This study reported results of PBT for 54 patients of inoperable stage I to III NSCLC with poor lung functions (defined as FEV1 ≤1.0 L, or FEV1≤50% predicted or DLco ≤50% or pulmonary fibrosis) due to underlying lung diseases. With a median follow up 19.8 months, this study reported no significant changes in the predicted FEV1 and DLco from baseline to month 7 after PBT treatment. Seven patients (13.0%) who developed grade 3 or higher pulmonary toxicities. The 2-year OS rate was 71.1%.