18F-Fluorodeoxyglucose-Positron Emission Tomography/Computed Tomography-Guided Chest Radiotherapy Improves Survival in Patients with Extracranial Oligometastatic Non-small Cell Lung Cancer

Abstract

Background This retrospective study compared positron emission tomography (PET)/computed tomography (CT) and CT in the treatment of extracranial oligometastatic non-small-cell lung cancer (NSCLC) and explored the impact of chest radiotherapy (CRT) on patient survival.

Methods We reviewed the medical records of Chinese patients with stage IV extracranial oligometastatic NSCLC who underwent PET/CT or CT at two centers. Propensity score matching (PSM) was used to control differences in patient characteristics between the maintenance chemotherapy alone and CRT plus maintenance chemotherapy groups.

Results We analyzed 192 eligible patients. The median survival time was significantly longer in the CRT plus maintenance group than in the chemotherapy alone group (13 months vs. 8 months, p < 0.001). Subgroup analysis showed no statistical difference in survival between both groups in patients who underwent CT examinations (n=98, 8 months vs. 5 months, p = 0.180). However, differences in survival between both groups were statistically significant in patients who underwent PET/CT examinations (n=94, 25 months vs. 11 months, p < 0.001). A multifactorial analysis revealed a more favorable prognosis in patients who underwent PET/CT evaluation (HR: 0.343, 95% CI: 0.250-0.471, p < 0.001) and CRT (HR: 0.624, 95% CI: 0.464-0.840, p = 0.002) than in those who did not. The conclusion obtained after PSM is consistent with the above mentioned results.

Conclusions PET/CT-guided CRT is associated with improved clinical outcomes in patients with stage IV extracranial oligometastatic NSCLC.

1. Introduction

Oligometastasis is a distant metastasis with a limited number and distribution of tumors. Patients with oligometastasis usually have a maximum of 3-5 metastatic sites, excluding the primary site, and no more than 3 different organs that can be treated locally using surgery, radiation therapy, and other treatments to improve survival [1]. Currently, oligometastasis is considered to be the transitional stage of advanced tumors from inducing limited to extensive metastases, with the former metastases having a significantly different biology and treatment options than the latter metastases [2]. This is a shift from the conventional oncological thinking, which considers metastasis as an end-stage disease with limited treatment options. In patients with non-small-cell lung cancer (NSCLC) receiving systemic therapy, such as chemotherapy, the most common progression site is the original disease site [3-6]. Our previous studies have shown that aggressive local consolidation therapy may destroy lesions, slow disease progression, and even improve patient survival [7].

Radiation therapy is a local NSCLC treatment. The clinical outline of the gross tumor volume (GTV) is based on morphological information, such as tissue density and anatomical structure, as provided by computed tomography (CT), but not on corresponding metabolic information. Consequently, potential occult metastases are often not detected, and the information provided is not sufficient to allow clinical radiation therapy[8]. ¹⁸F-fluorodeoxyglucose positron emission tomography-CT (¹⁸FDG-PET /CT) is of great value for early lung cancer detection, differential diagnoses of isolated nodules in lungs and mediastinum lymph nodes, clinical staging and detection of distant metastases, treatment efficacy determination, recurrence and metastasis follow-ups, and prognosis [9, 10]. ¹⁸FDG, the most widely used PET imaging agent in clinical practice, is brought into the cell, and then ¹⁸FDG-PET imaging distinguishes between benign and malignant lesions based on glucose metabolism, which is significantly correlated with tumor pathological response [11]. Most lung cancers are highly metabolic in nature, and abnormal cancer cell proliferation requires glucose overutilization. Accordingly, ¹⁸FDG-PET/CT has the advantages of both functional and anatomical structure imaging. ¹⁸FDG-PET/CT is better than traditional imaging methods and is increasingly recognized by clinicians for the diagnosis, treatment, and treatment efficacy evaluation of lung cancer and other tumors [12-14].

We demonstrated that local consolidation therapy (LCT) is superior to maintenance therapy in patients with oligometastatic NSCLC receiving PET/CT guidance [7]. However, we did not directly compare lung cancer patients who underwent PET/CT to those who underwent conventional CT, and there is no direct evidence to confirm a significant difference in the prognostic benefit between these two screening methods in patients with oligometastatic NSCLC. Undertreatment may significantly contribute to treatment failure or tumor recurrence, but overtreatment can cause serious side effects and physical damage. Minimizing unnecessary treatment while ensuring efficacy is important for the survival and quality of life of patients with advanced lung cancer. To further improve the diagnosis and treatment of oligometastatic NSCLC, we retrospectively studied the medical records of patients with oligometastatic NSCLC who underwent PET/CT and conventional CT at two centers to investigate the differences between both examination methods and the impact of radiation therapy to the primary lung cancer site on patient prognosis.

2. Materials And Methods

2.1 Study population

We retrospectively analyzed Chinese patients with stage IV extracranial oligometastatic NSCLC who had underwent PET/CT or CT scans at the First Hospital of China Medical University and Liaoning Cancer Hospital from February 2013 to November 2019. From patient medical records, we assessed age, gender, smoking history, patient status, tumor size, lymphatic metastases, histologic type, primary tumor site, treatment choice, and date of death. Inclusion criteria were patients with extracranial oligometastatic NSCLC with complete medical records who received chest radiotherapy (CRT) with maintenance or only maintenance therapy. PET/CT or CT was conducted on selected patients within one month before treatment. Patients with major organ dysfunction, more than one primary tumor, unknown metastatic status, or multiple metastases (number of transferring stoves >5), malignant pleural effusion, or underwent targeted therapy or immunotherapy were excluded.

Propensity score matching (PSM, The PSM was performed using age and sex) was used to create a maintenance chemotherapy alone and a chest radiotherapy (CRT) plus maintenance chemotherapy groups to reduce the effects of selection bias and confounding variables. The PSM function of the IBM SPSS software (IBM Co., Armonk, NY, USA) was used to estimate the propensity score, and 1:1 nearest neighbor matching was used with a caliper width of 0.02 for PSM. A chi-square test was used to check the covariate balance of each subgroup before and after PSM.

2.2 Statistical analysis

The number and clinical characteristics of all eligible patients were counted, and the chi-square test was used. The time from each patient’s first oligometastatic diagnosis to the last follow-up or death was recorded as overall survival time. The survival curves of different variables were determined using the kaplan-meier method and then checked by a log-rank test. Results were reported as hazard ratios (HRs) and 95% confidence intervals (CIs). Forest plots were used to describe the subgroup analysis of all data. According to the univariate analysis results, significant variables ( p < 0.1 ) were selected and put into a Cox regression analysis and then a multivariate analysis to ensure that significant variables ( p < 0.05 ) were remarkably associated with the prognosis of patients with extracranial oligometastatic NSCLC. IBM SPSS was used for statistical analysis, and GraphPad Prism8 was used to draw the survival curve and forest plots.

3. Results

3.1 Patient Characteristics

From analyses, 192 of a total of 532 patients with advanced lung cancer at two medical centers were selected; 113 patients were male, and 79 were female, with a median age of 60 years. We used PSM to select 178 eligible patients of 192 patients with extracranial oligometastatic NSCLC and divided them into maintenance chemotherapy alone and CRT plus maintenance chemotherapy groups (Figure 1).

Among all patients, the one-year and two-year overall survival rates were 41.7% and 15.1%, respectively, and the median survival time was 10.5 months. Patient demographic and clinical characteristics are presented in Table 1.

3.2 Univariate analysis

Considering tumor heterogeneity, we did not examine the relationship between different chemotherapy regimens and survival time. Important variables included in the univariate analysis before and after PSM, including T and N classifications, PET/CT and CRT are presented in Table 2 and 3 (all p <0.1). The median survival time was better in patients who received PET/CT than in those who only received CT (before PSM: 16 months vs 6 months; and after PSM: 17 months vs 7 months, both, p<0.001). The median survival time was significantly better in the CRT plus maintenance group than in the chemotherapy alone group (before PSM: 13 months vs 8 months, p<0.001; and after PSM: 13 months vs 8 months, both, p =0.001).

3.3 Survival analysis and subgroup analysis

Figure 2 and Figure 3 showed that PET/CT and CRT were significantly associated with improved survival times. A subgroup analysis showed no statistical difference in survival between the chemotherapy alone and CRT plus maintenance groups for those underwent CT examination (before PSM: 8 months vs 5 months, p = 0.180 ; and after PSM: 8 months vs 5 months, p = 0.185). However, for those underwent PET/CT examination, there was a significant difference between the aforementioned groups (before PSM:25 months vs 11 months; and after PSM: 25 months vs 11 months, both, p <0.001).

4. Discussion

Traditional NSCLC staging is based primarily on history, fiberoptic bronchoscopy, CT, magnetic resonance imaging, and other routine imaging examinations. If the lung cancer stage is inaccurate, an inappropriate treatment regimen will inevitably have an adverse impact on patient survival. PET/CT is an advanced diagnostic medical imaging technique that shows local metabolic function at a refined anatomical level, with a superior sensitivity and specificity than other examination methods, and allows a more effective detection of asymptomatic metastatic foci and metastatic lymph nodes [15]. Staging is more accurate when both metabolic function and anatomical information are assessed [16]. PET/CT can also improve the accuracy of depicting the target area for radiotherapy, avoiding missed target areas or increased exposure. Using PET/CT during an NSCLC radiotherapy program improves target volume delineation reliability and allows higher radiation doses without increasing side effect risks [17]. Accordingly, the volume of the scheduled irradiation target can be more accurately limited to morphologically and functionally defined tumor areas. Fewer normal tissues can be irradiated and higher total tumor doses can be used to seek optimal therapeutic effects and develop more rational treatments. Studies report that occult extrathoracic metastases can be found in up to 37% of patients with advanced NSCLC, altering 14%-26% of NSCLC treatment decisions [18]. Although PET/CT benefits on NSCLC diagnosis have been widely reported, we compared the prognosis of patients who underwent PET/CT and CT and found that PET/CT-localized CRT improved survival, while CT imaging-guided radiotherapy did not, indicating a significant PET/CT advantage in extracranial oligometastatic NSCLC diagnosis. PET/CT has become the standard imaging tool for characterizing lung nodules [19], initial staging [20, 21], treatment planning, treatment response assessment [22], recurrence staging [23, 24], and lung cancer monitoring. The widespread clinical use of FDG-PET/CT in patients with lung cancer has improved staging and restaging accuracies, allowing for better treatment planning and treatment response assessments.

Local consolidation therapy is a common treatment for oligometastatic NSCLC. In 2016, Gomez et al. was the first to report the results of a phase II randomized trial that compared standard maintenance therapy (n=24) to local consolidation therapy (n=25) [25]. The median patient follow-up was 12.39 months, and the progression-free survival (PFS) was significantly better in the maintenance treatment group than in the local consolidation group (11.9 months vs. 3.9 months, p=0.0054). Adverse events were similar between both groups, with no treatment-induced grade 4 adverse events or deaths. Gomez et al. published the results of their latest long-term clinical study in 2019 and successfully confirmed that PFS (median, 14.2 months [95% CI, 7.4 to 23.1 months] with LCT vs. 4.4 months [ 95% CI, 2.2 to 8.3 months] with maintenance therapy or observation; P=0.022) and overall survival (median, 41.2 months [95% CI, 18.9 months to not reached] with LCT vs. 17.0 months [95% CI, 10.1 to 39.8 months] with maintenance therapy or observation; P=0.017) significantly improved by the early inclusion of LCT [26]. They also reported that both initial LCT before progression and delayed LCT after progression contributed to improved overall survival. In 2018, Iyengar et al. reported results from a phase II randomized trial comparing standard maintenance therapy with and without stereotactic ablative radiotherapy (SABR) in a patient population nearly identical to that of the trial reported by Gomez et al. and showed that PFS was significantly better in the SABR plus maintenance chemotherapy group than in the maintenance chemotherapy alone group (9.7 months vs. 3.5 months, p=0.01) [27]. Gomez et al. and Iyengar et al. both conducted phase II trials in patients with oligometastatic NSCLC to study the prognostic impact of aggressive local therapy. Both trials reported a significant increase in PFS by increasing aggressive local therapy. Moreover, our previous study showed that PET/CT-guided LCT was significantly efficient in patients with oligometastatic advanced lung cancer [7]. Overall survival rates were extremely higher in the LCT group than in the chemotherapy alone group (13 months vs. 7 months, p=0.002). Side effects incidence was similar between LCT and chemotherapy alone groups, and there were no treatment-related adverse outcomes or deaths. To explore more effective and personalized treatment options for oligometastatic lung cancer, we focused on evaluating the efficacy of extracranial oligometastases treated with CRT and revealed that similar efficacy was achieved with radiation therapy to the primary lung site alone compared to LCT, meaning that it is possible to achieve satisfactory outcomes with treatments that cause less damage to patients with extracranial oligometastatic NSCLC than maintenance treatments. There are several possible mechanisms that could explain the survival benefits of lung radiotherapy. First, after systemic treatment, hard-to-treat malignant cells, which are unlikely to be eliminated by subsequent maintenance therapy and can serve as a source of following metastatic spread, are left behind. However, in such cases, CRT may reduce the number of such cells. Second, CRT may enhance the effects of systemic therapy by possibly making residual lesions more sensitive to subsequent maintenance therapy. A third possibility is that radiotherapy kills tumor cells by modulating the immune system. Radiotherapists are often subjective in the process of outlining tumor target areas based on CT images, and the influence of experience on the accuracy of tumor target area outlining can further affect the efficacy of radiotherapy. In particular, patients with advanced lung cancer often have complications such as pulmonary atelectasis, which is difficult to identify with the tumor lesions. Therefore, we believe that CT plays a limited role in the process of target area outlining, and it is difficult to improve the prognosis of lung cancer patients. A well-defined lesion is shown in PET/CT, which can exclude the influence of subjective factors on the accuracy of target area outlining and thus improve the efficacy of radiotherapy.

Immunotherapy was not included in this study because the patient information we analyzed was first obtained in 2013. As research progresses, it is increasingly recognized that there are complex interactions between radiotherapy and the immune system. In addition to producing a local therapeutic effect at the irradiated tumor site, radiotherapy can also cause spontaneous tumor regression in non-irradiated lesions; this is known as the abscopal effect [28]. Although the abscopal effect was studied for decades, the exact mechanism of this phenomenon is still unclear, and Demaria et al. [29] first linked the distal effect of radiotherapy to an immune-mediated mechanism. Preclinical studies suggest that radiotherapy is equivalent to an "agonist" in immunotherapy, making tumor cells more susceptible to T-cell-mediated immune attacks by modulating the immune system. Radiotherapy can enhance anti-tumor immune effects by inducing the release of more neoantigens from damaged tumor cells, enhancing the expression of major histocompatibility complex (MHC) class I molecules, and upregulating chemokines, cell adhesion molecules, and other immunomodulatory cell surface molecules, thereby inducing immunogenic cell death [30]. In terms of increasing attention to immunotherapy, the possibility of combining radiotherapy with immunotherapy is worth exploring, and this combination to produce synergistic antitumor activity shows great application prospects and development potential in the future.

5. Conclusions

PET/CT-guided chest radiation therapy is associated with improved clinical outcomes in patients with stage IV extracranial oligometastatic NSCLC. Advances in radiotherapy technology improved radiotherapy precision and ensured treatment safety, while improving radiotherapy effectiveness. Undoubtedly, the continuous optimization and innovation of radiotherapy technology in positioning, target area outlining, planning, and beam projection will continue to be a strong driving force for applying and developing radiotherapy in patients with lung cancer in the future. Future basic researches and large-sample randomized clinical trials on how to further optimize combinations of radiotherapy and systemic therapy, specifically in terms of targeted therapy and immunotherapy, in patients with lung cancer will be conducted.

Declarations

Author Contributions: Writing the original draft: TLW,CSL and YQS; formal analysis: CSL and YQS; data curation: TLW; software management: CYW; project administration: ZW; validation: GL; visualization: LH; draft review & editing: all authors; funding acquisition: TLW, YQS, additional resources: TLW.

Funding: This study has received funding by the Cancer Research Program of National Cancer Center[NCC2017A08],the Liaoning Province Natural Science Foundation [20180550741,2020-ZLLH-47],the Liaoning Province Key Area Joint Open Fund [2019-KF-01-01],the Shenyang Major Scientific Research Projects [19-112-4-090],the Tumor Mass spectrometry Center project[ZP202013, ZP202008],Key Laboratory of Tumor Radiosensitization and Normal Tissue Radioprotection Project of Liaoning Province [2018225102].

Institutional Review Board Statement: This statistical study was thoroughly reviewed and approved by the ethics committees of the First Hospital of China Medical University and the Liaoning Cancer Hospital. The whole research process was legal, reasonable, open, transparent, and strictly complied with the Declaration of Helsinki of 1964 and its later amendments or comparable ethical standards. The requirement for informed consent was waived due to the retrospective nature of this study.

Informed Consent Statement: The requirement for informed consent was waived due to the retrospective nature of this study.

Data Availability Statement: All data in this study are true and reliable.

Acknowledgments: We would like to thank all patients and their families for their active cooperation and all hospital staff for their assistance in the study.

Conflicts of Interest: The authors of this manuscript declare no relationships with any companies, whose products or services may be related to the subject matter of the article.

References

1. Hellman S., Weichselbaum R R.(1995). Oligometastases. J Clin Oncol, 13(1), 8-10. doi:10.1200/JCO.1995.13.1.8.
2. Kalinauskaite Goda G,Tinhofer Ingeborg I,Kufeld Markus M, et al. Radiosurgery and fractionated stereotactic body radiotherapy for patients with lung oligometastases.[J] .BMC Cancer, 2020, 20: 404. doi:10.1186/s12885-020-06892-4.
3. Weichselbaum RR. The 46th David A. Karnofsky Memorial Award Lecture: Oligometastasis-From Conception to T reatment. J Clin Oncol 2018.
4. Foster CC, Pitroda SP , Weichselbaum RR. Staging the Metastatic Spectrum Through Integration of Clinical and Molecular Features. J Clin Oncol 2019;37:1270-6. doi:10.1200/JCO.18.02021.
5. Pitroda SP , Weichselbaum RR. Integrated molecular and clinical staging defines the spectrum of metastatic cancer. Nat Rev Clin Oncol 2019;16:581-8.doi:10.1038/s41571-019-0220-6.
6. Iyengar P , Wardak Z, Gerber DE, et al. Consolidative Radiotherapy for Limited Metastatic Non-Small-Cell Lung Cancer: A Phase 2 Randomized Clinical T rial. JAMA Oncol 2018;4:e173501.doi:10.1001/jamaoncol.2017.3501.
7. Song Ying-Qiu,Wang Nan,Qiao Yun, et al. Treatment patterns and survival after 18F-fluorodeoxyglucose positron emission tomography/computed tomography-guided local consolidation therapy for oligometastatic non-small cell lung cancer: a two-center propensity score-matched analysis.[J] .J Cancer Res Clin Oncol, 2020, 146: 1021-1031.doi:10.1007/s00432-020-03134-9.
8. Fletcher JW, Djulbegovic B, Soares HP, et al. Recommendations on the use of 18F-FDG PET in oncology. J Nucl Med. 2008;49(3):480-508. doi:10.2967/jnumed.107.047787
9. Spadafora M, Pace L, Evangelista L, et al. Risk-related 18F-FDG PET/CT and new diagnostic strategies in patients with solitary pulmonary nodule: the ITALIAN multicenter trial. Eur J Nucl Med Mol Imaging. 2018;45(11):1908-1914. doi:10.1007/s00259-018-4043-y.
10. Higashi K, Matsunari I, Ueda Y, et al. Value of whole-body FDG PET in management of lung cancer. Ann Nucl Med. 2003;17(1):1-14. doi:10.1007/BF02988253.
11. Kaira K, Serizawa M, Koh Y, et al. Biological significance of 18F-FDG uptake on PET in patients with non-small-cell lung cancer. Lung Cancer. 2014;83(2):197-204. doi:10.1016/j.lungcan.2013.11.025.
12. Grootjans W, de Geus-Oei LF, Troost EG, et al. PET in the management of locally advanced and metastatic NSCLC. Nat Rev Clin Oncol. 2015;12(7):395-407. doi:10.1038/nrclinonc.2015.75.
13. Eze C, Schmidt-Hegemann NS, Sawicki LM, et al. PET/CT imaging for evaluation of multimodal treatment efficacy and toxicity in advanced NSCLC-current state and future directions [published online ahead of print, 2021 Mar 24]. Eur J Nucl Med Mol Imaging. 2021;10.1007/s00259-021-05211-8. doi:10.1007/s00259-021-05211-8.
14. Salaün PY, Abgral R, Malard O, et al. Good clinical practice recommendations for the use of PET/CT in oncology. Eur J Nucl Med Mol Imaging. 2020;47(1):28-50. doi:10.1007/s00259-019-04553-8.
15. Grootjans W, de Geus-Oei LF, Troost EG, et al. PET in the management of locally advanced and metastatic NSCLC. Nat Rev Clin Oncol. 2015;12(7):395-407.
16. Lee KA, Rangaswamy G, Lavan NA, et al. ICORG 06-35: a prospective evaluation of PET-CT scan in patients with non-operable or non-resectable non-small cell lung cancer treated by radical 3-dimensional conformal radiation therapy: a phase II study. Ir J Med Sci. 2019;188(4):1155-61.
17. Kong FM, Ten Haken RK, Schipper M, et al. Effect of Midtreatment PET/CT-Adapted Radiation Therapy With Concurrent Chemotherapy in Patients With Locally Advanced Non-Small-Cell Lung Cancer: A Phase 2 Clinical Trial. JAMA Oncol. 2017;3(10):1358-1365. doi:10.1001/jamaoncol.2017.0982.
18. Oikonomou A, Khalvati F, Tyrrell PN, et al. Radiomics analysis at PET/CT contributes to prognosis of recurrence and survival in lung cancer treated with stereotactic body radiotherapy. Sci Rep. 2018;8(1):4003. Published 2018 Mar 5. doi:10.1038/s41598-018-22357-y.
19. Ruilong Z, Daohai X, Li G, et al. Diagnostic value of 18F-FDG-PET/CT for the evaluation of solitary pulmonary nodules: a systematic review and meta-analysis. Nucl Med Commun. 2017;38(1):67-75. doi:10.1097/MNM.0000000000000605.
20. Ung YC, Maziak DE, Vanderveen JA, et al. 18Fluoro-deoxyglucose positron emission tomography in the diagnosis and staging of lung cancer: a systematic review. J Natl Cancer Inst 2007;99(23):1753–67.
21. Huellner MW, de Galiza Barbosa F, Husmann L, et al. TNM Staging of Non-Small Cell Lung Cancer: Comparison of PET/MR and PET/CT. J Nucl Med. 2016;57(1):21-26. doi:10.2967/jnumed.115.162040.
22. Hicks RJ. Role of 18F-FDG PET in assessment of response in non-small cell lung cancer. J Nucl Med. 2009;50 Suppl 1:31S-42S. doi:10.2967/jnumed.108.057216.
23. Onishi Y, Ohno Y, Koyama H, et al. Non-small cell carcinoma: comparison of postoperative intra- and extrathoracic recurrence assessment capability of qualitatively and/or quantitatively assessed FDG-PET/CT and standard radiological examinations. Eur J Radiol. 2011;79(3):473-479. doi:10.1016/j.ejrad.2010.04.027.
24. van den Berg LL, Klinkenberg TJ, Groen HJM, et al. Patterns of Recurrence and Survival after Surgery or Stereotactic Radiotherapy for Early Stage NSCLC. J Thorac Oncol. 2015;10(5):826-831. doi:10.1097/JTO.0000000000000483.
25. Gomez DR, Blumenschein GR Jr, Lee JJ, et al. Local consolidative therapy versus maintenance therapy or observation for patients with oligometastatic non-small-cell lung cancer without progression after first-line systemic therapy: a multicentre, randomised, controlled, phase 2 study. Lancet Oncol. 2016;17(12):1672-1682. doi:10.1016/S1470-2045(16)30532-0.
26. Gomez DR, Tang C, Zhang J, et al. Local Consolidative Therapy Vs. Maintenance Therapy or Observation for Patients With Oligometastatic Non-Small-Cell Lung Cancer: Long-Term Results of a Multi-Institutional, Phase II, Randomized Study. J Clin Oncol. 2019;37(18):1558-1565. doi:10.1200/JCO.19.00201.
27. Iyengar P, Wardak Z, Gerber DE, et al. Consolidative Radiotherapy for Limited Metastatic Non-Small-Cell Lung Cancer: A Phase 2 Randomized Clinical Trial. JAMA Oncol. 2018;4(1):e173501. doi:10.1001/jamaoncol.2017.3501.
28. Abuodeh Y, Venkat P, Kim S. Systematic review of case reports on the abscopal effect. Curr Probl Cancer. 2016;40(1):25-37. doi:10.1016/j.currproblcancer.2015.10.001.
29. Demaria S, Ng B, Devitt ML, et al. Ionizing radiation inhibition of distant untreated tumors (abscopal effect) is immune mediated. Int J Radiat Oncol Biol Phys. 2004;58(3):862-870. doi:10.1016/j.ijrobp.2003.09.012.
30. Daly ME, Monjazeb AM, Kelly K. Clinical Trials Integrating Immunotherapy and Radiation for Non-Small-Cell Lung Cancer. J Thorac Oncol. 2015;10(12):1685-1693. doi:10.1097/JTO.0000000000000686.