This case-insensitive code system http://hl7.org/fhir/us/breast-radiology/CodeSystem/AsymmetryAbnormalityTypeCS defines the following codes:

Code	Display	Definition
Asymmetry	Asymmetry	Breast asymmetry refers to when one breast is a different size or shape than the other.
AsymmetryFocal	Asymmetry focal	Focal asymmetry is visible as a confined asymmetry with a similar shape on two views but does not fit the criteria of a mass: that is, it lacks convex outer borders and conspicuity. In contrast to global asymmetry, it occupies a volume of less than one quadrant of the breast and is of more concern. The frequency of finding focal asymmetry at screening mammography is less than 1%. [pubs.rsna.org]
AsymmetryGlobal	Asymmetry global	Global asymmetry in breast tissue is a form of breast asymmetry where at least one quadrant of a breast has a larger amount of fibroglandular density than the corresponding area in the contralateral breast.
DevelopingAsymmetry	Developing Asymmetry	A developing asymmetry is a focal asymmetry that is new or increased in conspicuity compared with the previous mammogram. It is challenging to evaluate, as it often looks similar to fibroglandular tissue at mammography. A developing asymmetry should be viewed with suspicion because it is an uncommon manifestation of breast cancer. [pubs.rsna.org]