Dosimetry protocol for the preclinical trials in white-beam minibeam radiation therapy

Real-time dose measurement in minibeam radiotherapy using radioluminescence imaging

PURPOSE

Minibeam radiotherapy (MBRT) uses small parallel beams of radiation to create a highly modulated dose pattern. The aim of this study is to develop an optical radioluminescence imaging (RLI) approach to perform real-time dose measurement for MBRT.

METHODS

MBRT was delivered using an image-guided small animal irradiator equipped with a custom collimator. Five slabs of plastic scintillators with a thicknesses of 0.5, 1, 2, 3 and 10 mm were placed on top of a mouse phantom, to localize and measure the delivered dose. A thin radioluminescence film (Gd2O2S:Tb) was used to obtain the mini beam dose profile that was compared against GafChromic (GC) films measurements. The RLI signal was detected with a CMOS camera placed at 90 deg with respect to the beam axis. Monte Carlo (MC) simulations were also performed using TOPAS for comparison with the experimental results.

RESULTS

The measured peak to valley dose ratio (PVDR) obtained with RLI was 16.7 in line with GC films measurements. The differences between peak and valley dimension were less that 3% with respect to GC measurements. Using RLI performed with the scintillator slabs, it was possible to localize and measure in real-time MBRT delivery on the mouse phantom.

CONCLUSIONS

We proposed a novel method for MBRT dose localization and measurement in real-time based on RLI. The results we obtained are in good agreement with GC film measurements.

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.

Challenges for the Implementation of Primary Standard Dosimetry in Proton Minibeam Radiation Therapy

Background/Objectives: Spatial fractionation of proton fields as sub-millimeter beamlets to treat cancer has shown better sparing of healthy tissue whilst maintaining the same tumor control. It is critical to ensure primary standard dosimetry is accurate and ready to support the modality's clinical implementation. Methods: This work provided a proof-of-concept, using the National Physical Laboratory's Primary Standard Proton Calorimeter (PSPC) to measure average absorbed dose-to-water in a pMBRT field. A 100 MeV mono-energetic field and a 2 cm wide SOBP were produced with a spot-scanned proton beam incident on a collimator comprising 15 slits of 400 µm width, each 5 cm long and separated by a center-to-center distance of 4 mm. Results: The results showed the uncertainty on the absorbed dose-to-water in the mono-energetic beam was dominated by contributions of 1.4% and 1.1% (k = 1) for the NPL PSPC and PTW Roos chambers, respectively, originating from the achievable positioning accuracy of the devices. In comparison, the uncertainty due to positioning in the SOBP for both the NPL PSPC and PTW Roos chambers were 0.4%. Conclusions: These results highlight that it may be more accurate and reliable to perform reference dosimetry measuring the Dose-Area Product or in an SOBP for spatially fractionated fields.

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.

Monocrystalline diamond detector for online monitoring during synchrotron microbeam radiotherapy

Microbeam radiation therapy (MRT) is a radiotherapy technique combining spatial fractionation of the dose distribution on a micrometric scale, X-rays in the 50-500 keV range and dose rates up to 16 × 103 Gy s-1. Nowadays, in vivo dosimetry remains a challenge due to the ultra-high radiation fluxes involved and the need for high-spatial-resolution detectors. The aim here was to develop a striped diamond portal detector enabling online microbeam monitoring during synchrotron MRT treatments. The detector, a 550 µm bulk monocrystalline diamond, is an eight-strip device, of height 3 mm, width 178 µm and with 60 µm spaced strips, surrounded by a guard ring. An eight-channel ASIC circuit for charge integration and digitization has been designed and tested. Characterization tests were performed at the ID17 biomedical beamline of the European Synchrotron Radiation Facility (ESRF). The detector measured direct and attenuated microbeams as well as interbeam fluxes with a precision level of 1%. Tests on phantoms (RW3 and anthropomorphic head phantoms) were performed and compared with simulations. Synchrotron radiation measurements were performed on an RW3 phantom for strips facing a microbeam and for strips facing an interbeam area. A 2% difference between experiments and simulations was found. In more complex geometries, a preliminary study showed that the absolute differences between simulated and recorded transmitted beams were within 2%. Obtained results showed the feasibility of performing MRT portal monitoring using a microstriped diamond detector. Online dosimetric measurements are currently ongoing during clinical veterinary trials at ESRF, and the next 153-strip detector prototype, covering the entire irradiation field, is being finalized at our institution.

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.

Radiation quality correction factors for improved dosimetry in preclinical minibeam radiotherapy

BACKGROUND

In reference dosimetry, radiation quality correction factors are used in order to account for changes in the detector's response among different radiation qualities, improving dosimetric accuracy.

PURPOSE

Reference dosimetry radiation quality corrections factors for the PTW microDiamond were calculated for preclinical X-ray and proton minibeams, and their impact in dosimetric accuracy was evaluated.

METHODS

A formalism for the calculation of radiation quality correction factors for absolute dosimetry in minibeam fields was developed. Following our formalism, radiation quality correction factors were calculated for the PTW microDiamond detector, using the Monte Carlo method. Models of the detector, and X-ray and proton irradiation platform, were imported into the TOPAS Monte Carlo simulation toolkit. The radiation quality correction factors were calculated in the following scenarios: (i) reference dosimetry open field to minibeam center of the central peak, (ii) different positions at the minibeam profile (along the peaks and valleys direction) to the center of the central minibeam, and (iii) some representative depth positions. In addition, the radiation quality correction factors needed for the calculation of the peak-to-valley dose ratio at different depths were calculated.

RESULTS

An important overestimation of the dose (about 10%) was found in the case of the open to minibeam field for both X-rays and proton beams, when the correction factors were used. Smaller differences were observed in the other cases.

CONCLUSIONS

The usage of the PTW microDiamond detector requires radiation quality correction factors in order to be used in minibeam reference dosimetry.

HyperArcTM Dosimetric Validation for Multiple Targets Using Ionization Chamber and RT-100 Polymer Gel

Multiple brain metastases single-isocenter stereotactic radiosurgery (SRS) treatment is increasingly employed in radiotherapy department. Before its use in clinical routine, it is recommended to perform end-to-end tests. In this work, we report the results of five HyperArcTM treatment plans obtained by both ionization chamber (IC) and polymer gel. The end-to-end tests were performed using a water equivalent Mobius Verification PhantomTM (MVP) and a 3D-printed anthropomorphic head phantom PseudoPatient® (PP) (RTsafe P.C., Athens, Greece); 2D and 3D dose distributions were evaluated on the PP phantom using polymer gel (RTsafe). Gels were read by 1.5T magnetic resonance imaging (MRI). Comparison between calculated and measured distributions was performed using gamma index passing rate evaluation by different criteria (5% 2 mm, 3% 2 mm, 5% 1 mm). Mean point dose differences of 1.01% [min −0.77%−max 2.89%] and 0.23% [min 0.01%−max 2.81%] were found in MVP and PP phantoms, respectively. For each target volume, the obtained results in terms of gamma index passing rate show an agreement >95% with 5% 2 mm and 3% 2 mm criteria for both 2D and 3D distributions. The obtained results confirmed that the use of a single isocenter for multiple lesions reduces the treatment time without compromising accuracy, even in the case of target volumes that are quite distant from the isocenter.

Preclinical dosimetry in proton minibeam radiation therapy: robustness analysis and guidelines

Purpose

Proton minibeam radiation therapy (pMBRT) is a new radiotherapy approach that has shown a significant increase in the therapeutic window in glioma‐bearing rats compared to conventional proton therapy. The dosimetry of pMBRT is challenging and error prone due to the submillimetric beamlet sizes used. The aim of this study was to perform a robustness analysis on the setup parameters utilized in current preclinical trials and provide guidelines for reproducible dosimetry. The results of this work are intended to guide upcoming implementations of pMBRT worldwide, as well as pave the way for future clinical implementations.

Methods

Monte Carlo simulations and experimental data were used to evaluate the impact of variations in setup parameters and uncertainties in collimator specifications on lateral pMBRT dose distributions. The value of each parameter was modified individually to evaluate their effect on dose distributions. Experimental dosimetry was performed by means of high‐resolution detectors, that is, radiochromic films, the IBA Razor and the Microdiamond detector. New guidelines were proposed to optimize the experimental setup in pMBRT studies and perform reproducible dosimetry.

Results

The sensitivity of dose distributions to uncertainties and variations in setup parameters was quantified. Quantities that define pMBRT lateral profiles (i.e., the peak‐to‐valley dose ratio [PVDR], peak and valley doses, and peak width) are significantly influenced by small‐scale fluctuations in several of those parameters. The setup implemented at the Orsay proton therapy center for pMBRT irradiation was optimized to increase PVDRs and peak symmetry. In addition, we proposed guidelines to perform accurate and reproducible dosimetry in preclinical studies.

Conclusions

This study revealed the importance of adopting guidelines and protocols tailored to the distinct dose delivery method and dose distributions in pMBRT. This new methodology leads to reproducible dosimetry, which is imperative in preclinical trials. The results and guidelines presented in this manuscript can ease the initiation of pMBRT investigations in other centers.

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.

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

A Potential Renewed Use of Very Heavy Ions for Therapy: Neon Minibeam Radiation Therapy

Simple Summary

The treatment of hypoxic tumours continues to be one of the main challenges for radiation therapy. Minibeam radiation therapy (MBRT) shows a highly promising reduction of to-xicity in normal tissue, so that very heavy ions, such as Neon (Ne) or Argon (Ar), with extremely high LET, might become applicable to clinical situations. The high LET in the target would be unrivalled to overcome hypoxia, while MBRT might limit the side effects normally preventing the use of those heavy ions in a conventional radiotherapeutic setting. The work reported in this manuscript is the first experimental proof of the remarkable reduction of normal tissue (skin) toxicities after Ne MBRT irradiations as compared to conventional Ne irradiations. This result might allow for a renewed use of very heavy ions for cancer therapy.

Abstract

(1) Background: among all types of radiation, very heavy ions, such as Neon (Ne) or Argon (Ar), are the optimum candidates for hypoxic tumor treatments due to their reduced oxygen enhancement effect. However, their pioneering clinical use in the 1970s was halted due to severe side effects. The aim of this work was to provide a first proof that the combination of very heavy ions with minibeam radiation therapy leads to a minimization of toxicities and, thus, opening the door for a renewed use of heavy ions for therapy; (2) Methods: mouse legs were irradiated with either Ne MBRT or Ne broad beams at the same average dose. Skin toxicity was scored for a period of four weeks. Histopathology evaluations were carried out at the end of the study; (3) Results: a significant difference in toxicity was observed between the two irradiated groups. While severe da-mage, including necrosis, was observed in the broad beam group, only light to mild erythema was present in the MBRT group; (4) Conclusion: Ne MBRT is significantly better tolerated than conventional broad beam irradiations.

Physics and biology of ultrahigh dose-rate (FLASH) radiotherapy: a topical review

Ultrahigh dose-rate radiotherapy (RT), or 'FLASH' therapy, has gained significant momentum following various in vivo studies published since 2014 which have demonstrated a reduction in normal tissue toxicity and similar tumor control for FLASH-RT when compared with conventional dose-rate RT. Subsequent studies have sought to investigate the potential for FLASH normal tissue protection and the literature has been since been inundated with publications on FLASH therapies. Today, FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. The goal of this review article is to present the current state of this intriguing RT technique and to review existing publications on FLASH-RT in terms of its physical and biological aspects. In the physics section, the current landscape of ultrahigh dose-rate radiation delivery and dosimetry is presented. Specifically, electron, photon and proton radiation sources capable of delivering ultrahigh dose-rates along with their beam delivery parameters are thoroughly discussed. Additionally, the benefits and drawbacks of radiation detectors suitable for dosimetry in FLASH-RT are presented. The biology section comprises a summary of pioneering in vitro ultrahigh dose-rate studies performed in the 1960s and early 1970s and continues with a summary of the recent literature investigating normal and tumor tissue responses in electron, photon and proton beams. The section is concluded with possible mechanistic explanations of the FLASH normal-tissue protection effect (FLASH effect). Finally, challenges associated with clinical translation of FLASH-RT and its future prospects are critically discussed; specifically, proposed treatment machines and publications on treatment planning for FLASH-RT are reviewed.

Minibeam radiation therapy at a conventional irradiator: Dose-calculation engine and first tumor-bearing animals irradiation

PURPOSE

Minibeam radiation therapy (MBRT) is a novel therapeutic strategy, whose exploration was hindered due to its restriction to large synchrotrons. Our recent implementation of MBRT in a wide-spread small animal irradiator offers the possibility of performing systematic radiobiological studies. The aim of this research was to develop a set of dosimetric tools to reliably guide biological experiments in the irradiator.

METHODS

A Monte Carlo (Geant4)-based dose calculation engine was developed. It was then benchmarked against a series of dosimetric measurements performed with gafchromic films. Two voxelized rat phantoms (ROBY, computer tomography) were used to evaluate the treatment plan of F98 tumor-bearing rats. The response of a group of 7 animals receiving a unilateral irradiation of 58 Gy was compared to a group of non-irradiated controls.

RESULTS

The good agreement between calculations and the experimental data allowed the validation of the dose-calculation engine. The latter was first used to compare the dose distributions in computer tomography images of a rat's head and in a digital model of a rat's head (ROBY), obtaining a good general agreement. Finally, with respect to the in vivo experiment, the increase of mean survival time of the treated group with respect to the controls was modest but statistically significant.

CONCLUSIONS

The developed dosimetric tools were used to reliably guide the first MBRT treatments of intracranial glioma-bearing rats outside synchrotrons. The significant tumor response obtained with respect to the non-irradiated controls, despite the heterogenous dose coverage of the target, might indicate the participation of non-targeted effects.

Effect of X-ray minibeam radiation therapy on clonogenic survival of glioma cells

The goal is to compare, in vitro, the efficiency of minibeam radiotherapy (MBRT) and standard RT in inducing clonogenic cell death in glioma cell lines. With this aim, we report on the first in vitro study performed in an X-ray Small Animal Radiation Research Platform (SARRP) modified for minibeam irradiations. F98 rat and U87 human glioma cells were irradiated with either an array of minibeams (MB) or with conventional homogeneous beams (broad beam, BB). A specially designed multislit collimator was used to generate the minibeams with a with of a center-to-center distance of 1465 (±10) μm, and a PVDR value of 12.4 (±2.3) measured at 1 cm depth in a water phantom. Cells were either replated for clonogenic assay directly (immediate plating, IP) or 24 h after irradiation (delayed plating, DP) to assess the effect of potentially lethal damage repair (PLDR) on cell survival. Our hypothesis is that with MBRT, a similar level of clonogenic cell death can be reached compared to standard RT, when using equal mean radiation doses. To prove this, we performed dose escalations to determine the minimum integrated dose needed to reach a similar level of clonogenic cell death for both treatments. We show that this minimum dose can vary per cell line: in F98 cells a dose of 19 Gy was needed to obtain similar levels of clonogenic survival, whereas in U87 cells there was still a slightly increased survival with MB compared to BB 19 Gy treatment. The results suggest also an impairment of DNA damage repair in F98 cells as there is no difference in clonogenic cell survival between immediately and delayed plated cells for each dose and irradiation mode. For U87 cells, a small IP-DP effect was observed in the case of BB irradiation up to a dose of 17 Gy. However, at 19 Gy BB, as well as for the complete dose range of MB irradiation, U87 cells did not show a difference in clonogenic survival between IP and DP. We therefore speculate that MBRT might influence PLDR. The current results show that X-ray MBRT is a promising method for treatment of gliomas: future preclinical and clinical studies should aim at reaching a minimum radiation (valley) dose for effective eradication of gliomas with increased sparing of normal tissues compared to standard RT.

Different Dosimeters/Detectors Used in Small-Field Dosimetry: Pros and Cons

With the advent of complex and precise radiation therapy techniques, the use of relatively small fields is needed. Using such field sizes can cause uncertainty in dosimetry; therefore, special attention is required both in dose calculations and measurements. There are several challenges in small-field dosimetry such as the steep gradient of the radiation field, volume averaging effect, lack of charged particle equilibrium, partial occlusion of radiation source, beam alignment, and unable to use a reference dosimeter. Due to these challenges, special dosimeters are needed for small-field dosimetry, and this review article discusses this topic.

Spatial fractionation of the dose in heavy ions therapy: An optimization study

PURPOSE

The alliance of charged particle therapy and the spatial fractionation of the dose, as in minibeam or Grid therapy, is an innovative strategy to improve the therapeutic index in the treatment of radioresistant tumors. The aim of this work was to assess the optimum irradiation configuration in heavy ion spatially fractionated radiotherapy (SFRT) in terms of ion species, beam width, center-to-center distances, and linear energy transfer (LET), information that could be used to guide the design of the future biological experiments. The nuclear fragmentation leading to peak and valley regions composed of different secondary particles, creates the need for a more complete dosimetric description that the classical one in SFRT.

METHODS

Monte Carlo simulations (GATE 6.2) were performed to evaluate the dose distributions for different ions, beam widths, and spacings. We have also assessed the 3D-maps of dose-averaged LET and proposed a new parameter, the peak-to-valley-LET ratio, to offer a more thorough physical evaluation of the technique.

RESULTS

Our results show that beam widths larger than 400 μm are needed in order to keep a ratio between the dose in the entrance and the dose in the target of the same order as in conventional irradiations. A large ctc distance (3500 μm) would favor tissue sparing since it provides higher PVDR, it leads to a reduced contribution of the heavier nuclear fragments and a LET value in the valleys a factor 2 lower than the LET in the ctc leading to homogeneous distributions in the target.

CONCLUSIONS

Heavy ions MBRT provide advantageous dose distributions. Thanks to the reduced lateral scattering, the use of submillimetric beams still allows to keep a ratio between the dose in the entrance and the dose in the target of the same order as in conventional irradiations. Large ctc distances (3500 μm) should be preferred since they lead to valley doses composed of lighter nuclear fragments resulting in a much reduced dose-averaged LET values in normal tissue, favoring its preservation. Among the different ions species evaluated, Ne stands out as the one leading to the best balance between high PVDR and PVLR in normal tissues and high LET values (close to 100 keV/μm) and a favorable oxygen enhancement ratio in the target region.

Transfer of Minibeam Radiation Therapy into a cost-effective equipment for radiobiological studies: a proof of concept

Minibeam radiation therapy (MBRT) is an innovative synchrotron radiotherapy technique able to shift the normal tissue complication probability curves to significantly higher doses. However, its exploration was hindered due to the limited and expensive beamtime at synchrotrons. The aim of this work was to develop a cost-effective equipment to perform systematic radiobiological studies in view of MBRT. Tumor control for various tumor entities will be addressable as well as studies to unravel the distinct biological mechanisms involved in normal and tumor tissues responses when applying MBRT. With that aim, a series of modifications of a small animal irradiator were performed to make it suitable for MBRT experiments. In addition, the brains of two groups of rats were irradiated. Half of the animals received a standard irradiation, the other half, MBRT. The animals were followed-up for 6.5 months. Substantial brain damage was observed in the group receiving standard RT, in contrast to the MBRT group, where no significant lesions were observed. This work proves the feasibility of the transfer of MBRT outside synchrotron sources towards a small animal irradiator.

Carbon and oxygen minibeam radiation therapy: An experimental dosimetric evaluation

PURPOSE

To perform dosimetric characterization of a minibeam collimator in both carbon and oxygen ion beams to guide optimal setup geometry and irradiation for future radiobiological studies.

METHODS

Carbon and oxygen minibeams were generated using a prototype tungsten multislit collimator presenting line apertures 700 μm wide, which are spaced 3500 μm centre-to-centre distance apart. Several radiation beam spots generated the desired field size of 15 × 15 mm² and production of a 50 mm long spread out Bragg peak (SOBP) centered at 80 mm depth in water. Dose evaluations were performed with two different detectors: a PTW microDiamond® single crystal diamond detector and radiochromic films (EBT3). Peak-to-valley dose ratio (PVDR) values, output factors (OF), penumbras, and full width at half maximum (FWHM) were measured.

RESULTS

Measured lateral dose profiles exhibited spatial fractionation of dose at depth in a water phantom in the expected form of peaks and valleys for both carbon and oxygen radiation fields. The diamond detector and radiochromic film provided measurements of PVDR in good agreement. PVDR values at shallow depth were about 60 and decreased to about 10 at 80 mm depth in water. OF in the center of the SOBP was about 0.4; this value is larger than the corresponding one in proton minibeam radiation therapy measured using a comparable collimator due to a reduced lateral scattering for carbon and oxygen minibeams.

CONCLUSIONS

Carbon and oxygen minibeams may be produced by a mechanical collimator. PVDR values and output factors measured in this first study of these minibeam radiation types indicate there is potential for their therapeutic use. Optimization of minibeam collimator design and the number and size of focal spots for irradiation are advocated to improve PDVR values and dose distributions for each specific applied use.

Theoretical dosimetric evaluation of carbon and oxygen minibeam radiation therapy

PURPOSE

Charged particles have several advantages over x-ray radiations, both in terms of physics and radiobiology. The combination of these advantages with those of minibeam radiation therapy (MBRT) could help enhancing the therapeutic index for some cancers with poor prognosis. Among the different ions explored for therapy, carbon ions are considered to provide the optimum physical and biological characteristics. Oxygen could be advantageous due to a reduced oxygen enhancement ratio along with a still moderate biological entrance dose. The aforementioned reasons justified an in-depth evaluation of the dosimetric features of carbon and oxygen minibeam radiation therapy to establish the interest of further explorations of this avenue.

MATERIALS AND METHODS

The GATE/Geant4 6.2 Monte Carlo simulation platform was employed to simulate arrays of rectangular carbon and oxygen minibeams (600 μm × 2 cm) at a water phantom entrance. They were assumed to be generated by means of a magnetic focusing. The irradiations were performed with a 2-cm-long spread-out Bragg peak (SOBP) centered at 7-cm-depth. Several center-to-center (c-t-c) distances were considered. Peak and valley doses, as well as peak-to-valley dose ratio (PVDR) and the relative contribution of nuclear fragments and electromagnetic processes were assessed. In addition, the type and proportion of the secondary nuclear fragments were evaluated in both peak and valley regions.

RESULTS

Carbon and oxygen MBRT lead to very similar dose distributions. No significant advantage of oxygen over carbon ions was observed from physical point of view. Favorable dosimetric features were observed for both ions. Thanks to the reduced lateral scattering, the standard shape of the depth dose curves (in the peaks) is maintained even for submillimetric beam sizes. When a narrow c-t-c is considered (910-980 μm), a (quasi) homogenization of the dose can be obtained at the target, while a spatial fractionation of the dose is maintained in the proximal normal tissues with low PVDR. In contrast when a larger c-t-c is used (3500 μm) extremely high PVDR (≥ 50) are obtained in normal tissues, corresponding to very low valley doses. This suggests that carbon and oxygen MBRT might lead to a significant reduction of normal tissue complication probability. The main participant to the valley doses are secondary nuclear products at all depths. Among them the highest yield in normal tissues corresponds to the lightest fragments, neutrons and protons. Heavier fragments are dominant in the valleys only at the target position, which might favor tumor control.

CONCLUSIONS

The computed dose distributions suggest that a spatial fractionation of the dose combined to the use of submillimetric field sizes might allow profiting from the high efficiency of carbon and oxygen ions for the treatment of radioresistant tumors, while preserving normal tissues. Only biological experiments could confirm the shifting of the normal tissue complication probability curves. The authors' results support the further exploration of this avenue.

Proton minibeam radiation therapy: Experimental dosimetry evaluation

PURPOSE

Proton minibeam radiation therapy (pMBRT) is a new radiotherapy (RT) approach that allies the inherent physical advantages of protons with the normal tissue preservation observed when irradiated with submillimetric spatially fractionated beams. This dosimetry work aims at demonstrating the feasibility of the technical implementation of pMBRT. This has been performed at the Institut Curie - Proton Therapy Center in Orsay.

METHODS

Proton minibeams (400 and 700 μm-width) were generated by means of a brass multislit collimator. Center-to-center distances between consecutive beams of 3200 and 3500 μm, respectively, were employed. The (passive scattered) beam energy was 100 MeV corresponding to a range of 7.7 cm water equivalent. Absolute dosimetry was performed with a thimble ionization chamber (IBA CC13) in a water tank. Relative dosimetry was carried out irradiating radiochromic films interspersed in a IBA RW3 slab phantom. Depth dose curves and lateral profiles at different depths were evaluated. Peak-to-valley dose ratios (PVDR), beam widths, and output factors were also assessed as a function of depth.

RESULTS

A pattern of peaks and valleys was maintained in the transverse direction with PVDR values decreasing as a function of depth until 6.7 cm. From that depth, the transverse dose profiles became homogeneous due to multiple Coulomb scattering. Peak-to-valley dose ratio values extended from 8.2 ± 0.5 at the phantom surface to 1.08 ± 0.06 at the Bragg peak. This was the first time that dosimetry in such small proton field sizes was performed. Despite the challenge, a complete set of dosimetric data needed to guide the first biological experiments was achieved.

CONCLUSIONS

pMBRT is a novel strategy in order to reduce the side effects of RT. This works provides the experimental proof of concept of this new RT method: clinical proton beams might allow depositing a (high) uniform dose in a brain tumor located in the center of the brain (7.5 cm depth, the worst scenario), while a spatial fractionation of the dose is retained in the normal tissues in the beam path, potentially leading to a gain in tissue sparing. This is the first complete experimental implementation of this promising technique. Biological experiments are needed in order to confirm the clinical potential of pMBRT.

Spatial fractionation of the dose using neon and heavier ions: A Monte Carlo study

PURPOSE

This work explores a new radiation therapy approach which might trigger a renewed use of neon and heavier ions to treat cancers. These ions were shown to be extremely efficient in radioresistant tumor killing. Unfortunately, the efficient region also extends into the normal tissue in front of the tumor. The strategy the authors propose is to profit from the well-established sparing effect of thin spatially fractionated beams, so that the impact on normal tissues might be minimized while a high tumor control is achieved. The main goal of this work is to provide a proof of concept of this new approach. With that aim, a dosimetric study was carried out as a first step to evaluate the interest of further explorations of this avenue.

METHODS

The gate/geant4 v.6.1 Monte Carlo simulation platform was employed to simulate arrays of rectangular minibeams (700 μm × 2 cm) of four ions (Ne, Si, Ar, and Fe). The irradiations were performed with a 2 cm-long spread-out Bragg peak centered at 7 cm-depth. Dose distributions in a water phantom were scored considering two minibeams center-to-center distances: 1400 and 3500 μm. Peak and valley doses, peak-to-valley dose ratios (PVDRs), beam penumbras, and relative contribution of nuclear fragments and electromagnetic processes were assessed as figures of merit. In addition, the type and proportion of the secondary nuclear fragments were evaluated in both peak and valley regions.

RESULTS

Extremely high PVDR values (>100) and low valley doses were obtained. The higher the atomic number (Z) of the primary ion is, the lower the valleys and the narrower the penumbras. Although the yield of secondary nuclear products increases with Z, the actual dose being deposited by the secondary nuclear fragments in the valleys starts to be the dominant contribution at deeper points, helping in the sparing of proximal normal tissues. Additionally, a wider center-to-center distance leads to a minimized contribution of heavier secondary fragments in valleys.

CONCLUSIONS

The computed dose distributions suggest that a spatial fractionation of the dose combined to the use of submillimetric field sizes might allow profiting from the high efficiency of neon and heavier ions for the treatment of radioresistant tumors, while preserving normal tissues. The authors' results support the further exploration of this avenue. Next steps include the realization of biological experiment to confirm the shifting of normal tissue complication probability curves.

Dosimetric evaluation of new approaches in GRID therapy using nonconventional radiation sources

PURPOSE

Spatial fractionation of the dose has proven to be a promising approach to increase the tolerance of healthy tissue, which is the main limitation of radiotherapy. A good example of that is GRID therapy, which has been successfully used in the management of large tumors with low toxicity. The aim of this work is to explore new avenues using nonconventional sources: GRID therapy by using kilovoltage (synchrotron) x-rays, the use of very high-energy electrons, and proton GRID therapy. They share in common the use of the smallest possible grid sizes in order to exploit the dose-volume effects.

METHODS

Monte Carlo simulations (penelope/peneasy and geant4/GATE codes) were used as a method to study dose distributions resulting from irradiations in different configurations of the three proposed techniques. As figure of merit, percentage (peak and valley) depth dose curves, penumbras, and central peak-to-valley dose ratios (PVDR) were evaluated. As shown in previous biological experiments, high PVDR values are requested for healthy tissue sparing. A superior tumor control may benefit from a lower PVDR.

RESULTS

High PVDR values were obtained in the healthy tissue for the three cases studied. When low energy photons are used, the treatment of deep-seated tumors can still be performed with submillimetric grid sizes. Superior PVDR values were reached with the other two approaches in the first centimeters along the beam path. The use of protons has the advantage of delivering a uniform dose distribution in the tumor, while healthy tissue benefits from the spatial fractionation of the dose. In the three evaluated techniques, there is a net reduction in penumbra with respect to radiosurgery.

CONCLUSIONS

The high PVDR values in the healthy tissue and the use of small grid sizes in the three presented approaches might constitute a promising alternative to treat tumors with such spatially fractionated radiotherapy techniques. The dosimetric results presented here support the interest of performing radiobiology experiments in order to evaluate these new avenues.

Early gene expression analysis in 9L orthotopic tumor-bearing rats identifies immune modulation in molecular response to synchrotron microbeam radiation therapy

Synchrotron Microbeam Radiation Therapy (MRT) relies on the spatial fractionation of the synchrotron photon beam into parallel micro-beams applying several hundred of grays in their paths. Several works have reported the therapeutic interest of the radiotherapy modality at preclinical level, but biological mechanisms responsible for the described efficacy are not fully understood to date. The aim of this study was to identify the early transcriptomic responses of normal brain and glioma tissue in rats after MRT irradiation (400Gy). The transcriptomic analysis of similarly irradiated normal brain and tumor tissues was performed 6 hours after irradiation of 9 L orthotopically tumor-bearing rats. Pangenomic analysis revealed 1012 overexpressed and 497 repressed genes in the irradiated contralateral normal tissue and 344 induced and 210 repressed genes in tumor tissue. These genes were grouped in a total of 135 canonical pathways. More than half were common to both tissues with a predominance for immunity or inflammation (64 and 67% of genes for normal and tumor tissues, respectively). Several pathways involving HMGB1, toll-like receptors, C-type lectins and CD36 may serve as a link between biochemical changes triggered by irradiation and inflammation and immunological challenge. Most immune cell populations were involved: macrophages, dendritic cells, natural killer, T and B lymphocytes. Among them, our results highlighted the involvement of Th17 cell population, recently described in tumor. The immune response was regulated by a large network of mediators comprising growth factors, cytokines, lymphokines. In conclusion, early response to MRT is mainly based on inflammation and immunity which appear therefore as major contributors to MRT efficacy.

Double-strand breaks on F98 glioma rat cells induced by minibeam and broad-beam synchrotron radiation therapy

PURPOSE

To assess the DNA damage induced by MBRT and BB radiations on glioma cells.

METHODS

The analysis of fluorescent intensity emitted per nucleus was plotted versus DNA content 2 and 17 h after irradiations. At around cell-doubling time (17 h) after exposures, the remaining DNA radiation damage could be correlated with cellular death.

RESULTS

A higher γH2AX IF intensity per cell could be detected 2 and 17 h after MBRT when compared with BB. 17 h after MBRT, misrepaired damaged cells remained arrested in both G1 and G2 phases.

CONCLUSIONS

A pronounced G2 phase arrest was detected at 17 h after MBRT and BB. However, only after MBRT, a dose-dependent increasing number of damaged cells appeared arrested also in the G1 phase, and a higher amount of cells more prone to undergo apoptosis were detected. The threshold dose required to enhance the effectiveness of both synchrotron radiation techniques was 12 Gy.

Proton-minibeam radiation therapy: a proof of concept

PURPOSE

This Monte Carlo simulation work aims at studying a new radiotherapy approach called proton-minibeam radiation therapy (pMBRT). The main objective of this proof of concept was the evaluation of the possible gain in tissue sparing, thanks to the spatial fractionation of the dose, which could be used to deposit higher and potentially curative doses in clinical cases where tissue tolerances are a limit for conventional methods.

METHODS

Monte Carlo simulations (GATE v.6) have been used as a method to calculate the ratio of the peak-to-valley doses (PVDR) for arrays of proton minibeams of 0.7 mm width and several center-to-center distances, at different depths in a water phantom. The beam penumbras were also evaluated as an important parameter for tissue sparing, for example, in the treatment of non-cancer diseases like epilepsy. Two proton energies were considered in this study: a clinically relevant energy (105 MeV) and a very high energy (1 GeV), to benefit from a reduced lateral scattering. For the latter case, an interlaced geometry was also evaluated.

RESULTS

Higher or similar PVDR than the ones obtained in x-rays minibeam radiation therapy were achieved in several pMBRT configurations. In addition, for the two energies studied, the beam penumbras are smaller than in the case of Gamma Knife radiosurgery.

CONCLUSIONS

The high PVDR obtained for some configurations and the small penumbras in comparison with existing radiosurgery techniques, suggest a potential gain in healthy tissue sparing in this new technique. Biological studies are warranted to assess the effects of pMBRT on both normal and tumoral tissues.

Monte Carlo simulation of stereotactic microbeam radiation therapy: evaluation of the usage of a linear accelerator as the x-ray source

The usage of linear accelerator-generated x-rays for the stereotactic microbeam radiation therapy technique was evaluated in this study. Dose distributions were calculated with the Monte Carlo code MCNPX. Unidirectional single beams and beam arrays were simulated in a cylindrical water phantom to observe the effects of x-ray energies and irradiation geometry on dose distributions. Beam arrays were formed with square pencil beams. Two orthogonally interlaced beam arrays were simulated in a detailed head phantom and dose distributions were compared with ones which had been calculated for a bidirectional interlaced microbeam therapy (BIMRT) technique that uses synchrotron-generated x-rays. A parallel pattern of the beams was preserved through the phantom; however an unsegmented dose region could not be formed at the target. Five orthogonally interlaced beam array pairs (ten beam arrays) were simulated in a mathematical head phantom and the unsegmented dose region was formed. However, the dose fall-off distance is longer than the one that had been calculated for the BIMRT technique. Besides, the peak-to-dose ratios between the phantom's outer surface and the target region are lower. Therefore, the advantages of the MRT technique may not be preserved with the usage of a linac as the x-ray source.

Monte Carlo-based dose calculation engine for minibeam radiation therapy

Minibeam radiation therapy (MBRT) is an innovative radiotherapy approach based on the well-established tissue sparing effect of arrays of quasi-parallel micrometre-sized beams. In order to guide the preclinical trials in progress at the European Synchrotron Radiation Facility (ESRF), a Monte Carlo-based dose calculation engine has been developed and successfully benchmarked with experimental data in anthropomorphic phantoms. Additionally, a realistic example of treatment plan is presented. Despite the micron scale of the voxels used to tally dose distributions in MBRT, the combination of several efficiency optimisation methods allowed to achieve acceptable computation times for clinical settings (approximately 2 h). The calculation engine can be easily adapted with little or no programming effort to other synchrotron sources or for dose calculations in presence of contrast agents.

Synchrotron-generated microbeam sensorimotor cortex transections induce seizure control without disruption of neurological functions

Synchrotron-generated X-ray microplanar beams (microbeams) are characterized by the ability to deliver extremely high doses of radiation to spatially restricted volumes of tissue. Minimal dose spreading outside the beam path provides an exceptional degree of protection from radio-induced damage to the neurons and glia adjacent to the microscopic slices of tissue irradiated. The preservation of cortical architecture following high-dose microbeam irradiation and the ability to induce non-invasively the equivalent of a surgical cut over the cortex is of great interest for the development of novel experimental models in neurobiology and new treatment avenues for a variety of brain disorders. Microbeams (size 100 µm/600 µm, center-to-center distance of 400 µm/1200 µm, peak entrance doses of 360-240 Gy/150-100 Gy) delivered to the sensorimotor cortex of six 2-month-old naïve rats generated histologically evident cortical transections, without modifying motor behavior and weight gain up to 7 months. Microbeam transections of the sensorimotor cortex dramatically reduced convulsive seizure duration in a further group of 12 rats receiving local infusion of kainic acid. No subsequent neurological deficit was associated with the treatment. These data provide a novel tool to study the functions of the cortex and pave the way for the development of new therapeutic strategies for epilepsy and other neurological diseases.