roentgenol 188:667–675.
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Previous Current Developing Asymmetry new, visible on 2 views, <1 quadrant in volume Focal Asymmetry visible on 2 views, <1 quadrant in volume
Global Asymmetry visible on 2 views, >1 quadrant in volume Asymmetry visible on only one view
FIG. 4.13 Schematics of asymmetry: asymmetry, global asymmetry, focal asymmetry, and developing asymmetry.
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FIG. 4.14 Asymmetry: asymmetry—summation artifact, stable. (A and B) Paired mediolateral oblique (A) and craniocaudal (CC) (B) screening mammograms shows an asymmetry (arrow) in the left inner breast that is visible only on left CC view. (C–E) Photography magnified left CC views of annual screening mammograms (2-year [C]
and 1-year [D] prior studies and the current study [E]) show this one-view asymmetry has been stable over years, which supports the diagnosis as a summation artifact of normal breast tissue. (F) On tomosynthesis (photography magnified left CC projection), this asymmetry has no central density, unlike a typical mass (see also Video 4.5).
asymmetric breast tissue and were not cancer if the asymmetry did not form a mass, was nonpalpable, and had no architectural distortion or calcifications. During the 36- to 42-month follow-up study period, 20 patients underwent excisional biopsy for clinical findings showing two breast cancer and one lymphoma cases, all of which were palpable. The remaining 17 biopsies were benign, and there were no breast cancers found in the remaining 201 patients with asymmetries. The study abstract concluded with the statement:
...an asymmetric volume of breast tissue, asymmetrically dense breast tissue, or asymmetrically prominent ducts that do not form a mass, do not contain microcalcifica- tions, or do not produce architectural distortion should be view with concern only when associated with a palpable asymmetry, and are otherwise normal variations.
BI-RADS 2013 states that a global asymmetry is large, con- taining one quadrant or more of fibroglandular-like breast tissue compared with the same location in the contralateral breast, and
is a real finding because it is displayed on two orthogonal projec- tions. Global asymmetries are usually interspersed with fat and have no convex outward borders to suggest a mass. Similar to the smaller benign asymmetry, global asymmetries are benign if they are not new and have no associated architectural distortion, palpable findings, or suspicious calcifications. The nonpalpable global asymmetry is either an intrinsic normal variant (Fig. 4.16 A) or is caused by surgical removal of glandular tissue in the contra- lateral breast (Fig. 4.16B). If nonpalpable, the global asymmetry can be assessed as a BI-RADS 2 Benign Finding and returned to screening. However, if the finding is new, palpable, or is actually a mass instead of a global asymmetry, it may represent cancer and needs workup.
A focal asymmetry is defined as a more fibroglandular-like density in one breast compared with the other, both in a cor- responding location, is seen on two orthogonal views, and is less than one quadrant in size (smaller than the global asymmetry).
The focal asymmetry also lacks outward convex borders seen in masses, and may be interspersed with fat. It may be challenging
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to identify focal asymmetries, because comparison to the contra- lateral breast is especially important to identify the asymmetry on two views, and the findings must be included in the field of view on both projections. Some focal asymmetries may be dis- missed as benign at screening (Fig. 4.17), whereas others require workup. Scientific publications assess the focal asymmetry as a BI-RADS category 3 Probably Benign finding if called back from screening and is worked up, with a 0.5% to 1% probability of cancer if there are no masses at workup and it is stable over a 2-to 3-year mammographic follow-up period (Sickles, 1991; Varas et al., 1992, 2002; Vizcaino et al., 2001).
As stated in BI-RADS 2013, focal asymmetries that are less than 1 cm are of concern because they may represent nonpalpable cancers (Fig. 4.18; Video 4.7). It is of particular concern if the focal asymmetry recalled from screening has associated architec- tural distortion or calcifications. On occasion a focal asymmetry is found to be a true mass at diagnostic mammography or tar- geted US and might be cancer (Fig. 4.19).
A developing asymmetry is a focal asymmetry that, when compared with older mammograms, is new, larger, or more con- spicuous than on prior studies (BI-RADS 2013; Fig. 4.20).
The 2007 study by Leung and Sickles (2007a) showed that developing asymmetries were present in 0.16% (292 cases) of 180,801 screening mammograms and 0.11% (32 cases) of diagnostic mammograms. At screening mammography, 12.8%
of the developing asymmetries were cancer. The Leung and Sickles study and a follow-up study by Venkatesan et al. (2009) showed that the developing asymmetry in the diagnostic set- ting has a likelihood of 26.7% of malignancy if found in follow- up of BI-RADS category 3 lesions, if shown after a benign con- cordant biopsy or if developing in the first 5 years after breast
conservation. Because the percentage of cancer is above the 2%
Probably Benign category 3 threshold, developing asymmetries should undergo biopsy if they are not normal overlapping tissue at workup.
Occasionally, nonpuerperal (nonlactating) mastitis may show a developing asymmetry with rare associated architectural dis- tortion (Tan H, 2013), which can be difficult to distinguish from breast carcinoma. Puerperal or postpartum mastitis caused by lactation also produces the same radiologic features as develop- ing asymmetry but clinically, women present with a clinical his- tory of fever and pain, and their mammograms commonly show breast edema with the developing asymmetry.
Architectural Distortion
Architectural distortion is defined as linear alterations of breast parenchyma pulled into a central focus, without a definite vis- ible mass, resulting in radiating spiculations or thin lines pointing toward the center, like a star (Fig. 4.21). Distortion also describes the pulling in or straightening of any edge of the glandular tis- sue boundary with fat (see Fig. 4.10). This was called the “tent sign” and attributed to Dr. Lazslo Tabar. When associated with asymmetry or calcifications, architectural distortion is even more suspicious for cancer. In contrast, architectural distortion associ- ated with a history of surgical biopsy represents a postbiopsy scar and is benign. Because postbiopsy scars can be indicated by placement of a linear metallic marker on the skin over the post- biopsy scar, many facilities place radiopaque skin markers on skin scars to show that underlying distortion represents a benign scar.
However, architectural distortion is suspicious for malignancy or radial scar if there is no history of trauma or surgery, and it
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FIG. 4.15 Asymmetry: asymmetry—summation artifact. (A and B) Paired mediolateral oblique (A) and craniocau- dal (CC; B) screening mammograms show an asymmetry (box) in the inner right breast that is visible only on the right CC view. (C) The one-view asymmetry (box) persists on the tomosynthesis slice (also see Video 4.6A–B) of the right CC projection caused by inadequate compression. (D) Spot compression right CC tomosynthesis study slice using greater compression on the asymmetry (also see Video 4.6C–D) shows normal overlapping normal tissue at the corresponding area (box), and US of the right breast was negative. The asymmetry was diagnosed as a summation artifact of overlapping normal breast tissue and shows the importance of good compression with tomosynthesis.
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