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Stereotactic ablative radiotherapy (SABR) to early stage lung cancer has been shown promising results, but the development of regional recurrences are not uncommon and salvage treatment strategies are not established. We aimed to investigate treatment patterns, prognostic factors, and survival outcomes for these patients.

Retrospective analysis was performed to 391 patients who received SABR to primary lung cancer from 2012 to 2019. Among these patients, 90 patients (23.0%) experienced recurrence; local recurrence which was defined 2cm within PTV (n = 9), regional recurrence (n = 33), distant metastasis (n = 57). Further analysis was performed only for patients with regional recurrence and 8 (24.2%) of them were accompanied by simultaneous distant metastasis (DM). Survival analysis was performed by Kaplan-Meier method in overall survival (OS), post-recurrence overall survival (PR-OS), 1^st progression free survival (1^st PFS) and 2^nd progression free survival (2^nd PFS). Median follow-up duration was 17.3 months.

Median age was 75 years and most patients received primary SABR due to poor lung function (69.7%). Various salvage treatments were applied for regional recurrences; chemotherapy (n = 15), radiotherapy (n = 7), concurrent chemoradiotherapy (n = 2), and best supportive care (n = 9). Overall, median 1^st PFS and 2^nd PFS were 6.6 and 10.0 months, respectively. Median OS and PR-OS were 22.9 and  11.2months, respectively.

In univariate analysis for PR-OS, trend for better survival was observed for patients with isolated recurrence (p = 0.065) and those received radiotherapy without chemotherapy (p = 0.080). The non-receipt of salvage treatment (p =0.493) and simultaneous DM (p = 0.950)were not associated with decreased PR-OS. In multivariate analysis, age ≤ 75 years (HR = 0.36, p = 0.040), isolated recurrence (HR = 0.34, p = 0.037) and radiotherapy without chemotherapy (HR = 0.25, p = 0.024) were significant prognostic factors for PR-OS. Patients with 1^st PFS < 7 months showed worse OS (p = 0.029). In patients receiving salvage chemotherapy, 4 patients (23.5%) developed grade 4 or more toxicities.

PR-OS was less than 1 year after regional recurrence in our frail patients group underwent primary SABR to lung despite of various salvage treatments. Toxicities of salvage chemotherapy could be quite severe requiring careful patient selection. Further research is needed to validate our findings.