Preferential Effect of Synchrotron Microbeam Radiation Therapy on Intracerebral 9L Gliosarcoma Vascular Networks

Neurologic Changes Induced by Whole-Brain Synchrotron Microbeam Irradiation: 10-Month Behavioral and Veterinary Follow-Up

PURPOSE

Novel radiation therapy approaches have increased the therapeutic efficacy for malignant brain tumors over the past decades, but the balance between therapeutic gain and radiotoxicity remains a medical hardship. Synchrotron microbeam radiation therapy, an innovative technique, deposes extremely high (peak) doses in micron-wide, parallel microbeam paths, whereas the diffusing interbeam (valley) doses lie in the range of conventional radiation therapy doses. In this study, we evaluated normal tissue toxicity of whole-brain microbeam irradiation (MBI) versus that of a conventional hospital broad beam (hBB).

METHODS AND MATERIALS

Normal Fischer rats (n = 6-7/group) were irradiated with one of the two modalities, exposing the entire brain to MBI valley/peak doses of 0/0, 5/200, 10/400, 13/520, 17/680, or 25/1000 Gy or to hBB doses of 7, 10, 13, 17, or 25 Gy. Two additional groups of rats received an MBI valley dose of 10 Gy coupled with an hBB dose of 7 or 15 Gy (groups MBI17* and MBI25*). Behavioral parameters were evaluated for 10 months after irradiation combined with veterinary observations.

RESULTS

MBI peak doses of ≥680 Gy caused acute toxicity and death. Animals exposed to hBB or MBI dose-dependently gained less weight than controls; rats in the hBB25 and MBI25* groups died within 6 months after irradiation. Increasing doses of MBI caused hyperactivity but no other detectable behavioral alterations in our tests. Importantly, no health concerns were seen up to an MBI valley dose of 17 Gy.

CONCLUSIONS

While acute toxicity of microbeam exposures depends on very high peak doses, late toxicity mainly relates to delivery of high MBI valley doses. MBI seems to have a low impact on normal rat behavior, but further tests are warranted to fully explore this hypothesis. However, high peak and valley doses are well tolerated from a veterinary point of view. This normal tissue tolerance to whole-brain, high-dose MBI reveals a promising avenue for microbeam radiation therapy, that is, therapeutic applications of microbeams that are poised for translation to a clinical environment.

Microbeam Radiation Therapy Opens a Several Days' Vessel Permeability Window for Small Molecules in Brain Tumor Vessels

PURPOSE

Synchrotron microbeam radiation therapy (MRT), based on an inhomogeneous geometric and microscopic irradiation pattern of the tissues with high-dose and high-dose-rate x-rays, enhances the permeability of brain tumor vessels. This study attempted to determine the time and size range of the permeability window induced by MRT in the blood-brain (tumor) barrier.

METHODS AND MATERIALS

Rats-bearing 9L gliomas were exposed to MRT, either unidirectional (tumor dose, 406 Gy) or bidirectional (crossfired) (2 × 203 Gy). We measured vessel permeability to molecules of 3 sizes (Gd-DOTA, Dotarem, 0.56 kDa; gadolinium-labeled albumin, ∼74 kDa; and gadolinium-labeled IgG, 160 kDa) by daily in vivo magnetic resonance imaging, from 1 day before to 10 days after irradiation.

RESULTS

An equivalent tumor dose of bidirectional MRT delivered from 2 orthogonal directions increased tumor vessel permeability for the smallest molecule tested more effectively than unidirectional MRT. Bidirectional MRT also affected the permeability of normal contralateral vessels to a different extent than unidirectional MRT. Conversely, bidirectional MRT did not modify the permeability of normal or tumor vessels for both larger molecules (74 and 160 kDa).

CONCLUSIONS

High-dose bidirectional (cross-fired) MRT induced a significant increase in tumor vessel permeability for small molecules between the first and the seventh day after irradiation, whereas permeability of vessels in normal brain tissue remained stable. Such a permeability window could facilitate an efficient and safe delivery of intravenous small molecules (≤0.56 kDa) to tumoral tissues. A permeability window was not achieved by molecules larger than gado-grafted albumin (74 kDa). Vascular permeability for molecules between these 2 sizes has not been determined.

Preclinical photon minibeam radiotherapy using a custom collimator: Dosimetry characterization and preliminary in-vivo results on a glioma model

PURPOSE

The purpose of this study is to investigate the dosimetric characteristics of a collimator for minibeam radiotherapy (MBRT) with film dosimetry and Monte Carlo (MC) simulations. The outcome of MBRT with respect to conventional RT using a glioma preclinical model was also evaluated.

METHODS

A multi-slit collimator was designed to be used with commercial small animal irradiator. The collimator was built by aligning 0.6 mm wide and 5 mm thick parallel lead leaves at 0.4 mm intervals. Dosimetry characteristics were evaluated by Gafchromic (CG) films and TOPAS Monte Carlo (MC) code. An in vivo experiment was performed using a glioma preclinical model by injecting two million GL261cells subcutaneously and treating with 25 Gy, single fraction, with MBRT and conventional RT. Survival curves and acute radiation damage were measured to compare both treatments.

RESULTS

A satisfactory agreement between experimental results and MC simulations were obtained, the measured FWHM and distance between the peaks were respectively 0.431 and 1.098 mm. In vivo results show that MBRT can provide local tumor control for three weeks after RT treatment and a similar survival fraction of open beam radiotherapy. No severe acute effects were seen for the MBRT group.

CONCLUSIONS

We developed a minibeam collimator and presented its dosimetric features. Satisfactory agreement between MC and GC films was found with differences consistent with uncertainties due to fabrication and set-up errors. The survival curves of MBRT and open field RT are similar while atoxicity is dramatically lower with MBRT, preliminarily confirming the expected effect.

Neuro-Oncologic Veterinary Trial for the Clinical Transfer of Microbeam Radiation Therapy: Acute to Subacute Radiotolerance after Brain Tumor Irradiation in Pet Dogs

Synchrotron Microbeam Radiation Therapy (MRT) has repeatedly proven its superiority compared with conventional radiotherapy for glioma control in preclinical research. The clinical transfer phase of MRT has recently gained momentum; seven dogs with suspected glioma were treated under clinical conditions to determine the feasibility and safety of MRT. We administered a single fraction of 3D-conformal, image-guided MRT. Ultra-high-dose rate synchrotron X-ray microbeams (50 µm-wide, 400 µm-spaced) were delivered through five conformal irradiation ports. The PTV received ~25 Gy peak dose (within microbeams) per port, corresponding to a minimal cumulated valley dose (diffusing between microbeams) of 2.8 Gy. The dogs underwent clinical and MRI follow-up, and owner evaluations. One dog was lost to follow-up. Clinical exams of the remaining six dogs during the first 3 months did not indicate radiotoxicity induced by MRT. Quality of life improved from 7.3/10 [±0.7] to 8.9/10 [±0.3]. Tumor-induced seizure activity decreased significantly. A significant tumor volume reduction of 69% [±6%] was reached 3 months after MRT. Our study is the first neuro-oncologic veterinary trial of 3D-conformal Synchrotron MRT and reveals that MRT does not induce acute to subacute radiotoxicity in normal brain tissues. MRT improves quality of life and leads to remarkable tumor volume reduction despite low valley dose delivery. This trial is an essential step towards the forthcoming clinical application of MRT against deep-seated human brain tumors.

Minibeam Radiation Therapy Treatment (MBRT): Commissioning and First Clinical Implementation

PURPOSE

Minibeam radiation therapy (MBRT) is characterized by the delivery of submillimeter-wide regions of high "peak" and low "valley" doses throughout a tumor. Preclinical studies have long shown the promise of this technique, and we report here the first clinical implementation of MBRT.

METHODS AND MATERIALS

A clinical orthovoltage unit was commissioned for MBRT patient treatments using 3-, 4-, 5-, 8-, and 10-cm diameter cones. The 180 kVp output was spatially separated into minibeams using a tungsten collimator with 0.5 mm wide slits spaced 1.1 mm on center. Percentage depth dose (PDD) measurements were obtained using film dosimetry and plastic water for both peak and valley doses. PDDs were measured on the central axis for offsets of 0, 0.5, and 1 cm. The peak-to-valley ratio was calculated at each depth for all cones and offsets. To mitigate the effects of patient motion on delivered dose, patient-specific 3-dimensional-printed collimator holders were created. These conformed to the unique anatomy of each patient and affixed the tungsten collimator directly to the body. Two patients were treated with MBRT; both received 2 fractions.

RESULTS

Peak PDDs decreased gradually with depth. Valley PDDs initially increased slightly with depth, then decreased gradually beyond 2 cm. The peak-to-valley ratios were highest at the surface for smaller cone sizes and offsets. In vivo film dosimetry confirmed a distinct delineation of peak and valley doses in both patients treated with MBRT with no dose blurring. Both patients experienced prompt improvement in symptoms and tumor response.

CONCLUSIONS

We report commissioning results, treatment processes, and the first 2 patients treated with MBRT using a clinical orthovoltage unit. While demonstrating the feasibility of this approach is a crucial first step toward wider translation, clinical trials are needed to further establish safety and efficacy.

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.

In Vivo Microbeam Radiation Therapy at a Conventional Small Animal Irradiator

Microbeam radiation therapy (MRT) is a still pre-clinical form of spatially fractionated radiotherapy, which uses an array of micrometer-wide, planar beams of X-ray radiation. The dose modulation in MRT has proven effective in the treatment of tumors while being well tolerated by normal tissue. Research on understanding the underlying biological mechanisms mostly requires large third-generation synchrotrons. In this study, we aimed to develop a preclinical treatment environment that would allow MRT independent of synchrotrons. We built a compact microbeam setup for pre-clinical experiments within a small animal irradiator and present in vivo MRT application, including treatment planning, dosimetry, and animal positioning. The brain of an immobilized mouse was treated with MRT, excised, and immunohistochemically stained against γH2AX for DNA double-strand breaks. We developed a comprehensive treatment planning system by adjusting an existing dose calculation algorithm to our setup and attaching it to the open-source software 3D-Slicer. Predicted doses in treatment planning agreed within 10% with film dosimetry readings. We demonstrated the feasibility of MRT exposures in vivo at a compact source and showed that the microbeam pattern is observable in histological sections of a mouse brain. The platform developed in this study will be used for pre-clinical research of MRT.

Innovative minimally invasive options to treat drug-resistant epilepsies

Despite the regular discovery of new molecules, one-third of epileptic patients are resistant to antiepileptic drugs. Only a few can benefit from resective surgery, the current gold standard. Although effective in 50-70% of cases, this therapy remains risky, costly, and can be associated with long-term cognitive or neurological side effects. In addition, patients are increasingly reluctant to have a craniotomy, emphasizing the need for new less invasive therapies for focal drug-resistant epilepsies. Here, we review different minimally invasive approaches already in use in the clinic or under preclinical development to treat drug-resistant epilepsies. Localized thermolesion of the epileptogenic zone has been developed in the clinic using high-frequency thermo-coagulations or magnetic resonance imaging-guided laser or ultrasounds. Although less invasive, they have not yet significantly improved the outcomes when compared with resective surgery. Radiosurgery techniques have been used in the clinic for the last 20years and have proven efficiency. However, their efficacy is not better than resective surgery, and various side effects have been reported as well as the potential risk of sudden unexpected death associated with epilepsy. Recently, a new strategy of radiosurgery has emerged using synchrotron-generated X-ray microbeams: microbeam radiation therapy (MRT). The low divergence and high-flux of the synchrotron beams and the unique tolerance to MRT by healthy brain tissues, allows a precise targeting of specific brain regions with minimal invasiveness and limited behavioral or functional consequences in animals. Antiepileptic effects over several months have been recorded in animal models, and histological and synaptic tracing analysis suggest a reduction of neuronal connectivity as a mechanism of action. The possibility of transferring this approach to epileptic patients is discussed in this review.

The design and characterization of a novel dynamic collimator system for synchrotron radiotherapy applications

BACKGROUND

Novel synchrotron radiotherapy techniques are currently limited to using prefabricated beam-limiting blocks for field definition. For large experiments, a single square tungsten block is often used for every treatment since conformal blocks are both patient and field specific, and require long lead times for fabrication. Future synchrotron radiotherapy treatments would benefit from a dynamic collimator system.

PURPOSE

We developed and tested a novel collimator design for use on the Imaging and Medical Beamline (IMBL) at the ANSTO Australian Synchrotron.

METHODS

The maximum usable beam size on IMBL is 50-mm wide by 3-mm tall. Given the beam shape, targets must be vertically scanned through the synchrotron beam to cover the target volume. To shape the beam, a novel collimator design was developed, consisting of two semi-circular leaves made from 4-mm thick tungsten sheets, with each leaf capable of both vertical and horizontal movement. A software model was created to optimize motor trajectories and generate deliverable treatment fields. A series of geometric field shapes and clinical target volumes were delivered using the collimator and imaged with a digital imaging detector. Four similarity metrics (volumetric similarity, DICE, and the average and maximum Hausdorff distances) were used to measure differences between the input and planned fields, and the planned and delivered fields.

RESULTS

Differences between input and planned fields increased with delivery speed, and were worse for rectangular and square fields compared to circular fields. However, the differences between planned and delivered fields were small, where the maximum average deviation between the fields was 0.25 mm (one pixel). Field repeatability was consistent with no difference (σ = 0 for all metrics) observed in consecutively delivered fields.

CONCLUSIONS

We have successfully built and demonstrated a novel collimator for synchrotron radiotherapy applications on IMBL. Several design improvements have been highlighted and will be addressed in future revisions the collimator. However, in its current state, the collimator enables dynamically delivered conformal treatment fields to be utilized on IMBL, and is ready to support the forthcoming canine treatments on IMBL.

Effects of Microbeam Irradiation on Rodent Esophageal Smooth Muscle Contraction

BACKGROUND

High-dose-rate radiotherapy has shown promising results with respect to normal tissue preservation. We developed an ex vivo model to study the physiological effects of experimental radiotherapy in the rodent esophageal smooth muscle.

METHODS

We assessed the physiological parameters of the esophageal function in ex vivo preparations of the proximal, middle, and distal segments in the organ bath. High-dose-rate synchrotron irradiation was conducted using both the microbeam irradiation (MBI) technique with peak doses greater than 200 Gy and broadbeam irradiation (BBI) with doses ranging between 3.5-4 Gy.

RESULTS

Neither MBI nor BBI affected the function of the contractile apparatus. While peak latency and maximal force change were not affected in the BBI group, and no changes were seen in the proximal esophagus segments after MBI, a significant increase in peak latency and a decrease in maximal force change was observed in the middle and distal esophageal segments.

CONCLUSION

No severe changes in physiological parameters of esophageal contraction were determined after high-dose-rate radiotherapy in our model, but our results indicate a delayed esophageal function. From the clinical perspective, the observed increase in peak latency and decreased maximal force change may indicate delayed esophageal transit.

Good Timing Matters: The Spatially Fractionated High Dose Rate Boost Should Come First

Monoplanar microbeam irradiation (MBI) and pencilbeam irradiation (PBI) are two new concepts of high dose rate radiotherapy, combined with spatial dose fractionation at the micrometre range. In a small animal model, we have explored the concept of integrating MBI or PBI as a simultaneously integrated boost (SIB), either at the beginning or at the end of a conventional, low-dose rate schedule of 5x4 Gy broad beam (BB) whole brain radiotherapy (WBRT). MBI was administered as array of 50 µm wide, quasi-parallel microbeams. For PBI, the target was covered with an array of 50 µm × 50 µm pencilbeams. In both techniques, the centre-to-centre distance was 400 µm. To assure that the entire brain received a dose of at least 4 Gy in all irradiated animals, the peak doses were calculated based on the daily BB fraction to approximate the valley dose. The results of our study have shown that the sequence of the BB irradiation fractions and the microbeam SIB is important to limit the risk of acute adverse effects, including epileptic seizures and death. The microbeam SIB should be integrated early rather than late in the irradiation schedule.

A matter of space: how the spatial heterogeneity in energy deposition determines the biological outcome of radiation exposure

The outcome of the exposure of living organisms to ionizing radiation is determined by the distribution of the associated energy deposition at different spatial scales. Radiation proceeds through ionizations and excitations of hit molecules with an ~ nm spacing. Approaches such as nanodosimetry/microdosimetry and Monte Carlo track-structure simulations have been successfully adopted to investigate radiation quality effects: they allow to explore correlations between the spatial clustering of such energy depositions at the scales of DNA or chromosome domains and their biological consequences at the cellular level. Physical features alone, however, are not enough to assess the entity and complexity of radiation-induced DNA damage: this latter is the result of an interplay between radiation track structure and the spatial architecture of chromatin, and further depends on the chromatin dynamic response, affecting the activation and efficiency of the repair machinery. The heterogeneity of radiation energy depositions at the single-cell level affects the trade-off between cell inactivation and induction of viable mutations and hence influences radiation-induced carcinogenesis. In radiation therapy, where the goal is cancer cell inactivation, the delivery of a homogenous dose to the tumour has been the traditional approach in clinical practice. However, evidence is accumulating that introducing heterogeneity with spatially fractionated beams (mini- and microbeam therapy) can lead to significant advantages, particularly in sparing normal tissues. Such findings cannot be explained in merely physical terms, and their interpretation requires considering the scales at play in the underlying biological mechanisms, suggesting a systemic response to radiation.

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.

Microbeam Irradiation as a Simultaneously Integrated Boost in a Conventional Whole-Brain Radiotherapy Protocol

Microbeam radiotherapy (MRT), an experimental high-dose rate concept with spatial fractionation at the micrometre range, has shown a high therapeutic potential as well as good preservation of normal tissue function in pre-clinical studies. We investigated the suitability of MRT as a simultaneously integrated boost (SIB) in conventional whole-brain irradiation (WBRT). A 174 Gy MRT SIB was administered with an array of quasi-parallel, 50 µm wide microbeams spaced at a centre-to-centre distance of 400 µm either on the first or last day of a 5 × 4 Gy radiotherapy schedule in healthy adult C57 BL/6J mice and in F98 glioma cell cultures. The animals were observed for signs of intracranial pressure and focal neurologic signs. Colony counts were conducted in F98 glioma cell cultures. No signs of acute adverse effects were observed in any of the irradiated animals within 3 days after the last irradiation fraction. The tumoricidal effect on F98 cell in vitro was higher when the MRT boost was delivered on the first day of the irradiation course, as opposed to the last day. Therefore, the MRT SIB should be integrated into a clinical radiotherapy schedule as early as possible.

SyncMRT: a solution to image-guided synchrotron radiotherapy for quality assurance and pre-clinical trials

In this work, a new image guidance system and protocols for delivering image-guided radiotherapy (IGRT) on the Imaging and Medical Beamline (IMBL) at the ANSTO Australian Synchrotron are introduced. The image guidance methods used and the resulting accuracy of tumour alignment in in vivo experiments are often under-reported. Image guidance tasks are often complex, time-consuming and prone to errors. If unchecked, they may result in potential mis-treatments. We introduce SyncMRT, a software package that provides a simple, image guidance tool-kit for aligning samples to the synchrotron beam. We have demonstrated sub-millimetre alignment using SyncMRT and the small-animal irradiation platform (the DynamicMRT system) on the IMBL. SyncMRT has become the standard for carrying out IGRT treatments on the IMBL and has been used in all pre-clinical radiotherapy experiments since 2017. Further, we introduce two quality assurance (QA) protocols to synchrotron radiotherapy on the IMBL: the Winston-Lutz test and hidden target test. It is shown that the presented QA tests are appropriate for picking up geometrical setup errors and assessing the end-to-end accuracy of the image guidance process. Together, these tools make image guidance easier and provide a mechanism for reporting the geometric accuracy of synchrotron-based IGRT treatments. Importantly, this work is scalable to other delivery systems, and is in continual development to support the upcoming veterinary radiotherapy trials on the IMBL.

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.

Is Mini Beam Ready for Human Trials? Results of Randomized Study of Treating De-Novo Brain Tumors in Canines Using Linear Accelerator Generated Mini Beams

The main challenge in treating malignant brain neoplasms lies in eradicating the tumor while minimizing treatment-related damage. Conventional radiation treatments are associated with considerable side effects. Synchrotron generated micro-beam radiation (SMBRT) has shown to preserve brain architecture while killing tumor cells, however physical characteristics and limited facility access restrict its use. We have created a new clinical device which produces mini beams on a linear accelerator, to provide a new type of treatment called mini-beam radiation therapy (MBRT). The objective of this study is to compare the treatment outcomes of linear accelerator based MBRT versus standard radiation treatment (SRT), to evaluate the tumor response and the treatment-related changes in the normal brain with respect to each treatment type. Pet dogs with de-novo brain tumors were accrued for treatment. Dogs were randomized between standard fractionated stereotactic (9 Gy in 3 fractions) radiation treatment vs. a single fraction of MBRT (26 Gy mean dose). Dogs were monitored after treatment for clinical assessment and imaging. When the dogs were euthanized, a veterinary pathologist assessed the radiation changes and tumor response. We accrued 16 dogs, 8 dogs in each treatment arm. In the MBRT arm, 71% dogs achieved complete pathological remission. The radiation-related changes were all confined to the target region. Structural damage was not observed in the beam path outside of the target region. In contrast, none of the dogs in control group achieved remission and the treatment related damage was more extensive. Therapeutic superiority was observed with MBRT, including both tumor control and the normal structural preservation. The MBRT findings are suggestive of an immune related mechanism which is absent in standard treatment. These findings together with the widespread availability of clinical linear accelerators make MBRT a promising research topic to explore further treatment and clinical trial opportunities.

Radiobiological and Treatment-Related Aspects of Spatially Fractionated Radiotherapy

The continuously evolving field of radiotherapy aims to devise and implement techniques that allow for greater tumour control and better sparing of critical organs. Investigations into the complexity of tumour radiobiology confirmed the high heterogeneity of tumours as being responsible for the often poor treatment outcome. Hypoxic subvolumes, a subpopulation of cancer stem cells, as well as the inherent or acquired radioresistance define tumour aggressiveness and metastatic potential, which remain a therapeutic challenge. Non-conventional irradiation techniques, such as spatially fractionated radiotherapy, have been developed to tackle some of these challenges and to offer a high therapeutic index when treating radioresistant tumours. The goal of this article was to highlight the current knowledge on the molecular and radiobiological mechanisms behind spatially fractionated radiotherapy and to present the up-to-date preclinical and clinical evidence towards the therapeutic potential of this technique involving both photon and proton beams.

Meloxicam can Potentiate the Therapeutic Effects of Synchrotron Microbeam Radiation Therapy on High-Grade Glioma Bearing Rats

The microbeam radiation therapy (MRT), a spatially micro-fractionated synchrotron radiotherapy, leads to better control of incurable high-grade glioma than that obtained upon homogeneous radiotherapy. We evaluated the effect of meloxicam, a non-steroidal anti-inflammatory drug (NSAID), to increase the MRT response. Survival of rats bearing intracranial 9L gliosarcoma treated with meloxicam and/or MRT (400 Gy, 50 μm-wide microbeams, 200 μm spacing) was monitored. Tumor growth was assessed on histological tissue sections and COX-2 transcriptomic expression was studied 1 to 25 days after radiotherapy. Meloxicam significantly extended the median survival of microbeam-irradiated rats (from +10.5 to +20 days). Dual treatment led to last survivors until D90 (D39 for the MRT group) and to tumor 9.5 times smaller than MRT alone. No significant modification of COX-2 expression was induced by MRT in normal and tumor tissues. The meloxicam reinforced the anti-tumor effect of MRT for glioma treatment. Although the mechanisms of interaction between meloxicam and MRT remain to be elucidated, the addition of this NSAID, easily implemented as a supplement to water for example, is a very favorable therapeutic regimen since it doubled the survival benefit compared to MRT alone.

Divide and conquer: spatially fractionated radiation therapy

Spatially fractionated radiation therapy (SFRT) challenges some of the classical dogmas in conventional radiotherapy. The highly modulated spatial dose distributions in SFRT have been shown to lead, both in early clinical trials and in small animal experiments, to a significant increase in normal tissue dose tolerances. Tumour control effectiveness is maintained or even enhanced in some configurations as compared with conventional radiotherapy. SFRT seems to activate distinct radiobiological mechanisms, which have been postulated to involve bystander effects, microvascular alterations and/or immunomodulation. Currently, it is unclear which is the dosimetric parameter which correlates the most with both tumour control and normal tissue sparing in SFRT. Additional biological experiments aiming at parametrizing the relationship between the irradiation parameters (beam width, spacing, peak-to-valley dose ratio, peak and valley doses) and the radiobiology are needed. A sound knowledge of the interrelation between the physical parameters in SFRT and the biological response would expand its clinical use, with a higher level of homogenisation in the realisation of clinical trials. This manuscript reviews the state of the art of this promising therapeutic modality, the current radiobiological knowledge and elaborates on future perspectives.

Flying rats and microbeam paths crossing: the beauty of international interdisciplinary science

PURPOSE

Microbeam radiotherapy (MRT) is a still experimental radiotherapy approach. Two combined parameters contribute to an excellent normal tissue protection and an improved control of malignant tumors in small animal models, compared to conventional radiotherapy: dose deposition at a high dose rate and spatial fractionation at the micrometre level. The international microbeam research community expects to see clinical MRT trials within the next ten years.Physics-associated research is still widely regarded as a male domain. Thus, the question was asked whether this is reflected in the scientific contributions to the field of microbeam radiotherapy.

METHOD

A literature search was conducted using Pubmed, Semantic Scholar and other sources to look specifically for female contributors to the field of microbeam radiotherapy development.

CONCLUSION

The original idea for MRT was patented in 1994 by an all-male research team. In approximately 50% of all publications related to microbeam radiotherapy, however, either the first or the senior author is a woman. The contribution of those women who have been driving the development of both technical and biomedical aspects of MRT in the last two decades is highlighted.

Combining Nanocarrier-Assisted Delivery of Molecules and Radiotherapy

Cancer is responsible for a significant proportion of death all over the world. Therefore, strategies to improve its treatment are highly desired. The use of nanocarriers to deliver anticancer treatments has been extensively investigated and improved since the approval of the first liposomal formulation for cancer treatment in 1995. Radiotherapy (RT) is present in the disease management strategy of around 50% of cancer patients. In the present review, we bring the state-of-the-art information on the combination of nanocarrier-assisted delivery of molecules and RT. We start with formulations designed to encapsulate single or multiple molecules that, once delivered to the tumor site, act directly on the cells to improve the effects of RT. Then, we describe formulations designed to modulate the tumor microenvironment by delivering oxygen or to boost the abscopal effect. Finally, we present how RT can be employed to trigger molecule delivery from nanocarriers or to modulate the EPR effect.

Evaluation of silicon strip detectors in transmission mode for online beam monitoring in microbeam radiation therapy at the Australian Synchrotron

Successful transition of synchrotron-based microbeam radiation therapy (MRT) from pre-clinical animal studies to human trials is dependent upon ensuring that there are sufficient and adequate measures in place for quality assurance purposes. Transmission detectors provide researchers and clinicians with a real-time quality assurance and beam-monitoring instrument to ensure safe and accurate dose delivery. In this work, the effect of transmission detectors of different thicknesses (10 and 375 µm) upon the photon energy spectra and dose deposition of spatially fractionated synchrotron radiation is quantified experimentally and by means of a dedicated Geant4 simulation study. The simulation and experimental results confirm that the presence of the 375 µm thick transmission detector results in an approximately 1-6% decrease in broad-beam and microbeam peak dose. The capability to account for the reduction in dose and change to the peak-to-valley dose ratio justifies the use of transmission detectors as thick as 375 µm in MRT provided that treatment planning systems are able to account for their presence. The simulation and experimental results confirm that the presence of the 10 µm thick transmission detector shows a negligible impact (<0.5%) on the photon energy spectra, dose delivery and microbeam structure for both broad-beam and microbeam cases. Whilst the use of 375 µm thick detectors would certainly be appropriate, based upon the idea of best practice the authors recommend that 10 µm thick transmission detectors of this sort be utilized as a real-time quality assurance and beam-monitoring tool during MRT.

A Brief Overview of the Preclinical and Clinical Radiobiology of Microbeam Radiotherapy

Microbeam radiotherapy (MRT) is the delivery of spatially fractionated beams that have the potential to offer significant improvements in the therapeutic ratio due to the delivery of micron-sized high dose and dose rate beams. They build on longstanding clinical experience of GRID radiotherapy and more recently lattice-based approaches. Here we briefly overview the preclinical evidence for MRT efficacy and highlight the challenges for bringing this to clinical utility. The biological mechanisms underpinning MRT efficacy are still unclear, but involve vascular, bystander, stem cell and potentially immune responses. There is probably significant overlap in the mechanisms underpinning MRT responses and FLASH radiotherapy that needs to be further defined.

Microbeam Radiotherapy-A Novel Therapeutic Approach to Overcome Radioresistance and Enhance Anti-Tumour Response in Melanoma

Melanoma is the deadliest type of skin cancer, due to its invasiveness and limited treatment efficacy. The main therapy for primary melanoma and solitary organ metastases is wide excision. Adjuvant therapy, such as chemotherapy and targeted therapies are mainly used for disseminated disease. Radiotherapy (RT) is a powerful treatment option used in more than 50% of cancer patients, however, conventional RT alone is unable to eradicate melanoma. Its general radioresistance is attributed to overexpression of repair genes in combination with cascades of biochemical repair mechanisms. A novel sophisticated technique based on synchrotron-generated, spatially fractionated RT, called Microbeam Radiation Therapy (MRT), has been shown to overcome these treatment limitations by allowing increased dose delivery. With MRT, a collimator subdivides the homogeneous radiation field into an array of co-planar, high-dose microbeams that are tens of micrometres wide and spaced a few hundred micrometres apart. Different preclinical models demonstrated that MRT has the potential to completely ablate tumours, or significantly improve tumour control while dramatically reducing normal tissue toxicity. Here, we discuss the role of conventional RT-induced immunity and the potential for MRT to enhance local and systemic anti-tumour immune responses. Comparative gene expression analysis from preclinical tumour models indicated a specific gene signature for an 'MRT-induced immune effect'. This focused review highlights the potential of MRT to overcome the inherent radioresistance of melanoma which could be further enhanced for future clinical use with combined treatment strategies, in particular, immunotherapy.

Elke Bräuer-Krisch: dedication, creativity and generosity: May 17, 1961-September 10, 2018

PURPOSE

This extraordinary woman worked her professional way from a radiation protection engineer to become the successful principal investigator of a prestigious international European project for a new radiation therapy (ERC Synergy grant, HORIZON 2020). The evaluation of the submitted proposal was very positive. The panel proposed that it be funded. Elke tragically passed away a few days before this conclusion of the panel. The present account describes her gradual career development; it includes many episodes that Elke personally chronicled in her curriculum of 2017.

METHODS

An internet literature search was performed using Google Scholar and other sources to assist in the writing of this narrative review and account.

CONCLUSIONS

In parallel to the development of the new Biomedical Beamline ID17 at the European Synchrotron Radiation Facility in Grenoble in the late nineties, Elke focused her interest and her personal and professional priorities on MRT, particularly on its clinical goals. She outlined her main objectives in several documents: (1) develop a new paradigm of cancer care by broadening the foundation for MRT. (2) Filling the gaps in basic biological knowledge about the mechanisms of MRT effects on normal and neoplastic tissues. (3) Broaden the preclinical level of evidence for the low normal organ toxicity of MRT versus standard X-ray irradiations; preclinical experiments involved the application of MRT to animal tumor patients, to animals of larger size than laboratory rodents, using larger radiation field sizes, and irradiating in a real-time scenario comparable to the one planned for human patients. (4) To foster the specific purpose of radiosurgical MRT of tumor patients at the ESRF that required development of new, specific state of the art modalities and tools for treatment planning, dosimetry, dose calculation, patient positioning and, of particular importance, redundant levels of patient safety. Just as she was about to take responsibility as principal investigator for a prestigious international European project on a new radiation therapy, death called Elke in.

Quantification of Differential Response of Tumour and Normal Cells to Microbeam Radiation in the Absence of FLASH Effects

Microbeam radiotherapy (MRT) is a preclinical method of delivering spatially-fractionated radiotherapy aiming to improve the therapeutic window between normal tissue complication and tumour control. Previously, MRT was limited to ultra-high dose rate synchrotron facilities. The aim of this study was to investigate in vitro effects of MRT on tumour and normal cells at conventional dose rates produced by a bench-top X-ray source. Two normal and two tumour cell lines were exposed to homogeneous broad beam (BB) radiation, MRT, or were separately irradiated with peak or valley doses before being mixed. Clonogenic survival was assessed and compared to BB-estimated surviving fractions calculated by the linear-quadratic (LQ)-model. All cell lines showed similar BB sensitivity. BB LQ-model predictions exceeded the survival of cell lines following MRT or mixed beam irradiation. This effect was stronger in tumour compared to normal cell lines. Dose mixing experiments could reproduce MRT survival. We observed a differential response of tumour and normal cells to spatially fractionated irradiations in vitro, indicating increased tumour cell sensitivity. Importantly, this was observed at dose rates precluding the presence of FLASH effects. The LQ-model did not predict cell survival when the cell population received split irradiation doses, indicating that factors other than local dose influenced survival after irradiation.

Transient and Efficient Vascular Permeability Window for Adjuvant Drug Delivery Triggered by Microbeam Radiation

BACKGROUND

Microbeam Radiation Therapy (MRT) induces a transient vascular permeability window, which offers a novel drug-delivery system for the preferential accumulation of therapeutic compounds in tumors. MRT is a preclinical cancer treatment modality that spatially fractionates synchrotron X-rays into micrometer-wide planar microbeams which can induce transient vascular permeability, especially in the immature tumor vessels, without compromising vascular perfusion. Here, we characterized this phenomenon using Chicken Chorioallantoic Membrane (CAM) and demonstrated its therapeutic potential in human glioblastoma xenografts in mice.

METHODS

the developing CAM was exposed to planar-microbeams of 75 Gy peak dose with Synchrotron X-rays. Similarly, mice harboring human glioblastoma xenografts were exposed to peak microbeam doses of 150 Gy, followed by treatment with Cisplatin. Tumor progression was documented by Magnetic Resonance Imaging (MRI) and caliper measurements.

RESULTS

CAM exposed to MRT exhibited vascular permeability, beginning 15 min post-irradiation, reaching its peak from 45 min to 2 h, and ending by 4 h. We have deemed this period the "permeability window". Morphological analysis showed partially fragmented endothelial walls as the cause of the increased transport of FITC-Dextran into the surrounding tissue and the extravasation of 100 nm microspheres (representing the upper range of nanoparticles). In the human glioblastoma xenografts, MRI measurements showed that the combined treatment dramatically reduced the tumor size by 2.75-fold and 5.25-fold, respectively, compared to MRT or Cisplatin alone.

CONCLUSIONS

MRT provides a novel mechanism for drug delivery by increasing vascular transpermeability while preserving vessel integrity. This permeability window increases the therapeutic index of currently available chemotherapeutics and could be combined with other therapeutic agents such as Nanoparticles/Antibodies/etc.

Study of the X-ray radiation interaction with a multislit collimator for the creation of microbeams in radiation therapy

Microbeam radiation therapy (MRT) is a developing radiotherapy, based on the use of beams only a few tens of micrometres wide, generated by synchrotron X-ray sources. The spatial fractionation of the homogeneous beam into an array of microbeams is possible using a multislit collimator (MSC), i.e. a machined metal block with regular apertures. Dosimetry in MRT is challenging and previous works still show differences between calculated and experimental dose profiles of 10-30%, which are not acceptable for a clinical implementation of treatment. The interaction of the X-rays with the MSC may contribute to the observed discrepancies; the present study therefore investigates the dose contribution due to radiation interaction with the MSC inner walls and radiation leakage of the MSC. Dose distributions inside a water-equivalent phantom were evaluated for different field sizes and three typical spectra used for MRT studies at the European Synchrotron Biomedical beamline ID17. Film dosimetry was utilized to determine the contribution of radiation interaction with the MSC inner walls; Monte Carlo simulations were implemented to calculate the radiation leakage contribution. Both factors turned out to be relevant for the dose deposition, especially for small fields. Photons interacting with the MSC walls may bring up to 16% more dose in the valley regions, between the microbeams. Depending on the chosen spectrum, the radiation leakage close to the phantom surface can contribute up to 50% of the valley dose for a 5 mm × 5 mm field. The current study underlines that a detailed characterization of the MSC must be performed systematically and accurate MRT dosimetry protocols must include the contribution of radiation leakage and radiation interaction with the MSC in order to avoid significant errors in the dose evaluation at the micrometric scale.

Perspectives for microbeam irradiation at the SYRMEP beamline

It has been shown previously both in vitro and in vivo that microbeam irradiation (MBI) can control malignant tumour cells more effectively than the clinically established concepts of broad beam irradiation. With the aim to extend the international capacity for microbeam research, the first MBI experiment at the biomedical beamline SYRMEP of the Italian synchrotron facility ELETTRA has been conducted. Using a multislit collimator produced by the company TECOMET, arrays of quasi-parallel microbeams were successfully generated with a beam width of 50 µm and a centre-to-centre distance of 400 µm. Murine melanoma cell cultures were irradiated with a target dose of approximately 65 Gy at a mean photon energy of ∼30 keV with a dose rate of 70 Gy s-1 and a peak-to-valley dose of ∼123. This work demonstrated a melanoma cell reduction of approximately 80% after MBI. It is suggested that, while a high energy is essential to achieve high dose rates in order to deposit high treatment doses in a short time in a deep-seated target, for in vitro studies and for the treatment of superficial tumours a spectrum in the lower energy range might be equally suitable or even advantageous.

Unexpected Benefits of Multiport Synchrotron Microbeam Radiation Therapy for Brain Tumors

Simple Summary

We unveiled the potential of an innovative irradiation technique that ablates brain cancer while sparing normal tissues. Spatially fractionating the incident beam into arrays of micrometer-wide beamlets of X-rays (MRT for Microbeam Radiation Therapy) has led to significantly increased survival and tumor control in preclinical studies. Multiport MRT versus conventional irradiations, for the same background continuous dose, resulted in unexpectedly high equivalent biological effects in rats that have not been achieved with any other radiotherapeutic method. These hallmarks of multiport MRT, i.e., minimal impact on normal tissues and exceptional tumor control, may promote this method towards clinical applications, possibly increasing survival and improving long-term outcomes in neuro-oncology patients.

Abstract

Delivery of high-radiation doses to brain tumors via multiple arrays of synchrotron X-ray microbeams permits huge therapeutic advantages. Brain tumor (9LGS)-bearing and normal rats were irradiated using a conventional, homogeneous Broad Beam (BB), or Microbeam Radiation Therapy (MRT), then studied by behavioral tests, MRI, and histopathology. A valley dose of 10 Gy deposited between microbeams, delivered by a single port, improved tumor control and median survival time of tumor-bearing rats better than a BB isodose. An increased number of ports and an accumulated valley dose maintained at 10 Gy delayed tumor growth and improved survival. Histopathologically, cell death, vascular damage, and inflammatory response increased in tumors. At identical valley isodose, each additional MRT port extended survival, resulting in an exponential correlation between port numbers and animal lifespan (r² = 0.9928). A 10 Gy valley dose, in MRT mode, delivered through 5 ports, achieved the same survival as a 25 Gy BB irradiation because of tumor dose hot spots created by intersecting microbeams. Conversely, normal tissue damage remained minimal in all the single converging extratumoral arrays. Multiport MRT reached exceptional ~2.5-fold biological equivalent tumor doses. The unique normal tissue sparing and therapeutic index are eminent prerequisites for clinical translation.

Establishment of Microbeam Radiation Therapy at a Small-Animal Irradiator

PURPOSE

Microbeam radiation therapy is a preclinical concept in radiation oncology. It spares normal tissue more effectively than conventional radiation therapy at equal tumor control. The radiation field consists of peak regions with doses of several hundred gray, whereas doses between the peaks (valleys) are below the tissue tolerance level. Widths and distances of the beams are in the submillimeter range for microbeam radiation therapy. A similar alternative concept with beam widths and distances in the millimeter range is presented by minibeam radiation therapy. Although both methods were developed at large synchrotron facilities, compact alternative sources have been proposed recently.

METHODS AND MATERIALS

A small-animal irradiator was fitted with a special 3-layered collimator that is used for preclinical research and produces microbeams of flexible width of up to 100 μm. Film dosimetry provided measurements of the dose distributions and was compared with Monte Carlo dose predictions. Moreover, the micronucleus assay in Chinese hamster CHO-K1 cells was used as a biological dosimeter. The focal spot size and beam emission angle of the x-ray tube were modified to optimize peak dose rate, peak-to-valley dose ratio (PVDR), beam shape, and field homogeneity. An equivalent collimator with slit widths of up to 500 μm produced minibeams and allowed for comparison of microbeam and minibeam field characteristics.

RESULTS

The setup achieved peak entrance dose rates of 8 Gy/min and PVDRs >30 for microbeams. Agreement between Monte Carlo simulations and film dosimetry is generally better for larger beam widths; qualitative measurements validated Monte Carlo predicted results. A smaller focal spot enhances PVDRs and reduces beam penumbras but substantially reduces the dose rate. A reduction of the beam emission angle improves the PVDR, beam penumbras, and dose rate without impairing field homogeneity. Minibeams showed similar field characteristics compared with microbeams at the same ratio of beam width and distance but had better agreement with simulations.

CONCLUSION

The developed setup is already in use for in vitro experiments and soon for in vivo irradiations. Deviations between Monte Carlo simulations and film dosimetry are attributed to scattering at the collimator surface and manufacturing inaccuracies and are a matter of ongoing research.

Biological and Mechanical Synergies to Deal With Proton Therapy Pitfalls: Minibeams, FLASH, Arcs, and Gantryless Rooms

Proton therapy has advantages and pitfalls comparing with photon therapy in radiation therapy. Among the limitations of protons in clinical practice we can selectively mention: uncertainties in range, lateral penumbra, deposition of higher LET outside the target, entrance dose, dose in the beam path, dose constraints in critical organs close to the target volume, organ movements and cost. In this review, we combine proposals under study to mitigate those pitfalls by using individually or in combination: (a) biological approaches of beam management in time (very high dose rate “FLASH” irradiations in the order of 100 Gy/s) and (b) modulation in space (a combination of mini-beams of millimetric extent), together with mechanical approaches such as (c) rotational techniques (optimized in partial arcs) and, in an effort to reduce cost, (d) gantry-less delivery systems. In some cases, these proposals are synergic (e.g., FLASH and minibeams), in others they are hardly compatible (mini-beam and rotation). Fixed lines have been used in pioneer centers, or for specific indications (ophthalmic, radiosurgery,…), they logically evolved to isocentric gantries. The present proposals to produce fixed lines are somewhat controversial. Rotational techniques, minibeams and FLASH in proton therapy are making their way, with an increasing degree of complexity in these three approaches, but with a high interest in the basic science and clinical communities. All of them must be proven in clinical applications.

Technical and dosimetric realization of in vivo x-ray microbeam irradiations at the Munich Compact Light Source

PURPOSE

X-ray microbeam radiation therapy is a preclinical concept for tumor treatment promising tissue sparing and enhanced tumor control. With its spatially separated, periodic micrometer-sized pattern, this method requires a high dose rate and a collimated beam typically available at large synchrotron radiation facilities. To treat small animals with microbeams in a laboratory-sized environment, we developed a dedicated irradiation system at the Munich Compact Light Source (MuCLS).

METHODS

A specially made beam collimation optic allows to increase x-ray fluence rate at the position of the target. Monte Carlo simulations and measurements were conducted for accurate microbeam dosimetry. The dose during irradiation is determined by a calibrated flux monitoring system. Moreover, a positioning system including mouse monitoring was built.

RESULTS

We successfully commissioned the in vivo microbeam irradiation system for an exemplary xenograft tumor model in the mouse ear. By beam collimation, a dose rate of up to 5.3 Gy/min at 25 keV was achieved. Microbeam irradiations using a tungsten collimator with 50 μm slit size and 350 μm center-to-center spacing were performed at a mean dose rate of 0.6 Gy/min showing a high peak-to-valley dose ratio of about 200 in the mouse ear. The maximum circular field size of 3.5 mm in diameter can be enlarged using field patching.

CONCLUSIONS

This study shows that we can perform in vivo microbeam experiments at the MuCLS with a dedicated dosimetry and positioning system to advance this promising radiation therapy method at commercially available compact microbeam sources. Peak doses of up to 100 Gy per treatment seem feasible considering a recent upgrade for higher photon flux. The system can be adapted for tumor treatment in different animal models, for example, in the hind leg.

History and current perspectives on the biological effects of high-dose spatial fractionation and high dose-rate approaches: GRID, Microbeam & FLASH radiotherapy

The effects of various forms of ionising radiation are known to be mediated by interactions with cellular and molecular targets in irradiated and in some cases non-targeted tissue volumes. Despite major advances in advanced conformal delivery techniques, the probability of normal tissue complication (NTCP) remains the major dose-limiting factor in escalating total dose delivered during treatment. Potential strategies that have shown promise as novel delivery methods in achieving effective tumour control whilst sparing organs at risk involve the modulation of critical dose delivery parameters. This has led to the development of techniques using high dose spatial fractionation (GRID) and ultra-high dose rate (FLASH) which have translated to the clinic. The current review discusses the historical development and biological basis of GRID, microbeam and FLASH radiotherapy as advanced delivery modalities that have major potential for widespread implementation in the clinic in future years.

Simulation and measurement of microbeam dose distribution in lung tissue

Microbeam radiation therapy (MRT), a so far preclinical method in radiation oncology, modulates treatment doses on a micrometre scale. MRT uses treatment fields with a few ten micrometre wide high dose regions (peaks) separated by a few hundred micrometre wide low dose regions (valleys) and was shown to spare tissue much more effectively than conventional radiation therapy at similar tumour control rates. While preclinical research focused primarily on tumours of the central nervous system, recently also lung tumours have been suggested as a potential target for MRT. This study investigates the effect of the lung microstructure, comprising air cavities of a few hundred micrometre diameter, on the microbeam dose distribution in lung. In Monte Carlo simulations different models of heterogeneous lung tissue are compared with pure water and homogeneous air-water mixtures. Experimentally, microbeam dose distributions in porous foam material with cavity sizes similar to the size of lung alveoli were measured with film dosimetry at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Simulations and experiments show that the microstructure of the lung has a huge impact on the local doses in the microbeam fields. Locally, material inhomogeneities may change the dose by a factor of 1.7, and also average peak and valley doses substantially differ from those in homogeneous material. Our results imply that accurate dose prediction for MRT in lung requires adequate models of the lung microstructure. Even if only average peak and valley doses are of interest, the assumption of a simple homogeneous air-water mixture is not sufficient. Since anatomic information on a micrometre scale are unavailable for clinical treatment planning, alternative methods and models have to be developed.

Toward personalized synchrotron microbeam radiation therapy

Synchrotron facilities produce ultra-high dose rate X-rays that can be used for selective cancer treatment when combined with micron-sized beams. Synchrotron microbeam radiation therapy (MRT) has been shown to inhibit cancer growth in small animals, whilst preserving healthy tissue function. However, the underlying mechanisms that produce successful MRT outcomes are not well understood, either in vitro or in vivo. This study provides new insights into the relationships between dosimetry, radiation transport simulations, in vitro cell response, and pre-clinical brain cancer survival using intracerebral gliosarcoma (9LGS) bearing rats. As part of this ground-breaking research, a new image-guided MRT technique was implemented for accurate tumor targeting combined with a pioneering assessment of tumor dose-coverage; an essential parameter for clinical radiotherapy. Based on the results of our study, we can now (for the first time) present clear and reproducible relationships between the in vitro cell response, tumor dose-volume coverage and survival post MRT irradiation of an aggressive and radioresistant brain cancer in a rodent model. Our innovative and interdisciplinary approach is illustrated by the results of the first long-term MRT pre-clinical trial in Australia. Implementing personalized synchrotron MRT for brain cancer treatment will advance this international research effort towards clinical trials.

A proof of principle experiment for microbeam radiation therapy at the Munich compact light source

Microbeam radiation therapy (MRT), a preclinical form of spatially fractionated radiotherapy, uses an array of microbeams of hard synchrotron X-ray radiation. Recently, compact synchrotron X-ray sources got more attention as they provide essential prerequisites for the translation of MRT into clinics while overcoming the limited access to synchrotron facilities. At the Munich compact light source (MuCLS), one of these novel compact X-ray facilities, a proof of principle experiment was conducted applying MRT to a xenograft tumor mouse model. First, subcutaneous tumors derived from the established squamous carcinoma cell line FaDu were irradiated at a conventional X-ray tube using broadbeam geometry to determine a suitable dose range for the tumor growth delay. For irradiations at the MuCLS, FaDu tumors were irradiated with broadbeam and microbeam irradiation at integral doses of either 3 Gy or 5 Gy and tumor growth delay was measured. Microbeams had a width of 50 µm and a center-to-center distance of 350 µm with peak doses of either 21 Gy or 35 Gy. A dose rate of up to 5 Gy/min was delivered to the tumor. Both doses and modalities delayed the tumor growth compared to a sham-irradiated tumor. The irradiated area and microbeam pattern were verified by staining of the DNA double-strand break marker γH2AX. This study demonstrates for the first time that MRT can be successfully performed in vivo at compact inverse Compton sources.

Animal Models in Microbeam Radiation Therapy: A Scoping Review

BACKGROUND

Microbeam Radiation Therapy (MRT) is an innovative approach in radiation oncology where a collimator subdivides the homogeneous radiation field into an array of co-planar, high-dose beams which are tens of micrometres wide and separated by a few hundred micrometres.

OBJECTIVE

This scoping review was conducted to map the available evidence and provide a comprehensive overview of the similarities, differences, and outcomes of all experiments that have employed animal models in MRT.

METHODS

We considered articles that employed animal models for the purpose of studying the effects of MRT. We searched in seven databases for published and unpublished literature. Two independent reviewers screened citations for inclusion. Data extraction was done by three reviewers.

RESULTS

After screening 5688 citations and 159 full-text papers, 95 articles were included, of which 72 were experimental articles. Here we present the animal models and pre-clinical radiation parameters employed in the existing MRT literature according to their use in cancer treatment, non-neoplastic diseases, or normal tissue studies.

CONCLUSIONS

The study of MRT is concentrated in brain-related diseases performed mostly in rat models. An appropriate comparison between MRT and conventional radiotherapy (instead of synchrotron broad beam) is needed. Recommendations are provided for future studies involving MRT.

Synchrotron X-Ray Boost Delivered by Microbeam Radiation Therapy After Conventional X-Ray Therapy Fractionated in Time Improves F98 Glioma Control

PURPOSE

Synchrotron microbeam radiation therapy (MRT) is based on the spatial fractionation of the incident, highly collimated synchrotron beam into arrays of parallel microbeams depositing several hundred grays. It appears relevant to combine MRT with a conventional treatment course, preparing a treatment scheme for future patients in clinical trials. The efficiency of MRT delivered after several broad-beam (BB) fractions to palliate F98 brain tumors in rats in comparison with BB fractions alone was evaluated in this study.

METHODS AND MATERIALS

Rats bearing 10⁶ F98 cells implanted in the caudate nucleus were irradiated by 5 fractions in BB mode (3 × 6 Gy + 2 × 8 Gy BB) or by 2 boost fractions in MRT mode to a total of 5 fractions (3 × 6 Gy BB + MRT 2 × 8 Gy valley dose; peak dose 181 Gy [50/200 μm]). Tumor growth was evaluated in vivo by magnetic resonance imaging follow-up at T_-1, T₇, T₁₂, T₁₅, T₂₀, and T₂₅ days after radiation therapy and by histology and flow cytometry.

RESULTS

MRT-boosted tumors displayed lower cell density and cell proliferation compared with BB-irradiated tumors. The MRT boost completely stopped tumor growth during ∼4 weeks and led to a significant increase in median survival time, whereas tumors treated with BB alone recurred within a few days after the last radiation fraction.

CONCLUSIONS

The first evidence is presented that MRT, delivered as a boost of conventionally fractionated irradiation by orthovoltage broad x-ray beams, is feasible and more efficient than conventional radiation therapy alone.

FLASH and minibeams in radiation therapy: the effect of microstructures on time and space and their potential application to protontherapy

After years of lethargy, studies on two non-conventional microstructures in time and space of the beams used in radiation therapy are enjoying a huge revival. The first effect called “FLASH” is based on very high dose-rate irradiation (pulse amplitude ≥10⁶ Gy/s), short beam-on times (≤100 ms) and large single doses (≥10 Gy) as experimental parameters established so far to give biological and potential clinical effects. The second effect relies on the use of arrays of minibeams (e.g., 0.5–1 mm, spaced 1–3.5 mm). Both approaches have been shown to protect healthy tissues as an endpoint that must be clearly specified and could be combined with each other (e.g., minibeams under FLASH conditions). FLASH depends on the presence of oxygen and could proceed from the chemistry of peroxyradicals and a reduced incidence on DNA and membrane damage. Minibeams action could be based on abscopal effects, cell signalling and/or migration of cells between “valleys and hills” present in the non-uniform irradiation field as well as faster repair of vascular damage. Both effects are expected to maintain intact the tumour control probability and might even preserve antitumoural immunological reactions. FLASH in vivo experiments involving Zebrafish, mice, pig and cats have been done with electron beams, while minibeams are an intermediate approach between X-GRID and synchrotron X-ray microbeams radiation. Both have an excellent rationale to converge and be applied with proton beams, combining focusing properties and high dose rates in the beam path of pencil beams, and the inherent advantage of a controlled limited range. A first treatment with electron FLASH (cutaneous lymphoma) has recently been achieved, but clinical trials have neither been presented for FLASH with protons, nor under the minibeam conditions. Better understanding of physical, chemical and biological mechanisms of both effects is essential to optimize the technical developments and devise clinical trials.

Minibeam radiation therapy: A micro- and nano-dosimetry Monte Carlo study

PURPOSE

Minibeam radiation therapy (MBRT) is an innovative strategy based on a distinct dose delivery method that is administered using a series of narrow (submillimetric) parallel beams. To shed light on the biological effects of MBRT irradiation, we explored the micro- and nanodosimetric characteristics of three promising MBRT modalities (photon, electron, and proton) using Monte Carlo (MC) calculations.

METHODS

Irradiation with proton (100 MeV), electron (300 MeV), and photon (effective energy of 69 keV) minibeams were simulated using Geant4 MC code and the Geant4-DNA extension, which allows the simulation of energy transfer points with nanometric accuracy. As the target of the simulations, cells containing spherical nuclei with or without a detailed description of the DNA (deoxyribonucleic acid) geometry were placed at different depths in peak and valley regions in a water phantom. The energy deposition and number of events in the cell nuclei were recorded in the microdosimetry study, and the number of DNA breaks and their complexity were determined in the nanodosimetric study, where a multi-scale simulation approach was used for the latter. For DNA damage assessment, an adapted DBSCAN clustering algorithm was used. To compare the photon MBRT (xMBRT), electron MBRT (eMBRT), and proton MBRT (pMBRT) approaches, we considered the treatment of a brain tumor located at a depth of 75 mm.

RESULTS

Both mean energy deposition at micrometric scale and DNA damage in the "valley" cell nuclei were very low as compared with these parameters in the peak region at all depths for xMBRT and at depths of 0 to 30 mm and 0 to 50 mm for eMBRT and pMBRT, respectively. Only the charged minibeams were favorable for tumor control by producing similar effects in peak and valley cells after 70 mm. At the micrometer scale, the energy deposited per event pointed to a potential advantage of proton beams for tumor control, as more aggressive events could be expected at the end of their tracks. At the nanometer scale, all three MBRT modalities produced direct clustered DNA breaks, although the majority of damage (>93%) was composed of isolated single strand breaks. The pMBRT led to a significant increase in the proportion of clustered single strand breaks and double-strand breaks at the end of its range as compared to the entrance (7% at 75 mm vs 3% at 10 mm) in contrast to eMBRT and xMBRT. In the latter cases, the proportions of complex breaks remained constant, irrespective of the depth and region (peak or valley).

CONCLUSIONS

Enhanced normal tissue sparing can be expected with these three MBRT techniques. Among the three modalities, pMBRT offers an additional gain for radioresistant tumors, as it resulted in a higher number of complex DNA damage clusters in the tumor region. These results can aid understanding of the biological mechanisms of MBRT.

Technical advances in x-ray microbeam radiation therapy

In the last 25 years microbeam radiation therapy (MRT) has emerged as a promising alternative to conventional radiation therapy at large, third generation synchrotrons. In MRT, a multi-slit collimator modulates a kilovoltage x-ray beam on a micrometer scale, creating peak dose areas with unconventionally high doses of several hundred Grays separated by low dose valley regions, where the dose remains well below the tissue tolerance level. Pre-clinical evidence demonstrates that such beam geometries lead to substantially reduced damage to normal tissue at equal tumour control rates and hence drastically increase the therapeutic window. Although the mechanisms behind MRT are still to be elucidated, previous studies indicate that immune response, tumour microenvironment, and the microvasculature may play a crucial role. Beyond tumour therapy, MRT has also been suggested as a microsurgical tool in neurological disorders and as a primer for drug delivery. The physical properties of MRT demand innovative medical physics and engineering solutions for safe treatment delivery. This article reviews technical developments in MRT and discusses existing solutions for dosimetric validation, reliable treatment planning and safety. Instrumentation at synchrotron facilities, including beam production, collimators and patient positioning systems, is also discussed. Specific solutions reviewed in this article include: dosimetry techniques that can cope with high spatial resolution, low photon energies and extremely high dose rates of up to 15 000 Gy s-1, dose calculation algorithms-apart from pure Monte Carlo Simulations-to overcome the challenge of small voxel sizes and a wide dynamic dose-range, and the use of dose-enhancing nanoparticles to combat the limited penetrability of a kilovoltage energy spectrum. Finally, concepts for alternative compact microbeam sources are presented, such as inverse Compton scattering set-ups and carbon nanotube x-ray tubes, that may facilitate the transfer of MRT into a hospital-based clinical environment. Intensive research in recent years has resulted in practical solutions to most of the technical challenges in MRT. Treatment planning, dosimetry and patient safety systems at synchrotrons have matured to a point that first veterinary and clinical studies in MRT are within reach. Should these studies confirm the promising results of pre-clinical studies, the authors are confident that MRT will become an effective new radiotherapy option for certain patients.

Locomotion and eating behavior changes in Yucatan minipigs after unilateral radio-induced ablation of the caudate nucleus

The functional roles of the Caudate nucleus (Cd) are well known. Selective Cd lesions can be found in neurological disorders. However, little is known about the dynamics of the behavioral changes during progressive Cd ablation. Current stereotactic radiosurgery technologies allow the progressive ablation of a brain region with limited adverse effects in surrounding normal tissues. This could be of high interest for the study of the modified behavioral functions in relation with the degree of impairment of the brain structures. Using hypofractionated stereotactic radiotherapy combined with synchrotron microbeam radiation, we investigated, during one year after irradiation, the effects of unilateral radio-ablation of the right Cd on the behavior of Yucatan minipigs. The right Cd was irradiated to a minimal dose of 35.5 Gy delivered in three fractions. MRI-based morphological brain integrity and behavioral functions, i.e. locomotion, motivation/hedonism were assessed. We detected a progressive radio-necrosis leading to a quasi-total ablation one year after irradiation, with an additional alteration of surrounding areas. Transitory changes in the motivation/hedonism were firstly detected, then on locomotion, suggesting the influence of different compensatory mechanisms depending on the functions related to Cd and possibly some surrounding areas. We concluded that early behavioral changes related to eating functions are relevant markers for the early detection of ongoing lesions occurring in Cd-related neurological disorders.

Ultra high dose rate Synchrotron Microbeam Radiation Therapy. Preclinical evidence in view of a clinical transfer

This paper reviews the current state of the art of an emerging form of radiosurgery dedicated to brain tumour treatment and which operates at very high dose rate (kGy·s^-1). Microbeam Radiation Therapy uses synchrotron-generated X-rays which triggered normal tissue sparing partially mediated by FLASH effect.

High spatial resolution scintillator dosimetry of synchrotron microbeams

Microbeam radiation therapy is a novel pre-clinical external beam therapy that uses high-brilliance synchrotron X-rays to deliver the necessary high dose rates. The unique conditions of high dose rate and high spatial fractionation demand a new class of detector to experimentally measure important beam quality parameters. Here we demonstrate the highest spatial resolution plastic scintillator fibre-optic dosimeter found in the literature to date and tested it on the Imaging and Medical Beam-Line at the Australian Synchrotron in a X-ray beam where the irradiation dose rate was 4435 Gy/s. With a one-dimensional spatial resolution of 10 μm the detector is able to resolve the individual microbeams (53.7 ± 0.4 μm wide), and measure the peak-to-valley dose ratio to be 55 ± 17. We also investigate the role of radioluminescence in the optical fibre used to transport the scintillation photons, and conclude that it creates a significant contribution to the total light detected.

Introducing the concept of spiral microbeam radiation therapy (spiralMRT)

MOTIVATION

With interlaced microbeam radiation therapy (MRT) a first kilovoltage radiotherapy (RT) concept combining spatially fractionated entrance beams and homogeneous dose distribution at the target exists. However, this technique suffers from its high sensitivity to positioning errors of the target relative to the radiation source. With spiral microbeam radiation therapy (spiralMRT), this publication introduces a new irradiation geometry, offering similar spatial fractionation properties as interlaced MRT, while being less vulnerable to target positioning uncertainties.

METHODS

The dose distributions achievable with spiralMRT in a simplified human head geometry were calculated with Monte Carlo simulations based on Geant4 and the dependence of the result on the microbeam pitch, total field size, and photon energy were analysed. A comparison with interlaced MRT and conventional megavoltage tomotherapy was carried out.

RESULTS

SpiralMRT can deliver homogeneous dose distributions, while using spatially fractionated entrance beams. The valley dose of spiralMRT entrance beams is by up to 40% lower than the corresponding tomotherapy dose, thus indicating a better normal tissue sparing. The optimum photon energy is found to be around [Formula: see text].

CONCLUSIONS

SpiralMRT is a promising approach to delivering homogeneous dose distributions with spatially fractionated entrance beams, possibly decreasing normal tissue side effects in hypofractionated RT.