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In the treatment of concurrent chemoradiotherapy (CCRT) in limited-stage small cell lung cancer, the optimal once-daily radiotherapy (RT) dose/fractionation remain unclear although it is the most frequently used. Therefore, this study aimed to compare the treatment outcomes and toxicities of modest dose RT (≤54 Gy) with those of standard dose RT (>54 Gy) and investigate the benefit of the high dose based on patient factors.

Since 2004, our institution has gradually increased the thoracic RT dose. Among the 225 patients who underwent CCRT, 84 patients (37.3%) received >54 Gy. Because the patients treated with RT > 54 Gy were not randomly assigned, propensity score matching (PSM) was performed.

The proportion of patients treated with >54 Gy increased over time (p = 0.014). Multivariate analysis revealed that the overall tumor stage and dose > 54 Gy (hazard ratio 0.65, p = 0.029) were independent prognostic factors for overall survival (OS). PSM confirmed that thoracic RT doses of >54 Gy showed significantly improved progression-free survival (3-year rate, 40.0% vs 22.7%, p = 0.004) and OS (3-year rate, 52.4% vs 38.5%, p = 0.029). However, in patients with underlying lung disease, OS benefit from >54 Gy was not observed but considerable rates of severe pulmonary toxicities were observed (p = 0.001).

Delivery of a standard once-daily dose of 60 Gy should be considered in general. However, a modest dose of ~54 Gy may be sufficient for frail patients with underlying lung disease. Further study is needed to validate these results.