Regulatory T Cells Shape the Differential Impact of Radiation Dose-Fractionation Schedules on Host Innate and Adaptive Antitumor Immune Defenses

Radiotherapy, immunity, and immune checkpoint inhibitors

Radiotherapy exerts immunostimulatory and immunosuppressive effects, both locally, within the irradiated tumour microenvironment, and systemically, outside the radiation field. Inspired by preclinical data that showed synergy between radiotherapy and immune checkpoint inhibitors, multiple clinical trials were initiated with the hypothesis that combined treatment with radiotherapy and immune checkpoint inhibitors could stimulate a robust systemic immune response and improve clinical outcomes. However, despite early optimism, radioimmunotherapy trials in the curative and metastatic settings have met with little success. In this Review, we summarise the immunostimulatory effects of radiotherapy that provided the theoretical basis for trials of combination radiotherapy and immune checkpoint inhibitors. We also discuss findings from clinical trials incorporating radiotherapy and immune checkpoint inhibitors and examine the success of these trials in the context of the immunosuppressive effects of radiotherapy. We conclude by highlighting targets for relieving radiotherapy-induced immunosuppression with the goal of enhancing the combined effects of radiotherapy and immune checkpoint inhibitors.

Nano-Pulse Treatment Overcomes the Immunosuppressive Tumor Microenvironment to Elicit In Situ Vaccination Protection against Breast Cancer

We previously reported that nano-pulse treatment (NPT), a pulsed power technology, resulted in 4T1-luc mammary tumor elimination and a strong in situ vaccination, thereby completely protecting tumor-free animals against a second live tumor challenge. The mechanism whereby NPT mounts effective antitumor immune responses in the 4T1 breast cancer predominantly immunosuppressive tumor microenvironment (TME) remains unanswered. In this study, orthotopic 4T1 mouse breast tumors were treated with NPT (100 ns, 50 kV/cm, 1000 pulses, 3 Hz). Blood, spleen, draining lymph nodes, and tumors were harvested at 4-h, 8-h, 1-day, 3-day, 7-day, and 3-month post-treatment intervals for the analysis of frequencies, death, and functional markers of various immune cells in addition to the suppressor function of regulatory T cells (Tregs). NPT was verified to elicit strong in situ vaccination (ISV) against breast cancer and promote both acute and long-term T cell memory. NPT abolished immunosuppressive dominance systemically and in the TME by substantially reducing Tregs, myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs). NPT induced apoptosis in Tregs and TAMs. It also functionally diminished the Treg suppression capacity, explained by the downregulation of activation markers, particularly 4-1BB and TGFβ, and a phenotypic shift from predominantly activated (CD44+CD62L-) to naïve (CD44-CD62L+) Tregs. Importantly, NPT selectively induced apoptosis in activated Tregs and spared effector CD4+ and CD8+ T cells. These changes were followed by a concomitant rise in CD8+CD103+ tissue-resident memory T cells and TAM M1 polarization. These findings indicate that NPT effectively switches the TME and secondary lymphatic systems from an immunosuppressive to an immunostimulatory state, allowing cytotoxic T cell function and immune memory formation to eliminate cancer cells and account for the NPT in situ vaccination.

Design of a single-center, phase II trial to explore the efficacy and safety of 'R-ISV-RO' treatment in advanced tumors

Background: The authors' preclinical study has confirmed that RO adjuvant (composed of TLR 7 agonists [imiquimod/R837] and OX40 agonists) injected into local lesions induces the regression of both primary tumor and distant metastasis. The authors propose to realize local control and exert abscopal effect through an 'R-ISV-RO' in situ strategy plus anti-PD-1 monoclonal antibody in advanced tumors. Methods: This study is a single-center, exploratory, phase II trial to evaluate the efficacy and safety of R-ISV-RO plus anti-PD-1 monoclonal antibody in advanced tumors. 30 patients with one or more measurable extracerebral lesions that are accessible for radiation or injection will be enrolled. The primary endpoint is the objective response rate of target lesions. Discussion/Conclusion: The efficacy and safety of the novel strategy will be further validated through this clinical trial.Clinical trial registration: ChiCTR2100053870 (www.chictr.org.cn/).

Dynamic Responses of Circulating T Cells After Stereotactic Body Radiation Therapy for Bone Metastasis in Patients With Breast Cancer

PURPOSE

Preclinical studies have shown that radiation therapy modulates antitumor immune responses. However, circulating T-cell responses after radiation therapy in patients with cancer have been poorly characterized. This study aims to explore the changes in circulating T cells after stereotactic body radiation therapy (SBRT).

METHODS AND MATERIALS

Peripheral blood samples of 30 patients with breast cancer who underwent SBRT for bone metastasis were analyzed using multicolor flow cytometry. Phenotypes of PD-1+ CD8+ T cells and regulatory T (TREG) cells were examined. Additionally, plasma protein levels were analyzed using a bead-based immunoassay.

RESULTS

Circulating PD-1+ CD8+ T cells, which are enriched for tumor-specific clonotypes, were activated at 1 week after SBRT. However, circulating TREG cells were also activated after SBRT; this pattern was also evident among effector Foxp3hiCD45RA- TREG cells. We observed no difference in T-cell responses according to the fraction size and number. Notably, activation of TREG cells was more prominent in patients who experienced greater activation of PD-1+ CD8+ T cells. Plasma level changes in TGF-β1, soluble CTLA-4, and soluble 4-1BB at 1 week after SBRT were associated with PD-1+ CD8+ T-cell responses. Activation of TREG cells at 1 week after SBRT was associated with worse progression-free survival. Clinical factors including molecular subtype were not associated with the T-cell responses.

CONCLUSIONS

SBRT induced activation of both potentially tumor-specific CD8+ T cells and TREG cells, which were tightly associated with each other. These results may support the use of TREG cell-modulating strategies with SBRT to improve the antitumor immune response.

C-C chemokine receptor 4 (CCR4)-positive regulatory T cells interact with tumor-associated macrophages to facilitate metastatic potential after radiation

PURPOSE

Our previous study revealed that elevated C-C motif chemokine ligand 2 (CCL2) secretion by irradiated cancer cells recruited C-C motif chemokine receptor 2 (CCR2)-positive myeloid cells and polarized M2-type tumor-associated macrophages (TAMs), promoting lung metastasis in an established mouse model. This study investigated the impact of CCL2 and TAMs on adaptive immunity.

METHODS

We assessed the influence of CCL2 and TAMs on adaptive immunity through two ectopic allograft mouse models constructed with MB49 bladder cancer cells and Lewis lung carcinoma cells. Both models exhibited delayed primary tumor growth following radiation therapy (RT), but RT promoted the development of pulmonary metastases in C57BL/6 mice. Additionally, we employed a direct coculture system to investigate the interaction between macrophages and target cells in the context of adaptive immunity.

RESULTS

C-C motif chemokine receptor 4 (CCR4)-positive regulatory T cells (Tregs) were recruited to the postirradiated tumor microenvironment (TME). Utilizing a CCR4 antagonist to inhibit CCL2-CCR4 activation reversed the infiltration of CCR4 + Tregs and reduced the incidence of pulmonary metastases. In addition, a positive feedback loop between M2-type TAMs and Tregs was observed. The combined blockade of the CCL2-CCR4 and CCL2-CCR2 signaling pathways further decreased the risk of RT-promoted lung metastasis.

CONCLUSION

The recruitment of CCR4 + Tregs to the postirradiated TME increases the metastatic potential of tumor cells through increased interactions with M2-type TAMs. A significant reduction in post-RT lung metastases in ectopic mouse models was achieved by disrupting the recruitment of both CCR4 + Tregs and CCR2 + myeloid cells, which are TAM precursors.

The 29th Annual Prostate Cancer Foundation Scientific Retreat Report

BACKGROUND

The 29th Annual Prostate Cancer Foundation (PCF) Scientific Retreat was held from October 27 to 29, 2022, at the Omni La Costa Resort in Carlsbad, CA. This was the first-ever hybrid PCF Retreat.

METHODS

The Annual PCF Scientific Retreat is a prominent international scientific gathering centered on groundbreaking, unpublished, and influential studies in basic, translational, and clinical prostate cancer research. It also covers research from related fields with a strong potential for influencing prostate cancer research and patient care.

RESULTS

Key areas of research that were focused on at the 2022 PCF Retreat included: (i) the contributions of molecular and genomic factors to prostate cancer disparities; (ii) novel clinical trial updates; (iii) lessons from primary prostate cancer; (iv) lessons from single-cell studies; (v) genetic, epigenetic, epitranscriptomic and posttranslational mechanisms and clinical heterogeneity in prostate cancer; (vi) biology of neuroendocrine and lineage-plastic prostate cancer; (vii) next generation prostate cancer theranostics and combination therapies; (viii) the biology and therapeutic potential of targeting phosphoinositide 3-kinases pathways; (ix) combining immunomodulatory treatments for prostate cancer; (x) novel gamma delta (γδ) T-cell therapy platforms for oncology; and (xi) lessons from other cancers.

CONCLUSIONS

This article provides a summary of the presentations from the 2022 PCF Scientific Retreat. By disseminating this knowledge, we hope to enhance our understanding of the present research landscape and guide future strides in both prostate cancer research and patient care.

Combination of local radiotherapy and anti-glucocorticoid-induced tumor necrosis factor receptor (GITR) therapy augments PD-L1 blockade-mediated anti-tumor effects in murine breast cancer model

PURPOSE

In this study, we investigated whether local radiotherapy (RT) and an anti-glucocorticoid-induced tumor necrosis factor receptor (GITR) agonist could increase the efficacy of PD-L1 blockade.

METHODS AND MATERIALS

We analyzed a breast cancer dataset from the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) to determine the role of GITR in breast cancer. We used the 4T1 murine TNBC model (primary and secondary tumors) to investigate the efficacy of PD-L1 blockade, local RT, anti-GITR agonist, and their combinations. We assessed tumor growth by tumor volume measurements, in vivo bioluminescence imaging, and metastatic lung nodule counts to evaluate the effects of these treatments. Flow cytometry and immunohistochemistry determined the proportions and phenotypes of CD8+ T-cells and regulatory T-cells (Tregs) in the tumors and spleen. Plasma cytokine levels were measured by enzyme-linked immunosorbent assay.

RESULTS

In the METABRIC cohort, patients with high expression of TNFRSF18, which encodes GITR, had significantly better survival than those with low expression. Adding local RT or anti-GITR agonist to PD-L1 blockade did not significantly augment efficacy compared to PD-L1 blockade alone; however, adding both to PD-L1 blockade significantly reduced tumor growth and lung metastasis. The benefits of the triple combination were accompanied by increased CD8+ T-cells and decreased Tregs in the tumor microenvironment and spleen.

CONCLUSIONS

The combination of local RT and an anti-GITR agonist significantly enhanced the anti-tumor immune responses induced by PD-L1 blockade. These results provide the preclinical rationale for the combination of therapy.

Synergistic treatment strategy: combining CAR-NK cell therapy and radiotherapy to combat solid tumors

Immunotherapy, notably chimeric antigen receptor (CAR) modified natural killer (NK) cell therapy, has shown exciting promise in the treatment of hematologic malignancies due to its unique advantages including fewer side effects, diverse activation mechanisms, and wide availability. However, CAR-NK cell therapies have demonstrated limited efficacy against solid tumors, primarily due to challenges posed by the solid tumor microenvironment. In contrast, radiotherapy, a well-established treatment modality, has been proven to modulate the tumor microenvironment and facilitate immune cell infiltration. With these observations, we hypothesize that a novel therapeutic strategy integrating CAR-NK cell therapy with radiotherapy could enhance the ability to treat solid tumors. This hypothesis aims to address the obstacles CAR-NK cell therapies face within the solid tumor microenvironment and explore the potential efficacy of their combination with radiotherapy. By capitalizing on the synergistic advantages of CAR-NK cell therapy and radiotherapy, we posit that this could lead to improved prognoses for patients with solid tumors.

BRD4 Inhibition Enhances the Antitumor Effects of Radiation Therapy in a Murine Breast Cancer Model

Bromodomain-containing protein 4 (BRD4) is an intracellular protein that regulates expression of various cellular functions. This study investigated whether BRD4 inhibition can alter the immunomodulatory and antitumor effects of radiation therapy (RT). A murine breast cancer cell line was implanted into BALB/c mice. The dual-tumor model was used to evaluate the abscopal effects of RT. A total of 24 Gy was delivered and BRD4 inhibitor was injected intravenously. Tumor size was measured, and in vivo imaging was performed to evaluate tumor growth. Flow cytometry and immunohistochemistry were performed to examine immunologic changes upon treatment. The combination of BRD4 inhibitor and RT significantly suppressed tumor growth compared to RT alone. BRD4 inhibitor reduced the size of the unirradiated tumor, indicating that it may induce systemic immune responses. The expression of HIF-1α and PD-L1 in the tumor was significantly downregulated by the BRD4 inhibitor. The proportion of M1 tumor-associated macrophages (TAMs) increased, and the proportion of M2 TAMs decreased upon BRD4 inhibition. BRD4 inhibitor expanded CD4+ and CD8+ T cell populations in the tumor microenvironment. Additionally, splenic monocytic myeloid derived suppressor cells, which were increased by RT, were reduced upon the addition of BRD4 inhibitor. Therefore, the addition of BRD4 inhibitor significantly enhanced the systemic antitumor responses of local RT.

Modulation of Radiation Doses and Chimeric Antigen Receptor T Cells: A Promising New Weapon in Solid Tumors-A Narrative Review

Tumor behavior is determined by its interaction with the tumor microenvironment (TME). Chimeric antigen receptor (CART) cell therapy represents a new form of cellular immunotherapy (IT). Immune cells present a different sensitivity to radiation therapy (RT). RT can affect tumor cells both modifying the TME and inducing DNA damage, with different effects depending on the low and high doses delivered, and can favor the expression of CART cells. CART cells are patients' T cells genetically engineered to recognize surface structure and to eradicate cancer cells. High-dose radiation therapy (HDRT, >10-20 Gy/fractions) converts immunologically "cold" tumors into "hot" ones by inducing necrosis and massive inflammation and death. LDRT (low-dose radiation therapy, >5-10 Gy/fractions) increases the expansion of CART cells and leads to non-immunogenetic death. An innovative approach, defined as the LATTICE technique, combines a high dose in higher FDG- uptake areas and a low dose to the tumor periphery. The association of RT and immune checkpoint inhibitors increases tumor immunogenicity and immune response both in irradiated and non-irradiated sites. The aim of this narrative review is to clarify the knowledge, to date, on CART cell therapy and its possible association with radiation therapy in solid tumors.

Low-Dose Non-Targeted Effects and Mitochondrial Control

Non-targeted effects (NTE) have been generally regarded as a low-dose ionizing radiation (IR) phenomenon. Recently, regarding long distant abscopal effects have also been observed at high doses of IR) relevant to antitumor radiation therapy. IR is inducing NTE involving intracellular and extracellular signaling, which may lead to short-ranging bystander effects and distant long-ranging extracellular signaling abscopal effects. Internal and "spontaneous" cellular stress is mostly due to metabolic oxidative stress involving mitochondrial energy production (ATP) through oxidative phosphorylation and/or anaerobic pathways accompanied by the leakage of O₂^- and other radicals from mitochondria during normal or increased cellular energy requirements or to mitochondrial dysfunction. Among external stressors, ionizing radiation (IR) has been shown to very rapidly perturb mitochondrial functions, leading to increased energy supply demands and to ROS/NOS production. Depending on the dose, this affects all types of cell constituents, including DNA, RNA, amino acids, proteins, and membranes, perturbing normal inner cell organization and function, and forcing cells to reorganize the intracellular metabolism and the network of organelles. The reorganization implies intracellular cytoplasmic-nuclear shuttling of important proteins, activation of autophagy, and mitophagy, as well as induction of cell cycle arrest, DNA repair, apoptosis, and senescence. It also includes reprogramming of mitochondrial metabolism as well as genetic and epigenetic control of the expression of genes and proteins in order to ensure cell and tissue survival. At low doses of IR, directly irradiated cells may already exert non-targeted effects (NTE) involving the release of molecular mediators, such as radicals, cytokines, DNA fragments, small RNAs, and proteins (sometimes in the form of extracellular vehicles or exosomes), which can induce damage of unirradiated neighboring bystander or distant (abscopal) cells as well as immune responses. Such non-targeted effects (NTE) are contributing to low-dose phenomena, such as hormesis, adaptive responses, low-dose hypersensitivity, and genomic instability, and they are also promoting suppression and/or activation of immune cells. All of these are parts of the main defense systems of cells and tissues, including IR-induced innate and adaptive immune responses. The present review is focused on the prominent role of mitochondria in these processes, which are determinants of cell survival and anti-tumor RT.

Combination of CAR‑T cell therapy and radiotherapy: Opportunities and challenges in solid tumors (Review)

Chimeric antigen receptor (CAR) T cell therapy has emerged as a new and breakthrough cancer immunotherapy. Although CAR-T cell therapy has made significant progress clinically in patients with refractory or drug-resistant hematological malignancies, there are numerous challenges in its application to solid tumor therapy, including antigen escape, severe toxic reactions, abnormal vascularization, tumor hypoxia, insufficient infiltration of CAR-T cells and immunosuppression. As a conventional mode of anti-tumor therapy, radiotherapy has shown promising effects in combination with CAR-T cell therapy by enhancing the specific immunity of endogenous target antigens, which promoted the infiltration and expansion of CAR-T cells and improved the hypoxic tumor microenvironment. This review focuses on the obstacles to the application of CAR-T technology in solid tumor therapy, the potential opportunities and challenges of combined radiotherapy and CAR-T cell therapy, and the review of recent literature to evaluate the best combination for the treatment of solid tumors.

Mechanistic rationales for combining immunotherapy with radiotherapy

Immunotherapy consisted mainly of immune checkpoint inhibitors (ICIs) has led to significantly improved antitumor response. However, such response has been observed only in tumors possessing an overall responsive tumor immune micro-environment (TIME), in which the presence of functional tumor-infiltrating lymphocytes (TILs) is critical. Various mechanisms of immune escape from immunosurveillance exist, leading to different TIME phenotypes in correlation with primary or acquired resistance to ICIs. Radiotherapy has been shown to induce antitumor immunity not only in the irradiated primary tumor, but also at unirradiated distant sites of metastases. Such antitumor immunity is mainly elicited by radiation's stimulatory effects on antigenicity and adjuvanticity. Furthermore, it may be significantly augmented when irradiation is combined with immunotherapy, such as ICIs. Therefore, radiotherapy represents one potential therapeutic strategy to restore anti-tumor immunity in tumors presenting with an unresponsive TIME. In this review, the generation of anti-tumor immunity, its impairment, radiation's immunogenic properties, and the antitumor effects of combining radiation with immunotherapy will be comprehensively discussed.

Emerging Role of Immunomonitoring to Predict the Clinical Outcome of Patients With Malignant Pleural Mesothelioma Treated With Radical Radiation Therapy

PURPOSE

The present study aimed at evaluating the baseline immune profile and the immunomodulating effects of radical hemithoracic radiation therapy (RT) in patients affected by malignant pleural mesothelioma (MPM) to identify potential predictive biomarkers of therapy response, toxicity development, and eligibility for further immunotherapeutic treatments.

METHODS AND MATERIALS

Blood samples were collected from 55 patients with MPM, enrolled in a phase 3 trial comparing radical hemithoracic RT (interventional arm, n = 28) with local palliative RT (control arm, n = 27). Immunomonitoring was performed before RT, at the end of treatment, and 1 month after therapy, characterizing natural killer cells, B and T lymphocytes, activated CD4 and CD8 T cells, interferon-γ- and tumor necrosis factor-α-producing T helper (Th) 1 cells, regulatory T cells, and Th17 and Th22 lymphocytes, through flow cytometry. Serum levels of interleukin (IL)-6, -8, -10 and mesothelin were quantified through Enzyme-Linked Immunosorbent Assay (ELISA) assays at the same time points. Variations in the immune parameters were investigated by Friedman test and Wilcoxon signed rank post hoc test with Bonferroni correction for multiple testing, while the prognostic effect of immune biomarkers was evaluated through Kaplan-Meier method and Spearman's correlation analysis.

RESULTS

Major immune variations were noticed after radical RT compared with palliative treatment, in particular an improvement in activated T cells and in interferon-γ-producing Th1 cells after RT. In the interventional arm, baseline high levels of Th22 and IL-10 and an increase in T cells were associated with an improved survival, whereas a fold increase in serum mesothelin correlated with the development of severe toxicity. An improvement of immunosuppressive regulatory T cells was observed in both arms of treatment.

CONCLUSIONS

The immunomonitoring performed in patients with MPM revealed potential prognostic biomarkers for radical hemithoracic RT treatment and identified specific immune signatures induced by RT immunomodulation, which could suggest a synergistic effect with an immunotherapeutic treatment.

The mutual relationship between the host immune system and radiotherapy: stimulating the action of immune cells by irradiation

The effects of irradiation on tumor tissue and the host immune system are interrelated. The antitumor effect of irradiation is attenuated in the immunocompromised hosts. In addition, radiation alone positively and negatively influences the host immune system. The positive effects of radiation are summarized by the ability to help induce and enhance tumor-antigen-specific immune responses. The cancer-immunity cycle is a multistep framework that illustrates how the tumor-antigen-specific immune responses are induced and how the induced antigen-specific immune cells exert their functions in tumor tissues. Irradiation affects each step of this cancer-immunity cycle, primarily in a positive manner. In contrast, radiation also has negative effects on the immune system. The first is that irradiation has the possibility to kill irradiated effector immune cells. The second is that irradiation upregulates immunosuppressive molecules in the tumor microenvironment, whereas the third is that irradiation to the tumor condenses immunosuppressor cells in the tumor microenvironment. When used in conjunction with radiotherapy, immune checkpoint inhibitors can further leverage the positive effects of radiation on the immune system and compensate for the negative effects of irradiation, which supports the rationale for the combination of radiotherapy and immune checkpoint inhibitors. In this review, we summarize the preclinical evidence for the reciprocal effects of radiation exposure and the immune system, and up-front topics of the combination therapy of immune checkpoint inhibitors and radiotherapy.

Long-term expression changes of immune-related genes in prostate cancer after radiotherapy

The expression of immune-related genes in cancer cells can alter the anti-tumor immune response and thereby impact patient outcomes. Radiotherapy has been shown to modulate immune-related genes dependent on the fractionation regimen. To identify long-term changes in gene expression after irradiation, PC3 (p53 deleted) and LNCaP (p53 wildtype) prostate cancer cells were irradiated with either a single dose (SD, 10 Gy) or a fractionated regimen (MF) of 10 fractions (1 Gy per fraction). Whole human genome arrays were used to determine gene expression at 24 h and 2 months after irradiation. Immune pathway activation was analyzed with Ingenuity Pathway Analysis software. Additionally, 3D colony formation assays and T-cell cytotoxicity assays were performed. LNCaP had a higher basal expression of immunogenic genes and was more efficiently killed by cytotoxic T-cells compared to PC3. In both cell lines, MF irradiation resulted in an increase in multiple immune-related genes immediately after irradiation, while at 2 months, SD irradiation had a more pronounced effect on radiation-induced gene expression. Both immunogenic and immunosuppressive genes were upregulated in the long term in PC3 cells by a 10 Gy SD irradiation but not in LNCaP. T-cell-mediated cytotoxicity was significantly increased in 10 Gy SD PC3 cells compared to the unirradiated control and could be further enhanced by treatment with immune checkpoint inhibitors. Irradiation impacts the expression of immune-related genes in cancer cells in a fractionation-dependent manner. Understanding and targeting these changes may be a promising strategy for primary prostate cancer and recurrent tumors.

Basic cancer immunology for radiation oncologists

Although the impressive clinical responses seen with modern cancer immunotherapy are currently limited to a subset of patients, the underlying paradigm shift has resulted in now hardly a segment in oncology that has not been touched by the immuno‐oncology revolution. A growing body of data indicates that radiation therapy (RT) can modulate the tumour immune microenvironment and complement cancer immunotherapy via non‐overlapping mechanisms to reinvigorate immunity against cancer. Thus, increasingly RT is viewed as a highly unique partner for immunotherapy across the spectrum of cancer settings, as radiobiology and cancer immunology foreseeably become more intertwined. Considering these developments, this review summarises the key concepts and terminology in immunology for the radiation oncologist, with a focus on the cancer setting and with reference to important recent advances. These concepts will provide a starting point for understanding the strategies that underlie current and emerging immunotherapy trials, as well as the indirect effects of RT by which immune responses against cancer are shaped.

CAR-T Plus Radiotherapy: A Promising Combination for Immunosuppressive Tumors

Radiotherapy (RT) is the standard-of-care treatment for more than half of cancer patients with localized tumors and is also used as palliative care to facilitate symptom relief in metastatic cancers. In addition, RT can alter the immunosuppressive tumor microenvironment (TME) of solid tumors to augment the anti-tumor immune response of immune checkpoint blockade (ICB). The rationale of this combination therapy can also be extended to other forms of immunotherapy, such as chimeric antigen receptor T cell (CAR-T) therapy. Similar to ICB, the efficacy of CAR-T therapy is also significantly impacted by the immunosuppressive TME, leading to compromised T cell function and/or insufficient T cell infiltration. In this review, we will discuss some of the key barriers to the activity of CAR-T cells in the immunosuppressive TME and focus on how RT can be used to eliminate or bypass these barriers. We will present the challenges to achieving success with this therapeutic partnership. Looking forward, we will also provide strategies currently being investigated to ensure the success of this combination strategy in the clinic.

Inoperable Early-Stage Non-Small-Cell Lung Cancer: Stereotactic Ablative Radiotherapy and Rationale for Systemic Therapy

Stereotactic ablative radiotherapy (SABR) is the standard treatment for medically inoperable, early-stage non-small-cell lung cancer. SABR results in high rates of in-field tumor control, but among larger and more biologically aggressive tumors, regional and distant failures are problematic. Cytotoxic chemotherapy is rarely used in this patient population and the benefit is unclear. Alternative systemic therapy options with a milder side-effect profile are of considerable interest, and several randomized phase III trials are currently testing immune checkpoint inhibitors in this setting. We review the rationale, data, and ongoing studies evaluating systemic therapy in medically inoperable, early-stage non-small-cell lung cancer treated with SABR.

Particle radiotherapy and molecular therapies: mechanisms and strategies towards clinical applications

Immunotherapy and targeted therapy are now commonly used in clinical trials in combination with radiotherapy for several cancers. While results are promising and encouraging, the molecular mechanisms of the interaction between the drugs and radiation remain largely unknown. This is especially important when switching from conventional photon therapy to particle therapy using protons or heavier ions. Different dose deposition patterns and molecular radiobiology can in fact modify the interaction with drugs and their effectiveness. We will show here that whilst the main molecular players are the same after low and high linear energy transfer radiation exposure, significant differences are observed in post-exposure signalling pathways that may lead to different effects of the drugs. We will also emphasise that the problem of the timing between drug administration and radiation and the fractionation regime are critical issues that need to be addressed urgently to achieve optimal results in combined treatments with particle therapy.

Dose-Response Effect and Dose-Toxicity in Stereotactic Radiotherapy for Brain Metastases: A Review

Simple Summary

Brain metastases are one of the most frequent complications for cancer patients. Stereotactic radiosurgery is considered a cornerstone treatment for patients with limited brain metastases and the ideal dose and fractionation schedule still remain unknown. The aim of this literature review is to discuss the dose-effect relation in brain metastases treated by stereotactic radiosurgery, accounting for fractionation and technical considerations.

Abstract

For more than two decades, stereotactic radiosurgery has been considered a cornerstone treatment for patients with limited brain metastases. Historically, radiosurgery in a single fraction has been the standard of care but recent technical advances have also enabled the delivery of hypofractionated stereotactic radiotherapy for dedicated situations. Only few studies have investigated the efficacy and toxicity profile of different hypofractionated schedules but, to date, the ideal dose and fractionation schedule still remains unknown. Moreover, the linear-quadratic model is being debated regarding high dose per fraction. Recent studies shown the radiation schedule is a critical factor in the immunomodulatory responses. The aim of this literature review was to discuss the dose–effect relation in brain metastases treated by stereotactic radiosurgery accounting for fractionation and technical considerations. Efficacy and toxicity data were analyzed in the light of recent published data. Only retrospective and heterogeneous data were available. We attempted to present the relevant data with caution. A BED10 of 40 to 50 Gy seems associated with a 12-month local control rate >70%. A BED10 of 50 to 60 Gy seems to achieve a 12-month local control rate at least of 80% at 12 months. In the brain metastases radiosurgery series, for single-fraction schedule, a V12 Gy < 5 to 10 cc was associated to 7.1–22.5% radionecrosis rate. For three-fractions schedule, V18 Gy < 26–30 cc, V21 Gy < 21 cc and V23 Gy < 5–7 cc were associated with about 0–14% radionecrosis rate. For five-fractions schedule, V30 Gy < 10–30 cc, V 28.8 Gy < 3–7 cc and V25 Gy < 16 cc were associated with about 2–14% symptomatic radionecrosis rate. There are still no prospective trials comparing radiosurgery to fractionated stereotactic irradiation.