AbstractPurposeThe therascreenPIK3CAmutation assay and the alpha-specific PI3K inhibitor alpelisib are FDA-approved for identifying and treating patients with advancedPIK3CA-mutated (PIK3CAmut) breast cancer (BC). However, it is currently unknown to what extend this assay detects mostPIK3CAmutations in BC. This information is critical as patients and clinicians are using this and other genomic assays to indicate alpelisib.MethodsData from 6338 patients with BC was explored across 10 publicly available studies. The primary objective was to evaluate the proportion and distribution ofPIK3CAmutations in BC. Secondary objectives were (1) to evaluate in silico the spectrum ofPIK3CAmutations in BC that would be captured by the therascreen panel; (2) to evaluate the proportion and distribution ofPIK3CAmutations in hormone receptor-positive/HER2-negative (HR+/HER2−), HER2+, and triple-negative BC (TNBC); and (3) to explore the identification ofPIK3CAmutations in a cohort of 48 HR+/HER2− advanced BC patients by the Guardant B360 circulating tumor DNA (ctDNA) assay.ResultsPatients withPIK3CAmut tumors represented 35.7% (2261/6338). FivePIK3CAmutations comprised 73% of allPIK3CAmutations: H1047R (35%), E545K (17%), E542K (11%), N345K (6%), and H1047L (4%). Therascreen gene list would capture 72% of allPIK3CAmutations and 80% of patients with a knownPIK3CAmut BC. Among patients with doublePIK3CAmut tumors (12% of allPIK3CAmut), the therascreen panel would capture 78% as harboring 1 singlePIK3CAmutation, 17% asPIK3CAmut undetected, and 5% asPIK3CAdouble-mut.PIK3CAmutation rates were lower in TNBC (16%) compared to HR+/HER2 (42%) and HER2+ (31%) BC; however, the distribution of the 4 mainPIK3CAmutations across subtypes was similar. Finally, 28% ofPIK3CAmutations identified in ctDNA in 48 patients with advanced HR+/HER2− BC were not part of the therascreen panel.ConclusionPIK3CAmutations in BC are heterogenous and ~ 20% of patients with a knownPIK3CAmutation, and 95% with a known doublePIK3CAmut tumor, would not be captured by the therascreen panel. Finally, the clinical utility ofPIK3CAmutations not present in the therascreen companion diagnostic assay or identified by other sequencing-based assays needs further investigation.