Based on our results, N Stage, PE, ILD, ipsilateral Radscore and contralateral Radscore were independent risk factors for the development of RP in LA-NSCLC patients after IMRT. Using three machine learning algorithms, we built three models based on these features to predict RP, and selected the random forest model as the most optimal model according to training and external validation results.
Dose-volume indices have been studied for decades and they are widely known as significant risk factors for RP. In particular, lung mean and lung V20 are generally recognized criteria[31]. However, we did not observe an association between the lung dosimetric parameter and the risk of RP in our cohort, which seemed to be against consensus. Several similar studies in recent years reached the same conclusion[32, 33]. The results could be explained by the technological advancement, which made the influence of dosimetric parameters minimized to a non-significant level. IMRT is a more sophisticated techniques of conformal RT technologies and is associated with a lower incidence of RILI compared with standard 3D-CRT. Based on previous research results from 3D-CRT era[10], strict lung dose-volume constraint is now particularly emphasized in the IMRT era. Previous studies [34]suggested that the V5 of total lung should be maintained at < 60–65%, V20 should be maintained at < 30% and mean lung dose < 20 Gy to reduce the risk of RP[9–15]. Within both cohorts in our study, no violations at V20 and MLD thresholds and only a few and mild surpasses at V5 and V30 were observed in the included patients. These results reflect to some extent our ability to implement precision radiotherapy by using IMRT technique. In this case, the conventional dosimetric parameters may no longer be useful as independent predictors for predicting RP. Although the variations of the above-mentioned dose-volume indices among individuals are minimized by the application of strict dose–volume constraints, the spatial dose distributions can vary widely due to different radiotherapy plans. Several studies had been conducted to explore more deeply the interrelated effects of dose and morphology. It is reported recently that spatial dose distributions, instead of conventional dosimetric parameters, are intimately correlate with the incidence RP[22, 35, 36]. On the other hand, individual variations warrant more concern in the estimation of RP risk when dose-volume indices are severely restricted to prevent RP.
In the current study, the N stage, ILD grade, PE grade had been confirmed to be closely related to the occurrence of RP [14, 17, 37]. Okubo M suggested a significant association between subclinical ILD and the occurrence of grade 2 or higher radiation pneumonitis[38]. Zhou and team observed a strong correlation between the severity of PE and the higher cumulative incidence of symptomatic or severe radiation pneumonitis in locally advanced NSCLC[39]. Thoracic radiation may induce lymphocyte-mediated hypersensitivity reactions, which could exacerbate acute ILD. Therefore, accurate pre-radiotherapy localization of the treatment area and precise assessment of ILD and PE are crucial. In patients with stage N2 and N3 NSCLC, the higher tumor burden often indicates a more complex pathophysiological process. Definitive radiotherapy must cover a wide area to ensure adequate treatment of both the tumor and its lymphatic metastases[40]. The extensive mediastinal lymph node metastasis (N2-N3) leads to higher doses of radiation being delivered to portions of the lung tissue. However, this unavoidably increases the risk of irradiating part of normal lung tissue. The CT radiomics signature of lungs may represent different lung phenotype resulting from underlying lung diseases, the baseline lung status and the underlying radiosensitivity of lung tissue[41, 42]. Previous studies have reported that CT-based radiomics features can help to predict pulmonary toxicity for NSCLC patients receiving radiotherapy or immune checkpoint inhibitor [22, 36, 43]. In this study, radiomics method was applied to extract large numbers of radiomics features from pretreatment CT images. The addition of CT radiomics signature provided a more comprehensive measurement of individual differences and improved the ability of models to predict RP with higher AUC values.
In this study, we built three models through different machine learning algorithms. The random forest model yielded excellent AUC values of 0.938 for the training set and 0.885 for the external validation set, outperformed the traditional logistic regression algorithm and decision tree algorithm. Machine learning, a scientific discipline that enables computers to learn from experiences, has recently been shown to have better performances over traditional statistical modeling approaches[44–46]. The random forest is a machine learning method based on an ensemble of decision trees generated by random feature sets. It has been introduced as an accurate machine learning method the prediction of radiation-induced toxicities [47, 48]. In a study of 203 patients with LA-NSCLC, performed multivariate analysis using random forest to assess the combined performance of important predictors of RP with an AUC of 0.66 (p = 0.0005) [47]. According to a recent secondary analysis of RTOG 0617 to identify dosimetric predictors of toxicity in patients with LA-NSCLC using machine learning and artificial intelligence, the random forest model showed the best ability to predict the pulmonary toxicities with an AUC of 0.706, outperforming logistic regression[48]. Our random forest model seems to have performed better than the previous established models using the same methods and other machine learning approaches [35, 49].
Our study has several limitations. First, this was a small sample retrospective study, a selection bias may exist. However, participant recruitment was conducted at two institutions in this study, which may increase some representativeness. It is necessary to validate these models in larger datasets to stabilize our models. Second, even though random forest model could be used to predict multiple outcomes, our model can only predict the presence or absence of RP ≥ 2 but not each grade. This is because the low diversity of RP grades does not allow sufficient statistical power to classify RP grades. Third, only risk factors at the clinical level were included in the established model. In fact, the mechanism underlying RP is multifaceted and needs deeper and more extensive research. Recently, predicting models integrating other omics such as proteomics and genomics have been established[32, 50, 51]. Integration of multi-omics and machine learning is an important next step for accurate RP risk prediction.