Asymmetric background parenchymal enhancement on contrast-enhanced mammography: associated factors, diagnostic workup, and clinical outcome

Objectives: To summarize our institutional experience with contrast-enhanced mammography (CEM) exams reporting asymmetric background parenchymal enhancement (BPE). Materials and methods: Consecutive CEMs performed between December 2012 and July 2023 were retrospectively reviewed to identify exams reporting asymmetric BPE. Associated factors, the level of reporting certainty, BI-RADS score, diagnostic workup, and clinical outcome were summarized. BPE grades and BI-RADS were compared between initial CEM vs. immediate MRI and 6-month follow-up CEM, when indicated, using the Sign test. Results: Overall, 175/12,856 (1.4%) CEMs (140 female patients, mean age, 46 ± 8.0 years) reported asymmetric BPE. Reporting certainty was mostly high (n = 86), then moderate (n = 59) and low (n = 30). Associated factors included contralateral irradiation (n = 94), recent ipsilateral breast treatment (n = 14), and unilateral breastfeeding (n = 4). BI-RADS scores were 0 (n = 21), 1/2 (n = 75), 3 (n = 67), 4 (n = 3), and 6 (n = 1), or given for a finding other than asymmetric BPE (n = 8). Initial diagnostic-workup often included targeted-US (n = 107). Immediate MRI (n = 65) and/or 6-month CEM follow-up (n = 69) downgraded most cases, with a significant decrease in BPE grade compared to the initial CEM (p < 0.01 for both). On follow-up, two underlying cancers were diagnosed in the area of questionable asymmetric BPE. Conclusion: Apparent asymmetric BPE is most often a benign finding with an identifiable etiology. However, rarely, it may mask an underlying malignancy presenting as non-mass enhancement, thus requiring additional scrutiny. Clinical relevance statement: The variability in the diagnostic-workup of apparent asymmetric background parenchymal enhancement stresses the clinical challenge of this radiological finding. Further studies are required to verify these initial observations and to establish standardized management guidelines. Key Points: Apparent asymmetric background parenchymal enhancement usually represents a benign clinical correlate, though rarely it may represent malignancy. Evaluation of asymmetric background parenchymal enhancement varied considerably in the metrics that were examined. Targeted US and MRI can be useful in evaluating unexplained asymmetric background parenchymal enhancement.