Objective:To investigate the value of peripheral blood neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR) and serum lactate dehydrogenase (LDH) levels for predicting the occurrence of radiation pneumonia (RP) in small cell lung cancer.Methods:A total of 84 patients with small cell lung cancer who received image-guided radiotherapy in Xuzhou Cancer Hospital between September 2019 and September 2022 were retrospectively analyzed. They were divided into an RP group ( n = 25) and a non-RP group ( n = 59) according to whether RP occurred. Peripheral blood NLR and PLR and serum LDH levels were compared between the two groups before and after radiotherapy. The receiver operating characteristic curve (ROC curve) was used to analyze the value of peripheral blood NLR, PLR, and serum LDH levels for the diagnosis of RP in small cell lung cancer. Results:Before radiotherapy, there were no significant differences in peripheral blood NLR and PLR between the two groups (both P > 0.05). After radiotherapy, peripheral blood NLR and PLR in the RP group were (3.39 ± 0.81) and (129.06 ± 24.90), respectively, which were significantly higher than those in the non-RP group [(2.54 ± 0.71), (104.76 ± 26.26), t = 3.61, 3.83, both P < 0.05]. The NLR (2.86 ± 0.30) and PLR (110.07 ± 10.05) were the lowest in patients with grade 2 RP and they were highest in patients with grade 4 RP [(4.49 ± 0.63), (168.88 ± 14.11)]. The grade of RP was positively correlated with peripheral blood NLR and PLR. The sensitivity of peripheral blood NLR in the diagnosis of RP was 88.0%, the specificity was 66.1%, and the area under the curve (AUC) was 0.791. The sensitivity of PLR in the diagnosis of RP was 48.0%, the specificity was 94.9%, and the AUC was 0.735. The sensitivity of NLR combined with PLR in the diagnosis of RP was 92.0%, the specificity was 59.3%, and the AUC was 0.801. There was no significant difference in serum LDH levels between the two groups before and after radiotherapy (both P > 0.05). Logistic regression analysis showed that NLR and PLR were risk factors for RP in patients with small cell lung cancer ( OR = 2.309, 1.037; 95% CI: 1.061-5.024, 1.004-1.071). Conclusion:In patients with small cell lung cancer who develop RP, peripheral blood NLR, and PLR are markedly elevated compared with those in patients who do not develop RP, and combined detection of peripheral blood NLR and PLR has a high value for early diagnosis of RP in patients with small cell lung cancer.