At present, severe RP is one of the most important clinically relevant toxicity of thoracic radiation for patients with locally-advanced NSCLC, which not only affect the following treatment of patients, but also severely influenced the life quality and long-term survival. Thus, reducing the occurrence of severe RP is a critical goal for clinicians. While nowadays, there were still no effective method to predict the incidence of severe RP. In the present study, we collected data from 351 patients with locally-advanced NSCLC patients and after a long-term follow-up of up to 52.9 months, our data indicated that subclinical ILD, contralateral V5 > 11%, ipsilateralV20 > 45%, pre-RT dNLR > 1.9 and post-RT SIRI > 3.4 were the independent prognosticators of severe RP among patients with locally-advanced NSCLC receiving thoracic RT. The internal validation of the constructed nomogram demonstrated its superiority compared with any single hematological, dosimetric or clinical factor alone. These findings can be used to accurately identify patients with high risk for severe RP, and indicated that individualized and precise radiotherapy regimens can significantly reduce the occurrence of severe RP and can improve the prognosis of RP and the quality of life of patients.
Compared to other similar researches[29, 32, 33], it must be pointed out that to the best of our knowledge, our study is the first to systematically integrate clinical factors, peripheral blood biomarkers and dose-volume parameters to develop a predictive model for the occurrence of severe RP in such a large sample of patients with locally-advanced NSCLC treated with thoracic radiotherapy.
Our study reported that 9.7% patients developed severe RP, which was in accordance with those reported in previous studies (about 5-11.7%) [34–36]. And in our study, severe RP conferred a worse overall survival comparing with mild RP (30.8m vs. NR, p = 0.027), similar conclusions were reached by other recent studies[37, 38], while there was no significant difference between RP and non-RP patients (20.4m vs. 17.6m, p = 0.330). It underlined that early detection and timely intervention of severe RP is of great importance, which could help prolong the survival of patients.
We confirmed that patients with ILD intended to have higher risk of severe RP, it was in line with our preliminary studies [39, 40], and similar results were also in reported by other researches[23, 41]. Due to similar mechanisms, lung cancer (LC) patients have a high incidence of ILD, and ILD patients are also prone to develop lung cancer[42, 43]. Thoracic radiotherapy can be a choice for LC-ILD patients, while radiotherapy is contra-indicated in severe ILD, producing RP rates of up to 43%[44]. So clinical radiotherapy decisions must be cautious about the risk of severe RP in patients with ILD, and more conservative limits of lung dose should be used with a diagnosis or radiologic evidence of ILD, and specific measures should be taken in the early period of RP.
For patients with locally advanced NSCLC treated with definitive chemo-radiotherapy, the NCCN guideline recommended lung dose–volume constraints for conventionally fractionated RT: total lung V20 ≤ 35–40% and MLD ≤ 20 Gy[5], while there were there is still no principle or recommendation for evidence-based medicine for the dose limits of contralateral or ipsilateral lung dose. In present study, contralateral V5(> 11%) and ipsilateral V20(> 45%) showed significant associations with severe RP incidence. Bongers’s research have shown that contraV5 were predictor for grade ≥ 3 RP[45], while the optimal cutoff point has not been confirmed. And Zhao’s research also indicated that contralateral V5 were related to the clinical outcome of patients with severe RP (p = 0.057)[33]. One study by Ramella showed that ipsiV20 could effectively dividing patients into high-risk and low-risk groups for RP with the threshold value as 52%[26]. And Agrawal’s study found that ipsiV20 were significantly correlating with RP on univariate analysis, and mean ipsiV20 were 60% for patients with RP[46]. The threshold of ipsiV20 in our study is lower than in mentioned studies, it may because with the development of multidisciplinary therapy, systemic administration may increase the occurrence of RP, resulting in a decrease in the threshold of ipsilateral lung dose.
A growing body of evidence indicates that blood-based biomarkers can be used to predict radiation-related toxicity [47]. Based on our analysis in present study, we found pre-RT dNLR greater than 1.9 and post-RT SIRI greater than 3.4 were the most significant risk hematological biomarker for the development of severe RP. For dNLR, several researches have shown its relation to worse OS and other treatment toxicities, for example, Hsiang found that elevated pre-RT dNLR is associated with worse OS and development of liver toxicity for patients with hepatocellular carcinoma after stereotactic body radiotherapy (SBRT), and dNLR ≥ 1.9 was an optimal cut-off value for determining liver toxicity risk[48], which was exactly consistent with our findings. Cox’s research showed that an elevated pre-treatment dNLR was an independent prognostic biomarker for OS and PFS in oesophageal cancer patients treated with definitive CRT with the cutoff of 2[49], which was close to our data. Although the potential mechanisms are unclear, it might be due to that RP is a kind of immune-mediated hypersensitive pneumonia actually[50], and T lymphocyte subsets play a dominant role in the cellular immune response and may be involved in RP[51]. What’s more, patients with lower lymphocyte count have been shown to have negative impact on the immune system, leading to the development of infections[52]. For SIRI, our previous study has demonstrated that pretreatment SIRI are independent predictors of OS in stage Ⅲ NSCLC[53]. As previously reported, decreased lymphocyte count after RT may be a clinical indicator in the occurrence of RP[29], and even the severity of RP was associated with the degree of lymphocyte decrease[54], and the potential mechanism of lymphopenia after RT might be the effect of local irradiation of circulating lymphocyte in the blood pool[55]. And in the early phase of RP, after alveolar and interstitial edema is formed, then inflammatory cells outside like monocyte-derived pulmonary macrophages and neutrophils are recruited and accumulated here to effect action[56]. Thus, the relationship between post-RT SIRI and RP are possibly related to different changes in immune cells, including the decrease of lymphocytes and the increase of neutrophils and monocytes after RT.
Several limitations should be mentioned here. Firstly, due to its retrospective nature, selective bias existed in the present study, thus more prospective studies are necessary in the future. Secondly, although the internal validation showed a relatively excellent AUC (0.782) of the present predictive model, the result would be more convincing if it had been verified by an external validation, and the model needs to be further modified and verified in future studies before applied to the clinic. What’s more, there are several biomarkers including the vitronectin (VTN) and the single nucleotide polymorphisms (SNPs) have also been investigated and considered as predictors of RP recently[57, 58], hence from the perspective of precision medicine, a more comprehensive prediction model for severe RP combining the individual genomic information, dosimetric, hematological and clinical parameters should be developed in the future.