e20015 Background: Lung cancer, a leading global cause of cancer-related mortality, necessitates enhanced prognostic markers for improved treatment outcomes. This study focuses on the predictive value of phosphorylated cytosolic arginine sensor for mTORC1 subunit 1 at S14 (pCASTOR1) in lung adenocarcinoma (LUAD) patients, specifically in males with KRAS mutations. Methods: This retrospective study, approved by University of Pittsburgh IRB, utilized formaline-fixed paraffin embedded primary tissues from two independent cohorts of patients who underwent thoracic surgical procedures at the University of Pittsburgh Medical Center (UPMC) from 2002 to 2011. Comprehensive clinical, demographical and pathological information was obtained from UPMC Cancer Registry, with cases lacking incomplete information excluded from analysis. Study endpoints included overall survival (OS) and relapse-free survival (RFS). An antibody targeting pCASTOR1 was employed to analyze LUAD tumor and adjacent non-tumor lung tissues from both cohorts. A pathologist, blinded to patient information and survival outcomes, conducted scoring in 5% increments. Scores were categorized as low or high pCASTOR1 based on second or third quartile. Correlations between pCASTOR1 scores and OS/RFS were examined, stratifying by clinical, pathological, demographic, or genotype data. Univariate OS and RFS analyses were conducted using the Kaplan-Meier method and log-rank test, while multivariate survival significance was assessed via Cox regression analysis. Additionally, unpaired T-tests were utilized to compare pCASTOR1 levels between non-tumor and tumor cells. A significance threshold of p < 0.05 was applied for all analyses. Results: Tumor cells exhibited significantly elevated pCASTOR1 scores compared to non-tumor cells in both cohorts (P < 0.05). High pCASTOR1 scores predicted poor OS (HR = 3.3, P = 0.0008) and RFS (HR = 3.0, P = 0.0035) in male patients with KRAS mutations in Cohort 1. pCASTOR1 remained an independent predictor for OS (HR = 4.1, P = 0.0047) and RFS (HR = 3.5, P = 0.0342) after controlling for other factors. Cohort 2, comprised solely of smokers with incomplete genotype information, validated these findings, with high pCASTOR1 scores correlating with worse OS (HR = 4.8, P = 0.0363). Notably, in early-stage LUAD, elevated pCASTOR1 scores were associated with significantly worse OS (HR = 3.3, P = 0.0176) and RFS (HR = 3.1, P = 0.0277) in male patients with KRAS mutations in Cohort 1, and worse OS (HR = 5.0, P = 0.0069) and RFS (HR = 5.0, P = 0.0069) in Cohort 2, akin to late-stage patients. Conclusions: Elevated pCASTOR1 score serves as a robust biomarker predicting poor OS and RFS in male LUAD patients with KRAS mutations, offering potential clinical utility in optimizing treatment strategies for this subgroup.