LAUSR

1016Background: Clonal evolution during progression to and treatment of mBC highlights the importance of genomic profiling of recent samples to guide clinical decision making. We undertook this study to characterize genomic alterations (GA) in ctDNA from pts with mBC during the course of clinical care. Methods: Hybrid-capture based genomic profiling of 62 genes (FoundationACT) was performed on ctDNA from 255 BC pts. The fraction of ctDNA in the blood was estimated using the maximum somatic allele frequency (MSAF) for each sample. Results: 168 pts were ER+ [16 HER2+, 134 HER2(-), 18 HER2 unknown (unk)]; 51 were ER(-) [7 HER2+, 38 triple negative (TNBC), 6 HER2 unk]; 36 were ER unk; 95% were stage 4. For pts with treatment information, 90% had prior chemotherapy and 90% ER+ pts had prior aromatase inhibitor therapy (AI). ≥1 GA was reported in 78% of all cases and in 88% of cases with evidence of ctDNA in the blood (MSAF >0). For 226 cases with MSAF >0, an average of 2.7 GA/case were reported. The most commo...