In vivo effects of lattice radiation therapy on local and distant lung cancer: potential role of immunomodulation

Proton GRID and LATTICE treatment planning techniques for clinical liver SFRT treatments

Objective. This study aims to develop and evaluate various treatment clinical proton spatially-fractionated-radiotherapy (pSFRT) planning techniques namely proton GRID (pGRID) and LATTICE (pLATTICE).Approach.pSFRT plans (27 Gy(RBE), single-fraction) were initially developed using phantom geometries and created retrospectively for a liver patient previously treated with photon virtual GRID (vGRID). pGRID plans varied in cylinder diameters (Dcyl= 0.4-1.4 cm) and center-to-center distances (c-t-c = 1.7-3.4 cm) and were evaluated by peak-to-valley dose ratios (PVDRs), equivalent uniform dose (EUD), andV27Gy.Dcyland c-t-c distances matching the vGRID EUD andV27Gyvalues guided pLATTICE sphere distributions. Various pLATTICE techniques, including different beam numbers, orientations, and sphere arrangements, were investigated. We also explored using collimating brass apertures to enhance the pGRID PVDR.Main results.pGRID plans with 3.4 cm c-t-c and 0.4 cmDcylresulted in 2.09%V27Gy, closely matching vGRID's 1.50%. The resultant pGRID PVDR was 8.92 compared to vGRID's 2.7-3.0. PVDRs were affected by spot sizes with reductions of 15.0% with range shifters and 76.0% from 7.5 cm to 27.5 cm depths. The highest PVDR of 4.17 was achieved with two-field pLATTICE plans with favorable beam angles, with a 44.6% reduction with unfavorable beam orientations and up to 24.7% reductions with an increasing number of beams. Non-maximal pLATTICE sphere packing arrangements increases the PVDR with a decrease inV27Gywarranting further investigation. pSFRT plans reduced the healthy liverV5Gyby 83.6%-90.7% compared to vGRID. Apertures enhanced the PVDR by 170% at the deepest depths but increased the skin D0.03ccfrom 26.77 Gy to 54.66 Gy.Significance and conclusion.We developed pGRID and pLATTICE plans, demonstrating that desired plan metrics was achieved by adjusting the geometrical arrangements of SFRT contours. The relative orientation of these contours with beam entrances was crucial for high-quality SFRT plans. pSFRT plans achieved superior PVDRs and better dose sparing to OARs compared to vGRID plans.

Progress of the application of spatially fractionated radiation therapy in palliative treatment of tumors

INTRODUCTION/BACKGROUND

Malignant tumors pose a serious threat to human health. As tumor volume increases, conventional external beam radiation therapy (cEBRT) faces challenges in tumor control and normal tissue toxicity. Spatially fractionated radiation therapy (SFRT) has emerged as an alternative approach.

MATERIALS AND METHODS

This article reviews the history of SFRT, including kilovolt X-ray based GRID, megavolt X-ray based GRID, MLC-shaped GRID, LATTICE radiation therapy (LRT), Bragg-peak based SFRT, microbeam, minibeam, SBRT-PATHY, and ISPART. It also explores its radiobiological mechanisms, such as immunomodulation, bystander and abscopal effects, and vascular response. Clinical studies of SFRT in palliative tumor treatment are summarized, and its limitations and future directions are discussed.

RESULTS

SFRT has shown high symptom remission rates, significant target volume reduction, and even tumor control and long-term survival in some cases across various tumor types. However, it has limitations like lack of standardized dosimetric parameters, complex implementation, small-scale clinical studies, and uncertain immunomodulatory potential.

CONCLUSION

Despite limitations, SFRT shows promise as a palliative radiation therapy technique. Future large-scale, multi-center clinical trials are needed to standardize dosimetric parameters, clarify immunomodulatory mechanisms, and simplify the technology for wider application.

The Radiosurgery Society Working Groups on GRID, LATTICE, Microbeam, and FLASH Radiotherapies: Advancements Symposium and Subsequent Progress Made

PURPOSE

Since the inaugural workshop "Understanding High-Dose, Ultra-High Dose Rate and Spatially Fractionated Radiotherapy." hosted by the National Cancer Institute and sponsored by the Radiosurgery Society (RSS), growing collaborations and investigations have ensued among experts, practitioners, and researchers. The RSS GRID, LATTICE, Microbeam and FLASH (GLMF) Working Groups were formed as a framework for these efforts and have focused on advancing the understanding of the biology, technical/physical parameters, trial design, and clinical practice of these new radiation therapy modalities.

METHODS AND MATERIALS

In view of the steadily increasing clinical interest in Spatially Fractionated Radiotherapy (SFRT) and FLASH, a full-day symposium entitled "Advancements in GRID, LATTICE, and FLASH Radiotherapy Symposium" was established in 2022 that immediately preceded the RSS scientific meeting. This well-attended symposium focused on clinical, technical, and physics approaches for SFRT, and closely examining relevant radiobiological underpinnings. Practical clinical trial development was a highlighted discussion. An additional section reviewed proton therapy and other particle-based techniques for the delivery of GRID and LATTICE therapy. A treatment planning and delivery tutorial for GRID, LATTICE, and proton GRID/LATTICE was directed toward the real-world considerations for the development of new clinical GRID or LATTICE programs. An overall similar approach was applied to the discussion of FLASH. This report summarizes the content of the first GLMF Symposium and related work of the RSS GLMF Working Groups in the field of heterogeneous and ultrahigh dose rate irradiation, over approximately 2 years.

RESULTS

The GLMF Working Groups have continued to expand in membership and attendance, and several resultant trial concepts, research efforts, academic discussions, and peer-reviewed publications have followed as the number of institutions and practitioners using SFRT and FLASH continues to grow.

CONCLUSIONS

The GLMF Working Groups and the RSS continue to demonstrate excellent progress in proliferating use of and improving understanding of SFRT and ultrahigh dose rate radiation therapy techniques.

Reported Early Clinical Outcomes of Forward-Planned Multileaf Collimator-Based 3-Dimensional Conformal Spatially Fractionated Radiation Therapy Technique for Large and Bulky Tumors

PURPOSE

Conventionally fractionated radiation therapy for large, bulky (≥8 cm), unresectable tumors has been hampered by radiation-induced morbidity, but application of spatially fractionated radiation therapy (SFRT) for both palliative and curative intent has been increasingly accepted. We report our clinical use of novel 3-dimensional conformal multileaf collimator (MLC)-based SFRT with same-day computed tomography simulation and forward-planning method, providing a safe, rapidly efficacious reduction in disease burden and pain, with minimal normal-tissue toxicity.

METHODS AND MATERIALS

Patients with large, unresectable bulky tumors received 15 Gy in 1 fraction to the gross tumor volume (GTV) within an hour of computed tomography simulation, using a forward-planned MLC-based SFRT technique. All patients subsequently received either 30 Gy in 10 fractions, generally 2 days after SFRT for palliative intent, or site-specific, full-prescription doses starting 2 to 3 days after SFRT for curative intent. Patients underwent follow-up examinations and imaging in 3-month intervals to assess tumor response, pain control, and radiation-associated toxicity.

RESULTS

Between November 2019 and January 2024, 24 large tumors in 23 patients were analyzed. Median follow-up was 6 months (range 3-36 months). After SFRT, 16 patients (69.5%) proceeded with palliative-intent radiation therapy, 6 patients (26.0%) underwent curative-intent radiation therapy, and 1 patient (4.3%) declined further radiation therapy. Seven patients (30.4%) reported acute radiation-associated toxicities. A total of 3 acute grade ≥3 toxicities (13.0%) were reported, but no grade 5 toxicities occurred. Complete or partial response was seen in 14 of 24 (58%) tumors; clinical benefit rate was 79.2%. Twenty of 23 patients (86.9%) reported pain relief from tumor burden.

CONCLUSIONS

Same-day 3-dimensional MLC-based SFRT method provides fast, safe, and effective management of large, bulky, unresectable tumors for both palliative and therapeutic intents across a wide range of tumor sites and histologies, reducing tumor burden and improving patient comfort and compliance. This method could be useful for rapid SFRT including adaptive treatment. We recommend commissioning and validating this method at other institutions, including community cancer centers, to expand the access of efficient, high-quality SFRT treatment to underserved patient cohorts.

Photon mini-GRID therapy for preoperative breast cancer tumor treatment: A treatment plan study

BACKGROUND

Breast cancer is the leading cause of female cancer mortality worldwide, accounting for 1 in 6 cancer deaths. Surgery, radiation, and systemic therapy are the three pillars of breast cancer treatment, with several strategies developed to combine them. The association of preoperative radiotherapy with immunotherapy may improve breast cancer tumor control by exploiting the tumor radio-induced immune priming. However, this requires the use of hypofractionated radiotherapy (3 × 8 Gy), increasing the risk of toxicity. Mini-GRID therapy (mini-GRT) is an innovative form of spatially fractionated radiation therapy (SFRT) characterized by narrow beam widths between 1 to 2 mm that promises a significant increase in normal tissue dose tolerances and could thereby represent a new alternative for preoperative breast cancer treatment. Mini-GRT has been successfully implemented at the Hospital de Santiago de Compostela (Spain) with a flattening filter-free LINAC (megavoltage x-rays).

PURPOSE

In this dosimetry proof-of-concept study, we evaluate the feasibility of photon mini-GRT for preoperative breast cancer treatment. We also assess the clinical potential of mini-GRT and compare it with the current treatment standard of intensity-modulated radiotherapy (IMRT).

METHODS

Seven unbiased breast cancer dosimetries of patients treated with stereotactic body radiotherapy (SBRT) (3 × 8 Gy, IMRT) were selected for the study. Photon mini-GRT was compared with SBRT using three main criteria: (i) the dose to organs at risk (OARs), (ii) the dose constraints dictated by normal tissue tolerance, and (iii) the lateral penumbra in OARs. Tumor coverage was evaluated in terms of normalized total dose at 8 Gy-fractions. The optimized SBRT by IMRT was realized at the Institut Curie, Paris, France. The dose in mini-GRT was calculated by means of Monte Carlo simulations based on the mini-GRT implementation realized at the University Hospital in Santiago de Compostela.

RESULTS

Compared to SBRT plans, mini-GRT resulted in a reduction of the mean dose to the lungs, heart, chest wall, and lymph nodes in the studied cases by a factor ranging from 50% to 100%. Additionally, valley, mean, and peak doses to normal tissues meet the dose tolerance limits for the considered OARs, the most challenging of all being the skin. The mean dose to the skin was reduced (20%-60% less) for most of the studied cases. Mini-GRT also yielded sharper lateral penumbras in the skin and lungs (size reduced by at least 50%). Similar tumor integral doses were obtained for the two treatment modalities.

CONCLUSION

Mini-GRT with megavoltage x-rays is an innovative treatment approach already implemented in a clinical context. In this proof-of-concept study, we evaluated mini-GRT for partial breast cancer irradiation, demonstrating its potential for preoperative treatment thanks to the high skin and normal tissue-sparing capabilities. These initial results represent a first step towards clinical use and encourage further prospective clinical studies.

Feasibility of optimal vertex size and spacing for lattice radiotherapy implementation using helical tomotherapy

Purpose

Lattice radiotherapy (LRT), a type of spatially fractionated radiotherapy (SFRT), delivers high dose at specific volumes of lattice structure within the tumor to create a low valley-to-peak dose ratio (VPDR). This study aims to evaluate the feasibility of implementing SFRT using helical tomotherapy and to investigate the effects of vertex size and spacing for attaining the VPDR.

Methods

A three-dimensional lattice structure with 3×3×3 vertices was designed in a cheese phantom. Vertex sizes of 0.5 cm, 1.0 cm, and 2.0 cm were assessed, with spacing from 1.0 cm to 5.0 cm. The prescribed dose was set to 20 Gy to the vertices in a single fraction. VPDR was calculated from dose profiles along lines connecting three vertices in the anterior-posterior (AP), lateral (LAT), and superior-inferior (SI) directions. The minimum, maximum, and mean dose for each vertex, as well as conformity, homogeneity and monitor unit (MU) analysis were also performed.

Results

VPDR decreased significantly with increasing vertex size and spacing. While the AP and LAT directions showed similar VPDR values, the SI direction consistently exhibited lower VPDR values across all configurations. Vertex sizes of 0.5 cm, 1.0 cm, and 2.0 cm required spacing of at least 3.0 cm, 2.0 cm, and 1.0 cm, respectively, to achieve VPDR values below 0.4. The conformity indices ranged from 1.0 to 4.02, and the homogeneity indices ranged from 1.20 to 1.57 across all configurations. Additionally, the MUs increased with both vertex size and spacing.

Conclusions

This study quantitatively analyzed the impact of various vertex sizes and spacings on VPDR in lattice radiotherapy using helical tomotherapy. VPDR decreased with increasing vertex size and spacing, with consistently lower values in the SI direction. These findings provide crucial insights for optimizing LRT plans. The identified relationships between the parameters and VPDR offer a foundation for developing more effective LRT protocols in helical tomotherapy, potentially improving therapeutic outcomes.

Proceedings of the National Cancer Institute Workshop on combining immunotherapy with radiotherapy: challenges and opportunities for clinical translation

Radiotherapy both promotes and antagonises tumour immune recognition. Some clinical studies show improved patient outcomes when immunotherapies are integrated with radiotherapy. Safe, greater than additive, clinical response to the combination is limited to a subset of patients, however, and how radiotherapy can best be combined with immunotherapies remains unclear. The National Cancer Institute-Immuno-Oncology Translational Network-Society for Immunotherapy of Cancer-American Association of Immunology Workshop on Combining Immunotherapy with Radiotherapy was convened to identify and prioritise opportunities and challenges for radiotherapy and immunotherapy combinations. Sessions examined the immune effects of radiation, barriers to anti-tumour immune response, previous clinical trial data, immunological and computational assessment of response, and next-generation radiotherapy-immunotherapy combinations. Panel recommendations included: developing and implementing patient selection and biomarker-guided approaches; applying mechanistic understanding to optimise delivery of radiotherapy and selection of immunotherapies; using rigorous preclinical models including companion animal studies; embracing data sharing and standardisation, advanced modelling, and multidisciplinary cross-institution collaboration; interrogating clinical data, including negative trials; and incorporating novel clinical endpoints and trial designs.

A preliminary study of linear accelerator-based spatially fractionated radiotherapy

Purpose

This study aimed to provide quantitative information for implementing Lattice radiotherapy (LRT) using a medical linear accelerator equipped with the Millennium 120 multi-leaf collimator (MLC). The research systematically evaluated the impact of varying vertex diameters and separations on dose distribution, peak-to-valley dose ratio (PVDR), and normal tissue dose.

Methods

A cylindrical Virtual Water™ phantom was used to create LRT treatments using the Eclipse version 16.0 treatment planning system (Varian, Palo Alto, USA). The plans were optimized employing a 3 × 3 × 3 lattice structure with vertex diameters ranging from 0.5 to 2.0 cm and separations from 1.0 to 5.0 cm. The prescribed dose was 20.0 Gy to 50% of the vertex volume in a single fraction. Peak-to-valley dose ratio (PVDR) was calculated along three orthogonal axes, and normal tissue dose and monitor units (MU) were analyzed. Additionally, the modulation complexity score (MCS) was calculated for each plan to quantitatively assess treatment plan complexity.

Results

The PVDR analysis demonstrated heterogeneous dose distribution, with optimal values below 30% in all directions for 5.0 cm separation. PVDR in the superior-inferior direction was consistently lower than in other directions. Normal tissue dose analysis revealed increasing mean dose with larger diameters and separations, while the volume receiving high doses decreased. MU analysis showed significant contributions from collimator angles of 315.0° and 45.0°. MCS values ranged from 0.02 to 0.17 for 0.5 cm vertex diameter and 0.08 to 0.20 for larger diameters (1.0-2.0 cm) across different separations, respectively.

Conclusions

This study demonstrates the technical feasibility of implementing LRT using a medical linear accelerator with Millennium 120 MLC. The findings provide insights into optimizing LRT treatment plans, offering a comprehensive quantitative reference for achieving desired dose heterogeneity while maintaining normal tissue protection.

Emerging Radiotherapy Technologies for Head and Neck Squamous Cell Carcinoma: Challenges and Opportunities in the Era of Immunotherapy

Radiotherapy (RT) is an integral component in the multidisciplinary management of patients with head and neck squamous cell carcinoma (HNSCC). Significant advances have been made toward optimizing tumor control and toxicity profiles of RT for HNSCC in the past two decades. The development of intensity modulated radiotherapy (IMRT) and concurrent chemotherapy established the standard of care for most patients with locally advanced HNSCC around the turn of the century. More recently, selective dose escalation to the most radioresistant part of tumor and avoidance of the most critical substructures of organs at risk, often guided by functional imaging, allowed even further improvement in the therapeutic ratio of IMRT. Other highly conformal RT modalities, including intensity modulated proton therapy (IMPT) and stereotactic body radiotherapy (SBRT) are being increasingly utilized, although there are gaps in our understanding of the normal tissue complication probabilities and their relative biological effectiveness. There is renewed interest in spatially fractionated radiotherapy (SFRT), such as GRID and LATTICE radiotherapy, in both palliative and definitive settings. The emergence of immune checkpoint inhibitors (ICIs) has revolutionized the treatment of patients with recurrent and metastatic HNSCC. Novel RT modalities, including IMPT, SBRT, and SFRT, have the potential to reduce lymphopenia and immune suppression, stimulate anti-tumor immunity, and synergize with ICIs. The next frontier in the treatment of HNSCC may lie in the exploration of combined modality treatment with new RT technologies and ICIs.

Mechanistic in silico explorations of the immunogenic and synergistic effects of radiotherapy and immunotherapy: a critical review

Immunotherapy is a rapidly evolving field, with many models attempting to describe its impact on the immune system, especially when paired with radiotherapy. Tumor response to this combination involves a complex spatiotemporal dynamic which makes either clinical or pre-clinical in vivo investigation across the resulting extensive solution space extremely difficult. In this review, several in silico models of the interaction between radiotherapy, immunotherapy, and the patient's immune system are examined. The study included only mathematical models published in English that investigated the effects of radiotherapy on the immune system, or the effect of immuno-radiotherapy with immune checkpoint inhibitors. The findings indicate that treatment efficacy was predicted to improve when both radiotherapy and immunotherapy were administered, compared to radiotherapy or immunotherapy alone. However, the models do not agree on the optimal schedule and fractionation of radiotherapy and immunotherapy. This corresponds to relevant clinical trials, which report an improved treatment efficacy with combination therapy, however, the optimal scheduling varies between clinical trials. This discrepancy between the models can be attributed to the variation in model approach and the specific cancer types modeled, making the determination of the optimum general treatment schedule and model challenging. Further research needs to be conducted with similar data sets to evaluate the best model and treatment schedule for a specific cancer type and stage.

Feasibility planning study of lattice radiotherapy for palliation in bulky tumors

Purpose

Lattice radiotherapy can potentially deliver high doses to the tumor core, while conventional doses to the periphery resulting in improved response rates in large tumors (> 5 cm). We assessed the feasibility of planning lattice radiotherapy and dosimetrically compared it with conventional radiotherapy.

Methods

This retrospective dosimetric study evaluated 10 patients with large tumors (> 5 cm) treated with palliative intent with a dose of 20Gy in five fractions. High-dose lattice points were created at doses of 50Gy in non-hepatic tumors and 35Gy in hepatic tumors. Lattice plans were compared with treatment plans regarding dose coverage and organ-at-risk dosimetry.

Results

Treated sites included soft tissue metastases to the neck, lungs, abdomen, pelvis, and liver. The mean lesion volume was 1103 cc (352-3173 cc). The maximum tumor size was 16 cm. The target volume coverage was > 95% in all but one case (88% to achieve organ constraints). Dosimetry and organ-at-risk doses were similar in both palliative treatment and simulated lattice plans.

Conclusion

Lattice radiotherapy is feasible in large tumors using volumetric-modulated arc therapy and achieves good coverage while meeting organ constraints. However, a prospective clinical evaluation is required to confirm its efficacy.

The significance of dose heterogeneity on the anti-tumor response of minibeam radiation therapy

BACKGROUND AND PURPOSE

Proton Minibeam Radiation Therapy (pMBRT) is an unconventional radiation technique based on a strong modulation of the dose deposition. Due to its specific pattern, pMBRT involves several dosimetry (peak and valley doses, peak-to-valley dose ratio (PVDR)) and geometrical parameters (beam width, spacing) that can influence the biological response. This study aims at contributing to the efforts to deepen the comprehension of how the various parameters relate to central biological mechanisms, particularly anti-tumor immunity, and how these correlations affect treatment outcomes with the goal to fully unleash the potential of pMBRT. We also evaluated the effects of X-ray MBRT to further elucidate the influence of peak dose and dose heterogeneity.

METHODS AND MATERIALS

An orthotopic rat model of glioblastoma underwent several pMBRT configurations. The impact of different dosimetric parameters on survival and on the modulation of crucial mechanisms for pMBRT, such as immune response, was investigated. The latter was assessed by immunohistochemistry and flow cytometry at 7 days post-irradiation.

RESULTS

Survival was improved across the various pMBRT regimens via maintaining a minimum valley dose as well as a higher dose heterogeneity, which is driven by peak dose. While the mean dose did not impact immune infiltration, a higher PVDR promoted a less immunosuppressive microenvironment.

CONCLUSIONS

Our results suggest that both tumor eradication, and immune infiltration are associated with higher dose heterogeneity. Higher dose heterogeneity was achieved by optimizing the peak dose, as well as maintaining a minimum valley dose. These parameters contributed to direct tumor eradication as well as reduction of immunosuppression, which is a departure from the more immunosuppressive tumor environment found in conventional proton therapy that delivers uniform dose distributions.

Comparative Analysis of Dose Gradients and Valley Doses in Pelvic Lattice Radiotherapy in RapidArc and IMRT

PURPOSE

This study evaluated the dose fall off and valley dose percentage in pelvic cancer Lattice Radiotherapy (LRT) using various treatment techniques.

METHODS

Forty five treatment plans were developed for 15 patients undergoing radiotherapy using a linear accelerator. Plans were categorized into three sets: RapidArc (RA), seven-field intensity-modulated radiation therapy (IMRT), and nine-field IMRT, both for high-dose (HD) vertices and the entire planning target volume (PTV). Dose fall-off indices were analyzed using the normalized dose fall-off index (ʌ) to compare the rate of dose decrease beyond HD vertices. Valley dose percentages were determined by analyzing dose profiles between HD vertices to quantify lower dose percentages. Analysis involved averaging normalized dose fall-off index (ʌ) values, valley dose percentages and grouping valley doses to assess variation with respect to center-to-center (CTC) intervals between HD vertices for all the techniques.

RESULTS

RA plans achieved sharper dose fall-off beyond HD vertices compared to seven-field and nine-field IMRT techniques, with decreasing values as distance from the central plane increased. RA plans also exhibited higher valley doses (62.05%) relative to nine-field IMRT (55.02%) and seven-field IMRT (56.56%) for an average CTC distance of 3.75 cm, showing significant variability across CTC intervals.

CONCLUSIONS

RA plans achieve steeper dose fall-off and higher valley doses compared to IMRT, effective for pelvic LRT but less suitable for grid therapy due to challenges with MLCs. Minimal differences in valley doses between nine-field and seven-field IMRT suggest limited impact from beam angles, guiding treatment optimization. The equations derived can be utilized for dosimetric evaluation and clinical planning in grid therapy, emphasizing their practical relevance in treatment strategy development.

Rotationally Intensified Proton Lattice: A Novel Lattice Technique Using Spot-Scanning Proton Arc Therapy

Purpose

The aim of this study was to explore the feasibility and dosimetric advantage of using spot-scanning proton arc (SPArc) for lattice radiation therapy in comparison with volumetric-modulated arc therapy (VMAT) and intensity modulated proton therapy (IMPT) lattice techniques.

Methods

Lattice plans were retrospectively generated for 14 large tumors across the abdomen, pelvis, lung, and head-and-neck sites using VMAT, IMPT, and SPArc techniques. Lattice geometries comprised vertices 1.5 cm in diameter that were arrayed in a body-centered cubic lattice with a 6-cm lattice constant. The prescription dose was 20 Gy (relative biological effectiveness [RBE]) in 5 fractions to the periphery of the tumor, with a simultaneous integrated boost of 66.7 Gy (RBE) as a minimum dose to the vertices. Organ-at-risk constraints per American Association of Physicists in Medicine Task Group 101were prioritized. Dose-volume histograms were extracted and used to identify maximum, minimum, and mean doses; equivalent uniform dose; D95%, D50%, D10%, D5%; V19Gy; peak-to-valley dose ratio (PVDR); and gradient index (GI). The treatment delivery time of IMPT and SPArc were simulated based on the published proton delivery sequence model.

Results

Median tumor volume was 577 cc with a median of 4.5 high-dose vertices per plan. Low-dose coverage was maintained in all plans (median V19Gy: SPArc 96%, IMPT 96%, VMAT 92%). SPArc generated significantly greater dose gradients as measured by PVDR (SPArc 4.0, IMPT 3.6, VMAT 3.2; SPArc-IMPT P = .0001, SPArc-VMAT P < .001) and high-dose GI (SPArc 5.9, IMPT 11.7, VMAT 17.1; SPArc-IMPT P = .001, SPArc-VMAT P < .01). Organ-at-risk constraints were met in all plans. Simulated delivery time was significantly improved with SPArc compared with IMPT (510 seconds vs 637 seconds, P < .001).

Conclusions

SPArc therapy was able to achieve high-quality lattice plans for various sites with superior gradient metrics (PVDR and GI) when compared with VMAT and IMPT. Clinical implementation is warranted.

Automated lattice radiation therapy treatment planning personalised to tumour size and shape

PURPOSE

Lattice radiation therapy (LRT) alternates regions of high and low doses inside the tumour. Whilst this technique reported positive results in tumour size reduction, optimal lattice parameters are still unknown. We introduce an automated LRT planning method personalised to tumour shape and designed to allow investigation of lattice geometry.

METHODS

Patients with retroperitoneal sarcoma were considered for inclusion. Automation was performed with the Eclipse Scripting Application Interface (v16, Varian Medical Systems, Palo Alto). By iterating over vertex size (V) and centre-to-centre distance (D), vertices were segmented within the gross tumour volume (GTV) in an alternating square pattern. Iterations stopped when the number of inserted vertices was contained between a prespecified lower and upper bound. Forty sets of lattices were considered, produced by varying V and D in five lower/upper bound pairs. Best-scoring sets were determined with a score favouring the maximization of GTV dose uniformity and heterogeneity whilst minimizing the maximum dose to organs at risk.

RESULTS

Fifty patients with tumour volumes between 150 cm3 and 10,000 cm3 were included. Best-scoring sets were characterised by a low number of vertices (<15). Based on the best-scoring set, the predicted parameters to use for new patients were V = 0.19 (GTV volume)1/3 and D = 2V, in centimetres. The number of vertices (N) to insert in the GTV can be estimated with N ≤ (24 × 3% GTV volume)/(4πV3).

CONCLUSIONS

The automated LRT treatment planning personalised to tumour size allows investigation of lattice geometry over a large range of GTV volumes.

Strategies for overcoming tumour resistance to immunotherapy: harnessing the power of radiation therapy

Immune checkpoint inhibitors (ICI) have revolutionized cancer treatment; yet their efficacy remains variable across patients. This review delves into the intricate interplay of tumour characteristics contributing to resistance against ICI therapy and suggests that combining with radiotherapy holds promise. Radiation, known for its ability to trigger immunogenic cell death and foster an in situ vaccination effect, may counteract these resistance mechanisms, enhancing ICI response and patient outcomes. However, particularly when delivered at high-dose, it may trigger immunosuppressive mechanism and consequent side-effects. Notably, low-dose radiotherapy (LDRT), with its capacity for tumour reprogramming and reduced side effects, offers the potential for widespread application. Preclinical and clinical studies have shown encouraging results in this regard.

Spatially Fractionated Radiotherapy in the Era of Immunotherapy

Spatially fractionated radiotherapy (SFRT) includes historical grid therapy approaches but more recently encompasses the controlled introduction of one or more cold dose regions using intensity modulation delivery techniques. The driving hypothesis behind SFRT is that it may allow for an increased immune response that is otherwise suppressed by radiation effects. With both two- and three-dimensional SFRT approaches, SFRT dose distributions typically include multiple dose cold spots or valleys. Despite its unconventional methods, reported clinical experience shows that SFRT can sometimes induce marked tumor regressions, even in patients with large hypoxic tumors. Preclinical models using extreme dose distributions (i.e., half-sparing) have been shown to nevertheless result in full tumor eradications, a more robust immune response, and systemic anti-tumor immunity. SFRT takes advantage of the complementary immunomodulatory features of low- and high-dose radiotherapy to integrate the delivery of both into a single target. Clinical trials using three-dimensional SFRT (i.e., lattice-like dose distributions) have reported both promising tumor and toxicity results, and ongoing clinical trials are investigating synergy between SFRT and immunotherapies.

The Effects of Radiation Dose Heterogeneity on the Tumor Microenvironment and Anti-Tumor Immunity

Radiotherapy elicits dose- and lineage-dependent effects on immune cell survival, migration, activation, and proliferation in targeted tumor microenvironments. Radiation also stimulates phenotypic changes that modulate the immune susceptibility of tumor cells. This has raised interest in using radiotherapy to promote greater response to immunotherapies. To clarify the potential of such combinations, it is critical to understand how best to administer radiation therapy to achieve activation of desired immunologic mechanisms. In considering the multifaceted process of priming and propagating anti-tumor immune response, radiation dose heterogeneity emerges as a potential means for simultaneously engaging diverse dose-dependent effects in a single tumor environment. Recent work in spatially fractionated external beam radiation therapy demonstrates the expansive immune responses achievable when a range of high to low dose radiation is delivered in a tumor. Brachytherapy and radiopharmaceutical therapies deliver inherently heterogeneous distributions of radiation that may contribute to immunogenicity. This review evaluates the interplay of radiation dose and anti-tumor immune response and explores emerging methodological approaches for investigating the effects of heterogeneous dose distribution on immune responses.

The Peaks and Valleys of Photon Versus Proton Spatially Fractionated Radiotherapy

Spatially-fractionated radiotherapy (SFRT) delivers high doses to small areas of tumor while sparing adjacent tissue, including intervening disease. In this review, we explore the evolution of SFRT technological advances, contrasting approaches with photon and proton beam radiotherapy. We discuss unique dosimetric considerations and physical properties of SFRT, as well as review the preclinical literature that provides an emerging understanding of biological mechanisms. We emphasize crucial areas of future study and highlight clinical trials that are underway to assess SFRT's safety and efficacy, with a focus on immunotherapeutic synergies. The review concludes with practical considerations for SFRT's clinical application, advocating for strategies that leverage its unique dosimetric and biological properties for improved patient outcomes.

Optimizing GRID and Lattice Spatially Fractionated Radiation Therapy: Innovative Strategies for Radioresistant and Bulky Tumor Management

Treating radioresistant and bulky tumors is challenging due to their inherent resistance to standard therapies and their large size. GRID and lattice spatially fractionated radiation therapy (simply referred to GRID RT and LRT) offer promising techniques to tackle these issues. Both approaches deliver radiation in a grid-like or lattice pattern, creating high-dose peaks surrounded by low-dose valleys. This pattern enables the destruction of significant portions of the tumor while sparing healthy tissue. GRID RT uses a 2-dimensional pattern of high-dose peaks (15-20 Gy), while LRT delivers a three-dimensional array of high-dose vertices (10-20 Gy) spaced 2-5 cm apart. These techniques are beneficial for treating a variety of cancers, including soft tissue sarcomas, osteosarcomas, renal cell carcinoma, melanoma, gastrointestinal stromal tumors (GISTs), pancreatic cancer, glioblastoma, and hepatocellular carcinoma. The specific grid and lattice patterns must be carefully tailored for each cancer type to maximize the peak-to-valley dose ratio while protecting critical organs and minimizing collateral damage. For gynecologic cancers, the treatment plan should align with the international consensus guidelines, incorporating concurrent chemotherapy for optimal outcomes. Despite the challenges of precise dosimetry and patient selection, GRID RT and LRT can be cost-effective using existing radiation equipment, including particle therapy systems, to deliver targeted high-dose radiation peaks. This phased approach of partial high-dose induction radiation therapy with standard fractionated radiation therapy maximizes immune modulation and tumor control while reducing toxicity. Comprehensive treatment plans using these advanced techniques offer a valuable framework for radiation oncologists, ensuring safe and effective delivery of therapy for radioresistant and bulky tumors. Further clinical trials data and standardized guidelines will refine these strategies, helping expand access to innovative cancer treatments.

Which Modality of SFRT Should be Considered First for Bulky Tumor Radiation Therapy, GRID or LATTICE?

Spatially fractionated radiation therapy (SFRT), also known as the GRID and LATTICE radiotherapy (GRT, LRT), the concept of treating tumors by delivering a spatially modulated dose with highly non-uniform dose distributions, is a treatment modality of growing interest in radiation oncology, physics, and radiation biology. Clinical experience in SFRT has suggested that GRID and LATTICE therapy can achieve a high response and low toxicity in the treatment of refractory and bulky tumors. Limited initially to GRID therapy using block collimators, advanced, and versatile multi-leaf collimators, volumetric modulated arc technologies and particle therapy have since increased the capabilities and individualization of SFRT and expanded the clinical investigation of SFRT to various dosing regimens, multiple malignancies, tumor types and sites. As a 3D modulation approach outgrown from traditional 2D GRID, LATTICE therapy aims to reconfigure the traditional SFRT as spatial modulation of the radiation is confined solely to the tumor volume. The distinctively different beam geometries used in LATTICE therapy have led to appreciable variations in dose-volume distributions, compared to GRID therapy. The clinical relevance of the variations in dose-volume distribution between LATTICE and traditional GRID therapies is a crucial factor in determining their adoption in clinical practice. In this Point-Counterpoint contribution, the authors debate the pros and cons of GRID and LATTICE therapy. Both modalities have been used in clinics and their applicability and optimal use have been discussed in this article.

Spatially fractionated radiation therapy: a critical review on current status of clinical and preclinical studies and knowledge gaps

Spatially fractionated radiation therapy (SFRT) is a therapeutic approach with the potential to disrupt the classical paradigms of conventional radiation therapy. The high spatial dose modulation in SFRT activates distinct radiobiological mechanisms which lead to a remarkable increase in normal tissue tolerances. Several decades of clinical use and numerous preclinical experiments suggest that SFRT has the potential to increase the therapeutic index, especially in bulky and radioresistant tumors. To unleash the full potential of SFRT a deeper understanding of the underlying biology and its relationship with the complex dosimetry of SFRT is needed. This review provides a critical analysis of the field, discussing not only the main clinical and preclinical findings but also analyzing the main knowledge gaps in a holistic way.

Tuning spatially fractionated radiotherapy dose profiles using the moiré effect

Spatially Fractionated Radiotherapy (SFRT) has demonstrated promising potential in cancer treatment, combining the advantages of reduced post-radiation effects and enhanced local control rates. Within this paradigm, proton minibeam radiotherapy (pMBRT) was suggested as a new treatment modality, possibly producing superior normal tissue sparing to conventional proton therapy, leading to improvements in patient outcomes. However, an effective and convenient beam generation method for pMBRT, capable of implementing various optimum dose profiles, is essential for its real-world application. Our study investigates the potential of utilizing the moiré effect in a dual collimator system (DCS) to generate pMBRT dose profiles with the flexibility to modify the center-to-center distance (CTC) of the dose distribution in a technically simple way.We employ the Geant4 Monte Carlo simulations tool to demonstrate that the angle between the two collimators of a DCS can significantly impact the dose profile. Varying the DCS angle from 10∘ to 50∘ we could cover CTC ranging from 11.8 mm to 2.4 mm, respectively. Further investigations reveal the substantial influence of the multi-slit collimator's (MSC) physical parameters on the spatially fractionated dose profile, such as period (CTC), throughput, and spacing between MSCs. These findings highlight opportunities for precision dose profile adjustments tailored to specific clinical scenarios.The DCS capacity for rapid angle adjustments during the energy transition stages of a spot scanning system can facilitate dynamic alterations in the irradiation profile, enhancing dose contrast in normal tissues. Furthermore, its unique attribute of spatially fractionated doses in both lateral directions could potentially improve normal tissue sparing by minimizing irradiated volume. Beyond the realm of pMBRT, the dual MSC system exhibits remarkable versatility, showing compatibility with different types of beams (X-rays and electrons) and applicability across various SFRT modalities.Our study illuminates the dual MSC system's potential as an efficient and adaptable tool in the refinement of pMBRT techniques. By enabling meticulous control over irradiation profiles, this system may expedite advancements in clinical and experimental applications, thereby contributing to the evolution of SFRT strategies.

Harnessing the cGAS-STING pathway to potentiate radiation therapy: current approaches and future directions

In this review, we cover the current understanding of how radiation therapy, which uses ionizing radiation to kill cancer cells, mediates an anti-tumor immune response through the cGAS-STING pathway, and how STING agonists might potentiate this. We examine how cGAS-STING signaling mediates the release of inflammatory cytokines in response to nuclear and mitochondrial DNA entering the cytoplasm. The significance of this in the context of cancer is explored, such as in response to cell-damaging therapies and genomic instability. The contribution of the immune and non-immune cells in the tumor microenvironment is considered. This review also discusses the burgeoning understanding of STING signaling that is independent of inflammatory cytokine release and the various mechanisms by which cancer cells can evade STING signaling. We review the available data on how ionizing radiation stimulates cGAS-STING signaling as well as how STING agonists may potentiate the anti-tumor immune response induced by ionizing radiation. There is also discussion of how novel radiation modalities may affect cGAS-STING signaling. We conclude with a discussion of ongoing and planned clinical trials combining radiation therapy with STING agonists, and provide insights to consider when planning future clinical trials combining these treatments.

The Next Chapter in Immunotherapy and Radiation Combination Therapy: Cancer-Specific Perspectives

Immunotherapeutic agents have revolutionized cancer treatment over the past decade. However, most patients fail to respond to immunotherapy alone. A growing body of preclinical studies highlights the potential for synergy between radiation therapy and immunotherapy, but the outcomes of clinical studies have been mixed. This review summarizes the current state of immunotherapy and radiation combination therapy across cancers, highlighting existing challenges and promising areas for future investigation.

Practice Patterns of Spatially Fractionated Radiation Therapy: A Clinical Practice Survey

Purpose

Spatially fractionated radiation therapy (SFRT) is increasingly used for bulky advanced tumors, but specifics of clinical SFRT practice remain elusive. This study aimed to determine practice patterns of GRID and Lattice radiation therapy (LRT)-based SFRT.

Methods and Materials

A survey was designed to identify radiation oncologists' practice patterns of patient selection for SFRT, dosing/planning, dosimetric parameter use, SFRT platforms/techniques, combinations of SFRT with conventional external beam radiation therapy (cERT) and multimodality therapies, and physicists' technical implementation, delivery, and quality procedures. Data were summarized using descriptive statistics. Group comparisons were analyzed with permutation tests.

Results

The majority of practicing radiation oncologists (United States, 100%; global, 72.7%) considered SFRT an accepted standard-of-care radiation therapy option for bulky/advanced tumors. Treatment of metastases/recurrences and nonmetastatic primary tumors, predominantly head and neck, lung cancer and sarcoma, was commonly practiced. In palliative SFRT, regimens of 15 to 18 Gy/1 fraction predominated (51.3%), and in curative-intent treatment of nonmetastatic tumors, 15 Gy/1 fraction (28.0%) and fractionated SFRT (24.0%) were most common. SFRT was combined with cERT commonly but not always in palliative (78.6%) and curative-intent (85.7%) treatment. SFRT-cERT time sequencing and cERT dose adjustments were variable. In curative-intent treatment, concurrent chemotherapy and immunotherapy were found acceptable by 54.5% and 28.6%, respectively. Use of SFRT dosimetric parameters was highly variable and differed between GRID and LRT. SFRT heterogeneity dosimetric parameters were more commonly used (P = .008) and more commonly thought to influence local control (peak dose, P = .008) in LRT than in GRID therapy.

Conclusions

SFRT has already evolved as a clinical practice pattern for advanced/bulky tumors. Major treatment approaches are consistent and follow the literature, but SFRT-cERT combination/sequencing and clinical utilization of dosimetric parameters are variable. These areas may benefit from targeted education and standardization, and knowledge gaps may be filled by incorporating identified inconsistencies into future clinical research.

Combining spatially fractionated radiation therapy (SFRT) and immunotherapy opens new rays of hope for enhancing therapeutic ratio

Spatially Fractionated Radiation Therapy (SFRT) is a form of radiotherapy that delivers a single large dose of radiation within the target volume in a heterogeneous pattern with regions of peak dosage and regions of under dosage. SFRT types can be defined by how the heterogeneous pattern of radiation is obtained. Immune checkpoint inhibitors (ICIs) have been approved for various malignant tumors and are widely used to treat patients with metastatic cancer. The efficacy of ICI monotherapy is limited due to the "cold" tumor microenvironment. Fractionated radiotherapy can achieve higher doses per fraction to the target tumor, and induce immune activation (immodulate tumor immunogenicity and microenvironment). Therefore, coupling ICI therapy and fractionated radiation therapy could significantly improve the outcome of metastatic cancer. This review focuses on both preclinical and clinical studies that use a combination of radiotherapy and ICI therapy in cancer.

Immune Priming with Spatially Fractionated Radiation Therapy

PURPOSE OF REVIEW

This review aims to summarize the current preclinical and clinical evidence of nontargeted immune effects of spatially fractionated radiation therapy (SFRT). We then highlight strategies to augment the immunomodulatory potential of SFRT in combination with immunotherapy (IT).

RECENT FINDINGS

The response of cancer to IT is limited by primary and acquired immune resistance, and strategies are needed to prime the immune system to increase the efficacy of IT. Radiation therapy can induce immunologic effects and can potentially be used to synergize the effects of IT, although the optimal combination of radiation and IT is largely unknown. SFRT is a novel radiation technique that limits ablative doses to tumor subvolumes, and this highly heterogeneous dose deposition may increase the immune-rich infiltrate within the targeted tumor with enhanced antigen presentation and activated T cells in nonirradiated tumors. The understanding of nontargeted effects of SFRT can contribute to future translational strategies to combine SFRT and IT. Integration of SFRT and IT is an innovative approach to address immune resistance to IT with the overall goal of improving the therapeutic ratio of radiation therapy and increasing the efficacy of IT.

Activation of STING in Response to Partial-Tumor Radiation Exposure

PURPOSE

To determine the mechanisms involved in partial volume radiation therapy (RT)-induced tumor response.

METHODS AND MATERIALS

We investigated 67NR murine orthotopic breast tumors in Balb/c mice and Lewis lung carcinoma (LLC cells; WT, Crispr/Cas9 Sting KO, and Atm KO) injected in the flank of C57Bl/6, cGAS, or STING KO mice. RT was delivered to 50% or 100% of the tumor volume using a 2 × 2 cm collimator on a microirradiator allowing precise irradiation. Tumors and blood were collected at 6, 24, and 48 hours post-RT and assessed for cytokine measurements.

RESULTS

There is a significant activation of the cGAS/STING pathway in the hemi-irradiated tumors compared with control and to 100% exposed 67NR tumors. In the LLC model, we determined that an ATM-mediated noncanonical activation of STING is involved. We demonstrated that the partial exposure RT-mediated immune response is dependent on ATM activation in the tumor cells and on the STING activation in the host, and cGAS is dispensable. Our results also indicate that partial volume RT stimulates a proinflammatory cytokine response compared with the anti-inflammatory profile induced by 100% tumor volume exposure.

CONCLUSIONS

Partial volume RT induces an antitumor response by activating STING, which stimulates a specific cytokine signature as part of the immune response. However, the mechanism of this STING activation, via the canonical cGAS/STING pathway or a noncanonical ATM-driven pathway, depends on the tumor type. Identifying the upstream pathways responsible for STING activation in the partial RT-mediated immune response in different tumor types would improve this therapy and its potential combination with immune checkpoint blockade and other antitumor therapies.

Inhomogeneously distributed ferroptosis with a high peak-to-valley ratio may improve the antitumor immune response

Combined radiotherapy (RT) and mild hyperthermia have been used clinically for decades to increase local control. Both modalities tend to achieve a homogeneous dose distribution within treatment targets to induce immunogenic cell death. However, marked, and long-lasting abscopal effects have not usually been observed. We proposed a hypothesis to emphasize the importance of the peak-to-valley ratio of the dose distribution inside the tumor to induce immunogenic ferrroptosis in peak area while avoid nonimmunogenic ferroptosis in valley area. Although inhomogeneous distributed energy absorption has been noted in many anticancer medical fields, the idea of sedulously created dose inhomogeneity related to antitumor immunity has not been discussed. To scale up the peak-to-valley ratio, we proposed possible implications by the combination of nanoparticles (NP) with conventional RT or hyperthermia, or the use of a high modulation depth of extremely low frequency hyperthermia or high resolution spatially fractionated radiotherapy (SFRT) to enhance the antitumor immune reactions.

Radio-immune response modelling for spatially fractionated radiotherapy

Objective.Radiation-induced cell death is a complex process influenced by physical, chemical and biological phenomena. Although consensus on the nature and the mechanism of the bystander effect were not yet made, the immune process presumably plays an important role in many aspects of the radiotherapy including the bystander effect. A mathematical model of immune response during and after radiation therapy is presented.Approach.Immune response of host body and immune suppression of tumor cells are modelled with four compartments in this study; viable tumor cells, T cell lymphocytes, immune triggering cells, and doomed cells. The growth of tumor was analyzed in two distinctive modes of tumor status (immune limited and immune escape) and its bifurcation condition.Main results.Tumors in the immune limited mode can grow only up to a finite size, named as terminal tumor volume analytically calculated from the model. The dynamics of the tumor growth in the immune escape mode is much more complex than the tumors in the immune limited mode especially when the status of tumor is close to the bifurcation condition. Radiation can kill tumor cells not only by radiation damage but also by boosting immune reaction.Significance.The model demonstrated that the highly heterogeneous dose distribution in spatially fractionated radiotherapy (SFRT) can make a drastic difference in tumor cell killing compared to the homogeneous dose distribution. SFRT cannot only enhance but also moderate the cell killing depending on the immune response triggered by many factors such as dose prescription parameters, tumor volume at the time of treatment and tumor characteristics. The model was applied to the lifted data of 67NR tumors on mice and a sarcoma patient treated multiple times over 1200 days for the treatment of tumor recurrence as a demonstration.

Monte Carlo study of the free radical yields in minibeam radiation therapy

BACKGROUND

Minibeam radiation therapy (MBRT) is a novel technique which has been shown to widen the therapeutic window through significant normal tissue sparing. Despite the heterogeneous dose distributions, tumor control is still ensured. Nevertheless the exact radiobiological mechanisms responsible for MBRT efficacy are not fully understood.

PURPOSE

Reactive oxygen species (ROS) resulting from water radiolysis were investigated given their implications not only on targeted DNA damage, but also for their role in the immune response and non-targeted cell signalling effects: two potential drivers of MBRT efficacy.

METHODS

Monte Carlo simulations were performed using TOPAS-nBio to carry out the irradiation of a water phantom with beams of protons (pMBRT), photons (xMBRT), 4 He ions (HeMBRT), and 12 C ions (CMBRT). Primary yields at the end of the chemical stage were calculated in spheres of 20 μm diameter, located in the peaks and valleys at various depths up to the Bragg peak. The chemical stage was limited to 1 ns to approximate biological scavenging, and the yield of · OH, H2 O2 , ande aq - ${\rm e}^{-}_{\rm aq}$ was recorded.

RESULTS

Beyond 10 mm, there were no substantial differences in the primary yields between peaks and valleys of the pMBRT and HeMBRT modalities. For xMBRT, there was a lower primary yield of the radical species · OH ande aq - ${\rm e}^{-}_{\rm aq}$ at all depths in the valleys compared to the peaks, and a higher primary yield of H2 O2 . Compared to the peaks, the valleys of the CMBRT modality were subject to a higher · OH ande aq - ${\rm e}^{-}_{\rm aq}$ yield, and lower H2 O2 yield. This difference between peaks and valleys became more severe in depth. Near the Bragg peak, the increase in the primary yield of the valleys over the peaks was 6% and 4% for · OH ande aq - ${\rm e}^{-}_{\rm aq}$ respectively, while there was a decrease in the yield of H2 O2 by 16%. Given the similar ROS primary yields in the peaks and valleys of pMBRT and HeMBRT, the level of indirect DNA damage is expected to be directly proportional to the peak to valley dose ratio (PVDR). The difference in the primary yields implicates a lower level of indirect DNA damage in the valleys compared to the peaks than what would be suggested by the PVDR for xMBRT, and a higher level for CMBRT.

CONCLUSIONS

These results highlight the notion that depending on the particle chosen, one can expect different levels of ROS in the peaks and valley that goes beyond what would be expected by the macroscopic PVDR. The combination of MBRT with heavier ions is shown to be particularly interesting as the primary yield in the valleys progressively diverges from the level observed in the peaks as the LET increases. While differences in the reported · OH yields of this work implicated the indirect DNA damage, H2 O2 yields particularly implicate non-targeted cell signalling effects, and therefore this work provides a point of reference for future simulations in which the distribution of this species at more biologically relevant timescales could be investigated.

Clinical aspects of spatially fractionated radiation therapy treatments

PURPOSE

To provide clinical guidance for centers wishing to implement photon spatially fractionated radiation therapy (SFRT) treatments using either a brass grid or volumetric modulated arc therapy (VMAT) lattice approach.

METHODS

We describe in detail processes which have been developed over the course of a 3-year period during which our institution treated over 240 SFRT cases. The importance of patient selection, along with aspects of simulation, treatment planning, quality assurance, and treatment delivery are discussed. Illustrative examples involving clinical cases are shown, and we discuss safety implications relevant to the heterogeneous dose distributions.

RESULTS

SFRT can be an effective modality for tumors which are otherwise challenging to manage with conventional radiation therapy techniques or for patients who have limited treatment options. However, SFRT has several aspects which differ drastically from conventional radiation therapy treatments. Therefore, the successful implementation of an SFRT treatment program requires the multidisciplinary expertise and collaboration of physicians, physicists, dosimetrists, and radiation therapists.

CONCLUSIONS

We have described methods for patient selection, simulation, treatment planning, quality assurance and delivery of clinical SFRT treatments which were built upon our experience treating a large patient population with both a brass grid and VMAT lattice approach. Preclinical research and patient trials aimed at understanding the mechanism of action are needed to elucidate which patients may benefit most from SFRT, and ultimately expand its use.

Application of radiomics in lung immuno-oncology

Radiomics is a rapidly evolving field of research that extracts and analyzes quantitative features within medical images. Those features are termed as radiomic features that can characterize a tumor in a comprehensive and quantitative manner with regard to its internal structure and heterogeneity. Radiomic features can be used, alone or in combination with demographic, histological, genomic, or proteomic data, for predicting prognosis or treatment response. Immunotherapy, or immune-oncology, is the study of cancer treatment by taking advantage of the body's immune system to prevent, control, and eliminate cancer. In this review, we first provide a brief introduction to both radiomics and immune-oncology in lung cancer. Then, we discuss the need for developing immune-oncology biomarkers, and the advantages of radiomics in identifying biomarkers related to immunotherapy. We also discuss potential areas in and out of tumors, such as the intra-tumoral hypoxic region and tumor microenvironment, where radiomic markers might be extracted, as well as a potential application of radiomic biomarkers in clinical lung cancer management. Finally, we present radiation and immune modulation in non-small cell lung cancer, clinical trials and their design to incorporate radiomic biomarkers, and radiomics-guided precision radiation therapy.

Novel unconventional radiotherapy techniques: Current status and future perspectives - Report from the 2nd international radiation oncology online seminar

•Improvement of therapeutic ratio by novel unconventional radiotherapy approaches.•Immunomodulation using high-dose spatially fractionated radiotherapy.•Boosting radiation anti-tumor effects by adding an immune-mediated cell killing.

Challenges in the combination of radiotherapy and immunotherapy for breast cancer

INTRODUCTION

Immunotherapy (IT) is showing promise in the treatment of breast cancer, but IT alone only benefits a minority of patients. Radiotherapy (RT) is usually included in the standard of care for breast cancer patients and is traditionally considered as a local form of treatment. The emerging knowledge of RT-induced systemic immune response, and the observation that the rare abscopal effect of RT on distant cancer metastases can be augmented by IT, have increased the enthusiasm for combinatorial immunoradiotherapy (IRT) for breast cancer patients. However, IRT largely follows the traditional sole RT and IT protocols and does not consider patient specificity, although patients' responses to treatment remain heterogeneous.

AREAS COVERED

This review discusses the rationale of IRT for breast cancer, the current knowledge, challenges, and future directions.

EXPERT OPINION

The synergy between RT and the immune system has been observed but not well understood at the basic level. The optimal dosages, timing, target, and impact of biomarkers are largely unknown. There is an urgent need to design efficacious pre-clinical and clinical trials to optimize IRT for cancer patients, maximize the synergy of radiation and immune response, and explore the abscopal effect in depth, taking into account patients' personal features.

Neoadjuvant Radiation Therapy with Interdigitated High-Dose LRT for Voluminous High-Grade Soft-Tissue Sarcoma

Purpose

To report a case of large extremity soft tissue sarcoma (2933 cc), safely treated with a novel approach of interdigitating high-dose LATTICE radiation therapy (LRT) with standard radiation therapy as a neoadjuvant treatment to surgery.

Patients and Methods

Four sessions of high-dose LRT were delivered in a weekly interval, interdigitated with standard radiation therapy. The LRT plan consisted of 15 high-dose vertices receiving a dose >12 Gy per session, with 2-3 Gy to the peripheral margin of the tumor. The patient underwent surgical excision 2 months after the new regimen of induction radiation therapy.

Results and Discussion

The patient tolerated the radiation therapy regimen well. The post-operative assessment revealed a negative surgical margin and over 95% necrosis of the total tumor volume. The post-surgical wound complication was mitigated by outpatient wound care. Interdigitating multiple sessions of high-dose LATTICE radiation treatments with standard neoadjuvant radiation therapy as a neoadjuvant therapy for soft tissue sarcoma was feasible and did not incur additional toxicity in this clinical case. A phase-I/II trial will be conducted to further evaluate the toxicity and efficacy of the new treatment strategy with the intent to increase the rate of pathologic necrosis, which has been shown to positively correlate with the overall survival.

Evaluation of the Role of the Immune System Response After Minibeam Radiation Therapy

PURPOSE

Minibeam radiation therapy (MBRT) is an innovative technique that uses a spatial dose modulation. The dose distribution consists of high doses (peaks) in the path of the minibeam and low doses (valleys). The underlying biological mechanism associated with MBRT efficacy remains currently unclear and thus we investigated the potential role of the immune system after treatment with MBRT.

METHODS AND MATERIALS

Rats bearing an orthotopic glioblastoma cell line were treated with 1 fraction of high dose conventional radiation therapy (30 Gy) or 1 fraction of the same mean dose in MBRT. Both immunocompetent (F344) and immunodeficient (Nude) rats were analyzed in survival studies. Systemic and intratumoral immune cell population changes were studied with flow cytometry and immunohistochemistry (IHC) 2 and 7 days after the irradiation.

RESULTS

The absence of response of Nude rats after MBRT suggested that T cells were key in the mode of action of MBRT. An inflammatory phenotype was observed in the blood 1 week after irradiation compared with conventional irradiation. Tumor immune cell analysis by flow cytometry showed a substantial infiltration of lymphocytes, specifically of CD8 T cells and B cells in both conventional and MBRT-treated animals. IHC revealed that MBRT induced a faster recruitment of CD8 and CD4 T cells. Animals that were cured by radiation therapy did not suffer tumor growth after reimplantation of tumoral cells, proving the long-term immunity response generated after a high dose of radiation.

CONCLUSIONS

Our findings show that MBRT can elicit a robust antitumor immune response in glioblastoma while avoiding the high toxicity of a high dose of conventional radiation therapy.

Salvage LATTICE radiotherapy for a growing tumour despite conventional radio chemotherapy treatment of lung cancer

A 40-year-old patient with cT4cN1M0 squamous cell lung cancer of the upper right lobe received preoperative induction chemotherapy. Systemic induction treatment failed to reverse tumour growth with the addition of conventional radiotherapy (RT). A salvage lattice RT boost of 12 Gy was administered immediately to increase the dose to the tumour. Conventional RT was resumed at the planned dose of 60 Gy. The tumour shrank rapidly, and the patient was surged. The postoperative pathology remained ypT0ypN0 status.

Dying of Stress: Chemotherapy, Radiotherapy, and Small-Molecule Inhibitors in Immunogenic Cell Death and Immunogenic Modulation

Innovative strategies to re-establish the immune-mediated destruction of malignant cells is paramount to the success of anti-cancer therapy. Accumulating evidence suggests that radiotherapy and select chemotherapeutic drugs and small molecule inhibitors induce immunogenic cell stress on tumors that results in improved immune recognition and targeting of the malignant cells. Through immunogenic cell death, which entails the release of antigens and danger signals, and immunogenic modulation, wherein the phenotype of stressed cells is altered to become more susceptible to immune attack, radiotherapies, chemotherapies, and small-molecule inhibitors exert immune-mediated anti-tumor responses. In this review, we discuss the mechanisms of immunogenic cell death and immunogenic modulation and their relevance in the anti-tumor activity of radiotherapies, chemotherapies, and small-molecule inhibitors. Our aim is to feature the immunological aspects of conventional and targeted cancer therapies and highlight how these therapies may be compatible with emerging immunotherapy approaches.

Longitudinal Changes and Predictive Value of Multiparametric MRI Features for Prostate Cancer Patients Treated with MRI-Guided Lattice Extreme Ablative Dose (LEAD) Boost Radiotherapy

We investigated the longitudinal changes in multiparametric MRI (mpMRI) (T2-weighted, Apparent Diffusion Coefficient (ADC), and Dynamic Contrast Enhanced (DCE-)MRI) of prostate cancer patients receiving Lattice Extreme Ablative Dose (LEAD) radiotherapy (RT) and the capability of their imaging features to predict RT outcome based on endpoint biopsies. Ninety-five mpMRI exams from 25 patients, acquired pre-RT and at 3-, 9-, and 24-months post-RT were analyzed. MRI/Ultrasound-fused biopsies were acquired pre- and at two-years post-RT (endpoint). Five regions of interest (ROIs) were analyzed: Gross tumor volume (GTV), normally-appearing tissue (NAT) and peritumoral volume in both peripheral (PZ) and transition (TZ) zones. Diffusion and perfusion radiomics features were extracted from mpMRI and compared before and after RT using two-tailed Student t-tests. Selected features at the four scan points and their differences (Δ radiomics) were used in multivariate logistic regression models to predict the endpoint biopsy positivity. Baseline ADC values were significantly different between GTV, NAT-PZ, and NAT-TZ (p-values < 0.005). Pharmaco-kinetic features changed significantly in the GTV at 3-month post-RT compared to baseline. Several radiomics features at baseline and three-months post-RT were significantly associated with endpoint biopsy positivity and were used to build models with high predictive power of this endpoint (AUC = 0.98 and 0.89, respectively). Our study characterized the RT-induced changes in perfusion and diffusion. Quantitative imaging features from mpMRI show promise as being predictive of endpoint biopsy positivity.

An International Consensus on the Design of Prospective Clinical–Translational Trials in Spatially Fractionated Radiation Therapy for Advanced Gynecologic Cancer

Simple Summary

Spatially fractionated radiation therapy (SFRT) delivers intentionally heterogenous dose to tumors. This is a major departure from current radiation therapy, which strives for uniform dose. Early pilot experience suggests promising treatment outcomes with SFRT in patients with challenging bulky tumors, including gynecologic cancer. Well-conducted prospective multi-institutional clinical trials are now needed to further test SFRT as a treatment modality. However, clinical trial development is hampered by the variabilities in SFRT approach and the overall unfamiliarity with heterogeneous dosing. A broad consensus among SFRT experts, potential investigators, and the wider radiation oncology community is needed so that clinical trials in SFRT can be successfully designed and carried out. We developed an international consensus guideline for the design parameters of clinical/translational trials in SFRT for gynecologic cancer. High-to-moderate consensus was achieved, and harmonized fundamental design parameters for SFRT trials in advanced gynecologic cancer were defined.

Abstract

Despite the unexpectedly high tumor responses and limited treatment-related toxicities observed with SFRT, prospective multi-institutional clinical trials of SFRT are still lacking. High variability of SFRT technologies and methods, unfamiliar complex dose and prescription concepts for heterogeneous dose and uncertainty regarding systemic therapies present major obstacles towards clinical trial development. To address these challenges, the consensus guideline reported here aimed at facilitating trial development and feasibility through a priori harmonization of treatment approach and the full range of clinical trial design parameters for SFRT trials in gynecologic cancer. Gynecologic cancers were evaluated for the status of SFRT pilot experience. A multi-disciplinary SFRT expert panel for gynecologic cancer was established to develop the consensus through formal panel review/discussions, appropriateness rank voting and public comment solicitation/review. The trial design parameters included eligibility/exclusions, endpoints, SFRT technology/technique, dose/dosimetric parameters, systemic therapies, patient evaluations, and embedded translational science. Cervical cancer was determined as the most suitable gynecologic tumor for an SFRT trial. Consensus emphasized standardization of SFRT dosimetry/physics parameters, biologic dose modeling, and specimen collection for translational/biological endpoints, which may be uniquely feasible in cervical cancer. Incorporation of brachytherapy into the SFRT regimen requires additional pre-trial pilot investigations. Specific consensus recommendations are presented and discussed.

Combining FLASH and spatially fractionated radiation therapy: The best of both worlds

FLASH radiotherapy (FLASH-RT) and spatially fractionated radiation therapy (SFRT) are two new therapeutical strategies that use non-standard dose delivery methods to reduce normal tissue toxicity and increase the therapeutic index. Although likely based on different mechanisms, both FLASH-RT and SFRT have shown to elicit radiobiological effects that significantly differ from those induced by conventional radiotherapy. With the therapeutic potential having been established separately for each technique, the combination of FLASH-RT and SFRT could therefore represent a winning alliance. In this review, we discuss the state of the art, advantages and current limitations, potential synergies, and where a combination of these two techniques could be implemented today or in the near future.

The intercellular communications mediating radiation-induced bystander effects and their relevance to environmental, occupational, and therapeutic exposures

PURPOSE

The assumption that traversal of the cell nucleus by ionizing radiation is a prerequisite to induce genetic damage, or other important biological responses, has been challenged by studies showing that oxidative alterations extend beyond the irradiated cells and occur also in neighboring bystander cells. Cells and tissues outside the radiation field experience significant biochemical and phenotypic changes that are often similar to those observed in the irradiated cells and tissues. With relevance to the assessment of long-term health risks of occupational, environmental and clinical exposures, measurable genetic, epigenetic, and metabolic changes have been also detected in the progeny of bystander cells. How the oxidative damage spreads from the irradiated cells to their neighboring bystander cells has been under intense investigation. Following a brief summary of the trends in radiobiology leading to this paradigm shift in the field, we review key findings of bystander effects induced by low and high doses of various types of radiation that differ in their biophysical characteristics. While notable mechanistic insights continue to emerge, here the focus is on the many means of intercellular communication that mediate these effects, namely junctional channels, secreted molecules and extracellular vesicles, and immune pathways.

CONCLUSIONS

The insights gained by studying radiation bystander effects are leading to a basic understanding of the intercellular communications that occur under mild and severe oxidative stress in both normal and cancerous tissues. Understanding the mechanisms underlying these communications will likely contribute to reducing the uncertainty of predicting adverse health effects following exposure to low dose/low fluence ionizing radiation, guide novel interventions that mitigate adverse out-of-field effects, and contribute to better outcomes of radiotherapeutic treatments of cancer. In this review, we highlight novel routes of intercellular communication for investigation, and raise the rationale for reconsidering classification of bystander responses, abscopal effects, and expression of genomic instability as non-targeted effects of radiation.

Monte Carlo evaluation of high-gradient magnetically focused planar proton minibeams in a passive nozzle

Objective. To investigate the potential of using a single quadrupole magnet with a high magnetic field gradient to create planar minibeams suitable for clinical applications of proton minibeam radiation therapy.Approach. We performed Monte Carlo simulations involving single quadrupole Halbach cylinders in a passively scattered nozzle in clinical use for proton therapy. Pencil beams produced by the nozzle of 10-15 mm initial diameters and particle range of ∼10-20 cm in water were focused by magnets with field gradients of 225-350 T m^-1and cylinder lengths of 80-110 mm to produce very narrow elongated (planar) beamlets. The corresponding dose distributions were scored in a water phantom. Composite minibeam dose distributions composed from three beamlets were created by laterally shifting copies of the single beamlet distribution to either side of a central beamlet. Modulated beamlets (with 18-30 mm nominal central SOBP) and corresponding composite dose distributions were created in a similar manner. Collimated minibeams were also compared with beams focused using one magnet/particle range combination.Main results. The focusing magnets produced planar beamlets with minimum lateral FWHM of ∼1.1-1.6 mm. Dose distributions composed from three unmodulated beamlets showed a high degree of proximal spatial fractionation and a homogeneous target dose. Maximal peak-to-valley dose ratios (PVDR) for the unmodulated beams ranged from 32 to 324, and composite modulated beam showed maximal PVDR ranging from 32 to 102 and SOBPs with good target dose coverage.Significance.Advantages of the high-gradient magnets include the ability to focus beams with phase space parameters that reflect beams in operation today, and post-waist particle divergence allowing larger beamlet separations and thus larger PVDR. Our results suggest that high gradient quadrupole magnets could be useful to focus beams of moderate emittance in clinical proton therapy.

Magnetic Resonance Imaging-Guided Online Adaptive Lattice Stereotactic Body Radiotherapy in Voluminous Liver Metastasis: Two Case Reports

Lattice Radiotherapy (LRT) is a technique in which heterogeneous doses are delivered to the target so large tumors can have optimal doses of radiation without compromising healthy tissue sparing. To date, case reports and case series documented its application for bulky tumors mainly in the pelvic region. LRT not only provides dosimetric advantages but also promotes tumor control by triggering some radiobiological and immunological pathways. We report two cases of giant liver metastases for whom other treatment options were not suitable. We treated both patients with Magnetic Resonance Image-Guided Radiotherapy (MRgRT) with online adaptive LRT (OALRT) technique. Adaptive plans were generated before each fraction. Tumors were observed to have regressed interfractionally so the location and number of spheres were adapted to tumor size and daily anatomy of the surrounding organs at risk (OAR). Both patients had good treatment compliance without any Grade 3+ side effects. They are both under follow-up and report improvement. By reporting the first application of OALRT by using MRgRT in liver metastases, we show that MRgRT is a promising modality for LRT technique with better target and OAR visualization as well as online adaptive planning before each fraction according to the daily anatomy of the patient.

Targeted Accumulation of Macrophages Induced by Microbeam Irradiation in a Tissue-Dependent Manner

Radiation therapy (RT) is a vital component of multimodal cancer treatment, and its immunomodulatory effects are a major focus of current therapeutic strategies. Macrophages are some of the first cells recruited to sites of radiation-induced injury where they can aid in tissue repair, propagate radiation-induced fibrogenesis and influence tumour dynamics. Microbeam radiation therapy (MRT) is a unique, spatially fractionated radiation modality that has demonstrated exceptional tumour control and reduction in normal tissue toxicity, including fibrosis. We conducted a morphological analysis of MRT-irradiated normal liver, lung and skin tissues as well as lung and melanoma tumours. MRT induced distinct patterns of DNA damage, reflecting the geometry of the microbeam array. Macrophages infiltrated these regions of peak dose deposition at variable timepoints post-irradiation depending on the tissue type. In normal liver and lung tissue, macrophages clearly demarcated the beam path by 48 h and 7 days post-irradiation, respectively. This was not reflected, however, in normal skin tissue, despite clear DNA damage marking the beam path. Persistent DNA damage was observed in MRT-irradiated lung carcinoma, with an accompanying geometry-specific influx of mixed M1/M2-like macrophage populations. These data indicate the unique potential of MRT as a tool to induce a remarkable accumulation of macrophages in an organ/tissue-specific manner. Further characterization of these macrophage populations is warranted to identify their organ-specific roles in normal tissue sparing and anti-tumour responses.