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Abstract Backgroud The underlying molecular and cellular mechanisms of radiation pneumonitis (RP) are very complex. Several biological factors need to be considered together with the well known dosimetric parameters for understanding the molecular events in developing RP in lung cancer patients. The aim of this study was to correlate the variations of the cytokine levels in lung cancer patients during radiation therapy (RT) with the occurrence of symptomatic RP. Methods Thirty-four lung cancer patients who received three-dimensional conformal radiation therapy were evaluated prospectively. Serial blood samples before, at the beginning, in the middle of, at the end of RT and 2 and 4 weeks after RT were analyzed for IL-1α, IL-6, IL-10, TNF-α and TGF-β1 by performing enzyme-linked immunosorbent assay. The predictive values of dosimetric factors for RP were evaluated, too. Results Overall, 8 patients (23.5%) had grade ≥ 2 RP. By serial measurement of cytokines level, only the TGF-β1 level showed a correlation to the symptomatic RP. None of the other cytokines, IL-1α, IL-6, IL-10 and TNF-α level was correlated with the risk of RP. The mean pretreatment TGF-β1 level did not differ between RP and non-RP groups. However, during the period of radiation treatment, the TGF-β1 level began to increase at the end of RT in the RP group and became significantly higher 4 weeks after RT (p = 0.007). Using an ANOVA model for repeated-measures, we found significant associations between the changes of TGF-β1 during the time course of the RT and the risk of developing RP (p < 0.001). Most of the dosimetric factors showed a significant association with RP. Conclusion Our results show that the changes of TGF-β1 could be correlated with RP and the incorporation of the biological parameters into the dosimetric data could be useful for predicting symptomatic RP.