Abstract Body (Do not enter title and authors here): Introduction: The decision to perform ascending thoracic aortic aneurysm (aTAA) repair is primarily guided by diameter thresholds, but the optimal timing remains debated among guidelines due to limited natural history data. We aimed to analyze ATAA outcomes in a large cohort of veterans.

Research Question: Among veterans with ATAA under clinical surveillance, how does baseline aneurysm diameter relate to the risk of adverse aortic events and mortality, and how might this inform decisions regarding prophylactic surgical repair?

Methods: We conducted a retrospective cohort study of ATAA patients with diameter ≥4.0cm who underwent surveillance between 1998 and 2024 at a Veterans Affairs medical center. Baseline clinical data, imaging, interventions, and outcomes were collected from electronic health records. Outcomes included surgical repair, all-cause mortality, and aortic events—aortic dissection or rupture. A competing risks regression model evaluated the association of diameter with all-cause mortality, adjusting for age, smoking, and aortic valve phenotype.

Results: We included 764 veterans (98.0% male) with median (IQR) age of 75.0 (9.3) years and ATAA diameter of 4.40 (0.50) cm. Median follow-up was 5.4 (6.1) years. Surgical repair occurred in 86/764 patients (11.3%). Aortic dissection occurred in two patients (0.3%), both within the 4.0–4.5cm group, with an incidence of 0.05 events per 100 person-years (95% CI, 0.00–0.11). All-cause mortality rates were 2.86 (2.20–3.56), 3.22 (2.36–4.15), 5.82 (3.81–8.06), and 24.60 (12.30–38.94) deaths per 100 person-years for ATAA diameters 4.0–4.4, 4.5–4.9, 5.0–5.4, and ≥5.5 cm, respectively (p<0.001). In multivariable analysis, all-cause mortality was independently associated with increasing ATAA diameter (HR, 1.40 per 0.5 cm increase; 95% CI, 1.17–1.68; p<0.001) and increasing age (HR, 1.07 per year; 1.05–1.09; p<0.001).

Conclusions: ATAA all-cause mortality is associated with baseline diameter, with a sharp seven-fold incidence increase in aneurysms ≥5.5cm. Our findings support the current 5.5-cm threshold for prophylactic ATAA repair and emphasize the need for selective intervention in smaller aneurysms.