This study aimed to examine late radiological changes after proton beam therapy (PBT) for early-stage non-small cell lung cancer (NSCLC) and to clarify correlations between mass-like radiological changes and patient characteristics. CT scans of patients who underwent passive scattering PBT for T1-2N0M0 NSCLC were analyzed retrospectively. Patients were considered eligible if follow-up CT was performed for at least 2 years, with no definite evidence of local recurrence. The following five periods were defined: (i) 6-12 months, (ii) 12-24 months, (iii) 24-36 months, (iv) 36-48 months and (v) 48-60 months after PBT. Late (≥6 months) radiological changes were scored by consensus of three radiation oncologists according to classifications set forth by Koenig (Radiation injury of the lung after three-dimensional conformal radiation therapy. AJR Am J Roentgenol 2002;178:1383-8.). CT scans of 113 patients (median follow-up, 36 months; range, 24-137 months) were evaluated. Late radiological changes during Periods (i), (ii), (iii), (iv) and (v) included modified conventional pattern (80%, 79%, 72%, 58% and 56%, respectively), mass-like changes (8%, 9%, 14%, 22% and 18%, respectively), scar-like changes (4%, 9%, 11%, 17% and 24%, respectively) and no increased density (8%, 3%, 3%, 2% and 2%, respectively). Mass-like changes were observed in 23 patients (20%). Among patients who developed mass-like changes, the median interval between the initiation of PBT and the onset of mass-like changes was 19 months (range, 6-62 months). In multivariate analysis, a peripheral location was found to be a significant factor (P = 0.035; odds ratio: 4.44; 95% confidence interval: 1.12-21.28). In conclusion, mass-like changes were observed in 20% of patients who underwent PBT. Patients with peripheral tumors showed a higher incidence of mass-like changes.