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Background: Genetic testing plays an important role in guiding screening, diagnosis, and precision treatment of breast cancer (BC). However, the appropriate genetic testing criteria remain controversial. The current study aims to facilitate the development of suitable strategies by analyzing the germline mutational profiles and clinicopathological features of large‐scale Chinese BC patients. Methods: BC patients who had undergone genetic testing at the Sun Yat‐sen University Cancer Center (SYSUCC) from September 2014 to March 2022 were retrospectively reviewed. Different screening criteria were applied and compared in the population cohort. Results: A total of 1035 BC patients were enrolled, 237 pathogenic or likely pathogenic variants (P/LPV) were identified in 235 patients, including 41 out of 203 (19.6%) patients tested only for BRCA1/2 genes, and 194 out of 832 (23.3%) received 21 genes panel testing. Among the 235 P/LPV carriers, 222 (94.5%) met the NCCN high‐risk criteria, and 13 (5.5%) did not. While using Desai's criteria of testing, all females diagnosed with BC by 60 years and NCCN criteria for older patients, 234 (99.6%) met the high‐risk standard, and only one did not. The 21 genes panel testing identified 4.9% of non‐BRCA P/LPVs and a significantly high rate of variants of uncertain significance (VUSs) (33.9%). The most common non‐BRCA P/LPVs were PALB2 (11, 1.3%), TP53 (10, 1.2%), PTEN (3, 0.4%), CHEK2 (3, 0.4%), ATM (3, 0.4%), BARD1 (3, 0.4%), and RAD51C (2, 0.2%). Compared with BRCA1/2 P/LPVs, non‐BRCA P/LPVs showed a significantly low incidence of NCCN criteria listed family history, second primary cancer, and different molecular subtypes. Conclusions: Desai's criteria might be a more appropriate genetic testing strategy for Chinese BC patients. Panel testing could identify more non‐BRCA P/LPVs than BRCA1/2 testing alone. Compared with BRCA1/2 P/LPVs, non‐BRCA P/LPVs exhibited different personal and family histories of cancer and molecular subtype distributions. The optimal genetic testing strategy for BC still needs to be investigated with larger continuous population studies. [ABSTRACT FROM AUTHOR]

publisher‐imprint‐name Springer volume‐issue‐count 1 issue‐article‐count 17 issue‐toc‐levels 0 issue‐pricelist‐year 2020 issue‐copyright‐holder The Author(s) issue‐copyright‐year 2020 article‐contains‐esm Yes article‐numbering‐style Unnumbered article‐registration‐date‐year 2020 article‐registration‐date‐month 2 article‐registration‐date‐day 4 article‐toc‐levels 0 toc‐levels 0 volume‐type Regular journal‐product ArchiveJournal numbering‐style Unnumbered article‐grants‐type OpenChoice metadata‐grant OpenAccess abstract‐grant OpenAccess bodypdf‐grant OpenAccess bodyhtml‐grant OpenAccess bibliography‐grant OpenAccess esm‐grant OpenAccess online‐first false pdf‐file‐reference BodyRef/PDF/40169_2020_Article_269.pdf pdf‐type Typeset target‐type OnlinePDF issue‐type Regular article‐type OriginalPaper journal‐subject‐primary Medicine & Public Health journal‐subject‐secondary Medicine/Public Health, general journal‐subject‐collection Medicine open‐access true --> Background: Despite the impressive anti‐tumor activity of osimertinib in epidermal growth factor receptor (EGFR) T790M‐positive non‐small cell lung cancer (NSCLC) patients, 30–40% of patients still show limited response. There is therefore a need to identify biomarkers that accurately predict the response to osimertinib therapy. In this study, 54 patients with targeted next‐generation sequencing of circulating tumor DNA before osimertinib treatment and known T790M positivity were included. We investigated the predictive value of baseline circulating tumor DNA‐derived biomarkers on osimertinib therapy. Results: Baseline maximum somatic allele frequency (MSAF) level was not associated with objective response rate (ORR) (P = 0.886) and progression‐free survival (PFS) (P = 0.370) of osimertinib treatment. T790M relative mutation purity (RMP, defined here as the ratio of T790M AF to MSAF) quartiles were found to be significantly associated with ORR (P for trend = 0.002) and PFS (P for trend = 0.006), and a cut off value of 0.24 identified two distinct prognostic groups [Hazard ratio (HR) = 0.36 for low T790M RMP, 95% confidence interval (CI) 0.18–0.72, P = 0.004). Additionally, although T790M relative mutation abundance (RMA, defined as T790M AF/EGFR driver AF) quartiles were not significantly associated with ORR (P for trend = 0.063), a cut off value of 0.30 also identified two distinct prognostic groups (HR = 0.43 for low T790M RMA, 95% CI 0.22–0.85, P = 0.015). However, in multivariate analysis, grouping of T790M RMP showed a better predictive value (HR = 0.46, 95% CI 0.20–1.05, P = 0.066) than T790M RMA (HR = 0.71, 95% CI 0.31–1.61, P = 0.409). Moreover, T790M RMP as continuous covariate was independently predictive of PFS (HR = 0.15, 95% CI 0.03–0.79, P =0.025), while T790M RMA was not (HR = 1.14, 95% CI 0.49–2.66, P =0.766). An external validation cohort further confirmed the T790M RMP was significantly associated with PFS of osimertinib therapy. Conclusions: This study established the independent predictive role of T790M RMP in NSCLC patients receiving osimertinib treatment. [ABSTRACT FROM AUTHOR]