Effect of aromatase inhibitors on background parenchymal enhancement and amount of fibroglandular tissue at breast MR imaging.

Purpose: To evaluate whether treatment with an aromatase inhibitor (AI) influences background parenchymal enhancement (BPE) or amount of fibroglandular tissue (FGT) at breast magnetic resonance (MR) imaging in postmenopausal women with prior history of breast cancer.

Materials And Methods: A waiver of authorization and patient consent was granted by the institutional review board for this HIPAA-compliant retrospective study. Postmenopausal women with breast cancer and MR imaging findings of the contralateral unaffected breast, before and during 6-12 months of AI treatment (anastrozole, letrozole, or exemestane), between August 1999 and June 2010 were retrospectively identified (n = 149). Two readers performed blinded side-by-side comparison of BPE and MR imaging-depicted FGT before and during treatment. BPE and FGT were classified as the same or greater on one of the two MR studies and by using categorical scales: minimal, mild, moderate, or marked for BPE and fatty, scattered, heterogeneously dense, or dense for FGT. Consensus was reached in cases of disagreement. The sign test was used to conduct a side-by-side comparison of BPE and FGT before and during AI treatment.

Results: A decrease in BPE occurred in 33.9% (37 of 109) of women during anastrozole treatment, while an increase occurred in only one (P < .0001); 28 of 37 decreases resulted in a category change of BPE. A decrease in MR imaging-depicted FGT occurred in 5.5% (six of 109) of women, while no increases occurred (P = .031). During letrozole treatment, a decrease in BPE occurred in 46% (15 of 33), while an increase occurred in one woman (P = .0003); a decrease in FGT occurred in only one woman, and no increases occurred. Similar results were seen when women also undergoing chemotherapy were excluded. Only seven women were treated with exemestane.

Conclusion: Treatment with 6-12 months of anastrozole or letrozole was associated with decreases in BPE, which occurred in a greater proportion of women than decreases in FGT.