Genomic characterization of STAS in stage 1 EGFR-mutated NSCLC and prognostic implications.

8029 Background: Spread through air spaces (STAS) is a poor prognostic factor and was recently introduced as a histologic descriptor in the TNM edition 9 for stage 1 lung cancer. While STAS-positivity (STAS+) is known to be associated with adenocarcinoma, the molecular epidemiology and determinants of STAS+ specific to epidermal growth factor receptor-mutated lung cancer (EGFRm) and prognostic implications remain unknown. Methods: Consecutive patients from National Cancer Centre Singapore diagnosed with AJCC8 stage 1 lung adenocarcinoma with minimum 3 years follow up post-surgery and known EGFR and STAS status (+ or -) were included. Fresh frozen tumour and normal samples were subject to whole exome sequencing (WES) at 400X and 100X coverage respectively, with 50 million paired-end reads for RNA-seq per sample. PD-L1 expression by immunohistochemistry was scored using SP263. Wilcoxon and Fisher’s exact tests were used for association analysis and Kaplan Meier method for survival. Results: Between 1/1/16-31/12/21, 300 patients were included (203 EGFRm; 97 EGFR-wildtype (EGFRwt)). While the incidence of STAS+ was similar between EGFRm and EGFRwt (49.8% versus (vs) 56.7%, p=0.316), 5-year disease-free survival (DFS) was significantly worse for STAS+ vs STAS- EGFRm (67.8% vs 93.2%, p=0.005) but not EGFRwt (78.9% vs 82.0%, p=0.6). Comparing STAS+ and STAS- EGFRm, there was no significant difference in the proportion of never-smokers (77.2% vs 76.4%, p=0.248) and females (52.5% vs 60.8%, p=0.292). Distribution of EGFR mutation subtype was also similar (ex19del 43.6% vs 41.2%; L858R 37.6% vs 44.1%; others 18.8% vs 14.7%, p=0.576). Lymphovascular invasion (LVI) (20.8% vs 3.9%, p<0.001) and high histological grade (32.7% vs 4.0%, p<0.001) were significantly more common in STAS+ vs STAS- EGFRm. Among EGFRm, 193 patients had available WES and RNA-seq. Incidence of TP53 co-mutations (60.7% vs 43.2%, p=0.020) and whole genome doubling (WGD) (34% vs 17%, p=0.013) was significantly more common among STAS+ than STAS- tumours. No other significant differences in co-mutations or copy number alterations were observed. The proportion of PD-L1 expression ≥1% (42.3% vs 21.2%, p=0.011) and non-TRU transcriptomic subtype (55.9% vs 22.2%, p<0.001) were also significantly higher in STAS+ compared to STAS- tumours. Controlling for stage (1A vs 1B), age, smoking status, gender, histological grade and LVI, STAS+ remained an independent predictor of inferior DFS in stage 1 EGFRm (hazard ratio: 4.0, 95% confidence interval: 1.1-15.2, p=0.04). Conclusions: Despite a similar incidence, STAS+ independently predicts for inferior DFS in patients with stage 1 EGFRm but not EGFRwt. STAS+ stage 1 EGFRm demonstrate a higher frequency of TP53 co-mutations, WGD, non-TRU subtype and PD-L1≥1% as compared to STAS- stage 1 EGFRm. Our findings highlight the molecular determinants of STAS+ and support STAS as a risk stratification factor for stage 1 EGFRm.