Mammographic Asymmetry Better Seen on CC View: Cancer Risk Assessment
Asymmetry that is better visualized on the craniocaudal (CC) view compared to the mediolateral oblique (MLO) view is MORE likely to represent summation artifact and therefore LESS likely to be cancer, but this finding alone should never be used to dismiss the need for proper diagnostic workup.
Understanding the Significance
The key principle is that true lesions persist across multiple views, while summation artifacts disappear or change significantly with different projections 1. When an asymmetry is more prominent or only visible on one view (such as the CC view), this suggests it may be caused by overlapping normal fibroglandular tissue rather than a true mass or architectural distortion 2.
Benign Asymmetry Characteristics
- Asymmetric breast tissue that does not form a mass, does not contain microcalcifications, and does not produce architectural distortion should be viewed as a normal variation when it lacks a palpable correlate 2
- In a prospective study of 8,408 mammograms, asymmetric breast tissue was found in 3% of cases, and no breast cancer was diagnosed in any patient without an associated palpable abnormality 2
- Most asymmetries are benign or caused by summation artifacts due to typical breast tissue superimposition during mammography 1
Critical Diagnostic Algorithm
Step 1: Additional Mammographic Views
Diagnostic mammography with spot compression and magnification views must be performed to characterize any asymmetry 3, 4. These additional views will:
- Determine if the asymmetry persists or disappears (indicating summation artifact) 3
- Better evaluate the subareolar region if asymmetry/focal asymmetry is present 4
- Assess for any associated microcalcifications 3
Step 2: Targeted Ultrasound
Concurrent targeted ultrasound of the area of concern should be performed 3. This helps identify:
Step 3: Risk Stratification Based on Findings
If the asymmetry disappears on additional views: This confirms summation artifact and is reassuring 1, 2
If the asymmetry persists but remains stable and has no suspicious features: Follow BI-RADS 3 protocol with clinical re-examination in 3-6 months and follow-up imaging every 6-12 months for 1-2 years 3
If the asymmetry is new or developing: This requires heightened suspicion, as developing asymmetry has a positive predictive value of 12.8% for cancer at screening and 42.9% when biopsy is recommended 6
Important Caveats and Pitfalls
The "Developing Asymmetry" Exception
A developing asymmetry (new or increased in conspicuity compared to prior) should be viewed with suspicion regardless of which view it appears on 5, 6. This is an uncommon but significant manifestation of breast cancer with:
- PPV1 of 12.8% at screening 6
- PPV2 of 42.9% when biopsy is recommended 6
- 23.8% of cancers presenting as developing asymmetry had no sonographic correlate 6
Associated Features That Increase Cancer Risk
Even if better seen on CC view, biopsy is indicated if the asymmetry is associated with 4, 3:
- Suspicious microcalcifications (fine pleomorphic or fine-linear branching)
- Architectural distortion
- A palpable abnormality 2
- New or increasing size compared to prior studies 6
Mammographic Findings in DCIS
Low and intermediate-grade DCIS can present as an asymmetry without calcifications 4, so the absence of calcifications does not exclude malignancy.
When Normal Sonography Is Not Sufficient
Lack of an ultrasound correlate should not preclude biopsy of a developing asymmetry 5. Normal sonographic findings do not exclude malignancy in cases of developing asymmetry 6. If no US correlate is identified and the mammographic finding remains suspicious, stereotactic biopsy or MRI-guided biopsy should be considered 5.
BI-RADS Classification and Management
For BI-RADS 4-5 findings (suspicious or highly suggestive of malignancy), tissue biopsy using core needle biopsy is recommended 3. The decision should be based on the totality of imaging findings, not solely on which view demonstrates the asymmetry best.