Spatial heterogeneity of MRI response to first-line hormonal therapy to predict progression-free survival in metastatic breast cancer.

Abstract

e12544 Background: Whole-body magnetic resonance imaging (WB-MRI) can positively identify response to systemic therapy in metastatic breast cancer (MBC) by the analysis of water diffusivity, cellularity and cell variability. We adapted a novel methodology that captures patient-level, spatial response heterogeneity using the METastasis Response Assessment Diagnostic System (MET-RADS) using WB-MRI. This study evaluated whether spatial heterogeneity seen at the first response assessment is predictive of duration of treatment (i.e. progression-free survival, PFS) in patients on first line hormonal therapy for MBC. Methods: Patients on first line hormonal therapy for MBC had baseline and on-treatment response assessment WB-MRI scans. All patients had a primary breast mass in situ. Patients showing unequivocal disease progression at their first response assessment scans were excluded from further analysis. Criteria for response assessment utilised the methodology described by MET-RADS. A Likert five-point response assessment category (RAC) score (1 = response highly likely, 5 = progression highly likely) was applied to 14 anatomic regions (7 bone & 7 soft tissue). Two scores reflecting the dominant and next most common response per region were recorded, capturing inter- and intra-region response heterogeneity. A novel Response Heterogeneity Index (RHI) summarised the response heterogeneity at the patient level. RHI and depth of response (mean RAC score for all involved regions) and therapy duration were analysed. Results: Twenty-one patients with primary breast mass in situ were analysed. Patients with higher levels of response heterogeneity (defined as RHI > 5; n = 11) had significantly shorter PFS than those with RHI ≤5 (n = 10; median PFS: 11 vs 27 months; log rank test p = 0.011). The depth of response and PFS were unrelated (mean RAC ≤2.5 vs mean RAC > 2.5; median PFS: 21 vs 18 months; log rank test p = 0.46). There were no correlations between RHI score and mean RAC (r2= 0.007). Conclusions: A low spatial heterogeneity of response is predictive of improved PFS in patients receiving first line hormonal therapy for MBC.