1. Introduction
Immune surveillance plays an important role in the pathogenesis and progression of non-small cell lung cancer (NSCLC). The failure of host immune surveillance mechanisms, in which lymphocytes play a pivotal role, is a key step in the early stages of tumor development [
1]. The outcome of cancer-immune interactions —“cancer immunogram”— is based on a number of largely unrelated parameters, such as tumor “foreignness” and T-cell inhibitory mechanisms, and the proposed cancer immunogram assumes that T-cell activity is the ultimate effector mechanism in human tumors [
2]. Among the parameters constituting a reasonable framework for building such an immunogram, targetable biomarkers for immunotherapy are the lymphocyte count, intratumoral T-cell infiltration, and the presence of T-cell checkpoints, such as cytotoxic T lymphocyte-associated protein 4 (CTLA4) and programmed cell death protein 1 (PD1) – known as immune checkpoints. Immune checkpoint inhibitors (ICIs) are approved for the treatment of advanced NSCLC [
3,
4].
Lymphocytes are highly sensitive to radiotherapy (RT) and a reduction in total blood lymphocyte count is a common consequence of irradiation [
5]. Reduced absolute lymphocyte count (ALC) and an elevated neutrophil-to-lymphocyte ratio (NLR) are independent negative prognostic factors for survival in many malignancies [6-11]. The occurrence of radiation-associated lymphopenia (RAL) is dependent on the field size, fraction number, and treatment duration [
12,
13]. In particular, exposure of immune-related organs, such as the lungs and heart, is associated with immunosuppression during treatment, resulting in worse patient outcomes [
8,
14]. Moreover, severe lymphopenia at the onset of immunotherapy is associated with poor survival in patients treated with ICIs [
15]. In clinical settings, following the concurrent administration of RT and ICIs, RAL at the onset of ICI therapy was found to be associated with increased mortality. The authors explained that the effector activity of ICIs relies on cytotoxic T lymphocytes, and that RAL might negate the activity of ICIs.
Therefore, in the present study, we aimed to identify blood lymphocytes as a prognostic factor for survival and identify any risk factors affecting clinically significant RAL.
4. Discussion
Platinum-based doublet concurrent chemoradiotherapy (CCRT) followed by durvalumab has been the standard treatment for eligible patients with locally advanced unresectable NSCLC, with good performance and minimal weight loss [18-20]. Suboptimal primary tumor control has led to radiation dose escalation studies aimed at achieving better treatment outcomes. RTOG 0617 was a landmark study that compared high-dose RT (74 Gy) and standard-dose RT (60 Gy) with or without cetuximab in patients with IIIA or IIIIB NSCLC [
21]. The median OS was 28.7 months for patients who received standard-dose radiotherapy and 20.3 months for those who received high-dose radiotherapy (p = 0.004). Unexpectedly, there were more treatment-related deaths among patients treated with high-dose RT.
A secondary study of the RTOG 0617 trial evaluated host immune function and tumor control [
22]. The effective dose to immune cells was modeled using the RT fractions and doses to the lungs, heart, and whole body. The 2-year OS of patients with a high effective dose to immune cells was poor. Immunotoxicity associated with RT has been shown to be a predictive factor for treatment outcomes. The immunosuppressive effects of RT include the inactivation of lymphocytes, recruitment of MDSCs and Treg lymphocytes, M2 polarization of macrophages, secretion of TGF-β, and induction of PDL1 expression on tumor cells [
23].
Lymphocytes are highly sensitive to radiation; indeed, the LD50 of lymphocytes (lethal dose required to reduce the surviving fraction of lymphocytes by 50%) is approximately 2 Gy, and the LD90 is 3 Gy [
24]. TLC remains stable during neoadjuvant chemotherapy, grade III–IV lymphocyte reduction was observed in nearly half of the patients 2 months after the initiation of radiation [
6]. Treatment-related lymphopenia is more likely to be radiation-related than chemotherapy-related. Our study showed that lymphopenia was the most common grade ≥ 3 hematologic toxicity during CCRT and lymphocytes decreased more markedly than the other blood cells immediately after CCRT. The median TLC reduction ratio was 0.74, indicating severe lymphopenia.
Joseph et al. demonstrated that lymphopenia is associated with poor survival in patients with lung cancer [
10]. In multivariate analysis, low post-treatment ALC, high pre-treatment ANC, and high PTV integral dose were associated with poor survival. The authors explained that a high pre-treatment ANC indicates cancer-induced inflammation and is useful for predicting aggressive tumor biology. Neutrophils promote tumor growth by inducing tumor growth and angiogenesis [
25]. The current study demonstrated that TLC reduction was significantly associated with PFS, and that the pretreatment NLR was significantly associated with DRFS. In many studies, a high NLR has been recognized as a poor prognostic indicator of solid cancers [
7,
11].
Tang et al. sought to determine factors associated with lymphopenia in patients receiving definitive radiotherapy for NSCLC [
14]. The results demonstrated that the GTV was better associated with the nadir of lymphocytes than with other WBCs, such as neutrophils and monocytes, during RT. In association with TLC reduction, pretreatment NLR, GTV, and heart V20 were significant predictive factors in the current study. To avoid multicollinearity, heart V2, V20, V50, and V60 were selected. Multivariate linear regression analysis showed that heart V20 was more significantly associated with TLC reduction than V60. A large volume of the critical lower dose could lead to more lymphocyte destruction (i.e., a greater “low-dose bath”) [
14]
. Contreras et al. demonstrated a relationship between lymphopenia and increased heart dose [
8]. Most patients (n = 310, 77%) underwent CCRT, and male sex, RT alone, percentage of the heart receiving ≥ 50 Gy, and a higher NLR at 4 months were found to be associated with reduced OS in multivariate analysis. In subgroup analyses of patients with stage III disease treated with CCRT, heart V50 > 25% was associated with an elevated NLR at 4 months after RT on multivariable logistic regression analysis.
When lung cancer is treated with RT, the lungs, heart, great vessel, and bone marrow are affected. We confirmed that RAL during CCRT was immediately apparent within 2–3 weeks of treatment. Nadir ALC during CCRT was significantly associated with OS, LRFS, and DRFS in the current study, although the significance disappeared in the multivariate analysis. In contrast, the NLR during CCRT was significantly associated with survival outcomes only on univariate analysis, whereas the pretreatment NLR was significantly associated with PFS and DRFS on multivariate analysis. Hence, we hypothesized that the patient’s immune status before treatment might be more important than that during or after treatment in predicting treatment outcomes.
High PDL1 expression predicts response to pembrolizumab in the primary treatment of advanced NSCLC [
26,
27]. In a meta-analysis, PDL1 expression was associated with sex, smoking status, histology, differentiation, tumor size, lymph node metastasis, TNM stage, and EGFR mutation [
28]. However, our data did not show a significant relationship between PDL1 expression and the aforementioned clinical parameters, except that patients with more advanced stage IIIB tumors tended to express PDL1. PDL1 does not appear to be a prognostic factor in patients with locally advanced NSCLC who have undergone CCRT alone [
29]. Moreover, our data did not reveal a relationship between PDL1 expression and survival outcomes; however, changes in PDL1 expression after CCRT have been shown to be associated with the prognosis of patients with locally advanced NSCLC [
30]. Indeed, the OS of patients with locally advanced NSCLC and increased PDL1 expression after CCRT was poorer than that of patients with decreased or unchanged PDL1 expression. Gong et al. showed that PDL1 expression increased after conventional fractionated radiation [
31]. Patients with negative PDL1 expression showed significantly higher objective response and disease control rates than those with positive PDL1 expression. This study demonstrated the possibility that radiotherapy plus anti-PDL1 antibody synergistically enhances antitumor immunity.
TILs are significantly associated with treatment outcomes in NSCLC and survival after therapy [
32]. Tokito et al. demonstrated that the density of CD8
+ TILs was an independent and significant predictive factor for PFS and OS in patients with locally advanced NSCLC who underwent CCRT [
29]. In the current study, CD8
+ TILs were significantly associated with PFS and marginally associated with OS. CD8
+ T cells are also significant predictors of OS and disease-free survival in early stage NSCLC [
33]. However, in the present study, no relationship was observed between CD8
+ TILs and peripheral circulating lymphocytes, and no correlation was noted between CD8+ expression and pretreatment lymphocyte count.
Previous studies have suggested that RAL may reduce the effectiveness of immunotherapy [
15,
34]. Pike et al. demonstrated the effect of radiation on lymphocyte counts and the survival of patients with metastatic cancer receiving a PD1 ICI [
15]. Severe lymphopenia at the time of ICI treatment initiation was associated with decreased survival. In a retrospective study of lymphopenia in patients receiving immunotherapy for NSCLC, radiation was a significant risk factor for peri-immunotherapy lymphopenia in multivariate logistic regression analysis [
34]. Peri-immunotherapy lymphopenia was a significant prognostic factor of both PFS and OS. Hence, greater efforts are needed to determine the optimal radiation technique that preserves ALC during radiotherapy in the era of standard immunotherapies, such as durvalumab, following CCRT in patients with unresectable NSCLC. Unfortunately, our data did not confirm a survival difference when the radiation techniques were considered. Patients treated with partial or full IMRT showed a trend towards better survival outcomes than those treated with 3D-CRT alone, albeit without statistical significance.
As we did not design a secondary analysis, we must consider some possible limitations of this study. First, although serial hematological parameters could be obtained faithfully, data on PDL1 and CD8+ TILs were obtained only from patients with available tissue specimens, and the post-radiation changes in these parameters could not be evaluated. Second, we did not scrutinize the effects of immunotherapy as a salvage therapy after tumor recurrence, which might affect overall survival. However, this is unlikely to have had a significant impact on the results because most patients in this study were enrolled before the era of immunotherapy.
In conclusion, TLC reduction during CCRT and pretreatment NLR are significant prognostic factors for PFS. In subgroup analysis, CD8+ TILs were significantly associated with PFS and marginally significantly associated with OS. Both LRFS and DRFS were significantly dependent on TLC reduction during CCRT, whereas DRFS was associated with pretreatment NLR. TLC reduction during CCRT is closely associated with GTV, pretreatment NLR, and heart V20. Thus, efforts are needed to reduce TLC by constraining the volume of the radiation dose to the entire heart. Further studies should focus on developing precise RT techniques to overcome RAL.
Author Contributions
Conceptualization, S.-J.A. and Y.-H.K.; methodology, S.-J.A. and Y.-H.K.; software, Y.-H.K.; validation, Y.-H.K., Y.-D.C., S.-J.A., Y.-C.K., I.-J.O., C.-K.P., T.-K.N., M.S.Y., J.-U.J., I.J.C., J.-Y.S., and S.C.; formal analysis, Y.-H.K., Y.-D.C. and S.-J.A.; investigation, Y.-H.K., Y.-D.C. and S.-J.A.; resources, S.-J.A., Y.-C.K., I.-J.O., C.-K.P., T.-K.N., M.S.Y. and J.-U.J.; data curation, Y.-H.K., Y.-D.C., S.-J.A., Y.-C.K., I.-J.O., C.-K.P., T.-K.N., M.S.Y. and J.-U.J.; writing—original draft preparation, Y.-H.K., Y.-D.C., S.-J.A., Y.-C.K., I.-J.O., C.-K.P., T.-K.N., M.S.Y., J.-U.J., I.J.C., J.-Y.S., and S.C.; writing—review and editing, Y.-H.K., Y.-D.C., S.-J.A., Y.-C.K., I.-J.O., C.-K.P., T.-K.N., M.S.Y., J.-U.J., I.J.C., J.-Y.S., and S.C.; visualization, Y.-H.K. and Y.-D.C.; supervision, Y.-H.K., Y.-D.C., S.-J.A., Y.-C.K., I.-J.O., C.-K.P., T.-K.N., M.S.Y. and J.-U.J.; project administration, S.-J.A. and Y.-H.K.; All authors have read and agreed to the published version of the manuscript.