Asymmetrical density in mammography

Asymmetrical mammographic density is a mammographic morphological descriptor. It is given when there is increased density in one of the breasts, on either one or both standard mammographic views but without evidence of a discrete mass.

The criteria for an asymmetry may include borders of the tissue are not convex, and or a lesion that is not centrally dense; and therefore the lesion will not meet the criteria for a mass.

An asymmetrical density can be further characterized as:


The most common cause for asymmetric breast density is the common normal variant of asymmetrically distributed breast parenchyma. The most important differential to exclude is neoplasia. A more extensive differential list follows. 

Benign causes
 Malignant causes

Radiographic features

Mammography: screening

In practice, an asymmetry which is stable and unchanged over years does not deserve attention. An asymmetry is usually seen on both views of the breast if it is not a summation shadow. Typically, summation shadows are seen on one view only and disappear when the view is repeated or a rolled CC of ML view of the breast is done. Ultrasound is not needed if you are working up a summation shadow.

Of more significance is a developing asymmetrical density. Remember, not all carcinomas present in imaging as a mass, some develop and present as asymmetry only with no noticeable mass. A developing asymmetry deserves your full attention and justifies workup. It is rarely helpful to make magnification views of an asymmetrical density, if you enlarge a blob you tend to get a bigger blob so you rarely get extra information to help you decide what you are dealing with. Its often more useful to do rolled CC and ML views of the breast and compare with the original mammogram. If the lesion was a summation shadow, it is not seen on the additional views. If it persists, it is a significant finding and ultrasound is indicated.

Mammography: diagnostic

Before you recall an asymmetry, go through the old images and decide whether the density is actually changing over time. In general, malignancies grow over time and benign lesions remain stable. 

A parenchymal distortion with a dark center is a BIRADS IV lesion.

A parenchymal distortion with a white center is a BIRADS V lesion.

Additional imaging may include:

  • spot-compression views: they rarely add information in practice.  Best used in looking at microcalcifications
  • step oblique mammography
  • rolled CC view: very little "roll" is actually needed, just a slight degree of obliquity will often give you the answer; if you simply repeat the view you just looked at the answer will be obvious
  • 90 degree lateral view/LM view: may help localize the abnormality, especially one that is better seen on MLO projection
Breast imaging and pathology
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Article information

rID: 15627
System: Breast, Oncology
Section: Gamuts
Synonyms or Alternate Spellings:
  • Asymmetrical mammographic density
  • Asymmetrical breast density
  • Asymmetric mammographic density

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