Reduced side effects by proton microchannel radiotherapy: study in a human skin model

Multi-collimator proton minibeam radiotherapy with joint dose and PVDR optimization

BACKGROUND

The clinical translation of proton minibeam radiation therapy (pMBRT) presents significant challenges, particularly in developing an optimal treatment planning technique. A uniform target dose is crucial for maximizing anti-tumor efficacy and facilitating the clinical acceptance of pMBRT. However, achieving a high peak-to-valley dose ratio (PVDR) in organs-at-risk (OAR) is essential for sparing normal tissue. This balance becomes particularly difficult when OARs are located distal to the beam entrance or require patient-specific collimators.

PURPOSE

This work proposes a novel pMBRT treatment planning method that can achieve high PVDR at OAR and uniform dose at target simultaneously, via multi-collimator pMBRT (MC-pMBRT) treatment planning method with joint dose and PVDR optimization (JDPO).

METHODS

MC-pMBRT utilizes a set of generic and premade multi-slit collimators with different center-to-center distances and does not need patient-specific collimators. The collimator selection per field is OAR-specific and tailored to maximize PVDR in OARs while preserving target dose uniformity. Then, the inverse optimization method JDPO is utilized to jointly optimize target dose uniformity, PVDR, and other dose-volume-histogram based dose objectives, which is solved by iterative convex relaxation optimization algorithm and alternating direction method of multipliers.

RESULTS

The need and efficacy of MC-pMBRT is demonstrated by comparing the single-collimator (SC) approach with the multi-collimator (MC) approach. While SC degraded either PVDR for OAR or dose uniformity for the target, MC provided a good balance of PVDR and target dose uniformity. The proposed JDPO method is validated in comparison with the dose-only optimization (DO) method for MC-pMBRT, in reference to the conventional (CONV) proton RT (no pMBRT). Compared to CONV, MC-pMBRT (DO and JDPO) preserved target dose uniformity and plan quality, while providing unique PVDR in OAR. Compared to DO, JDPO further improved PVDR via PVDR optimization during treatment planning.

CONCLUSION

A novel pMBRT treatment planning method called MC-pMBRT is proposed that utilizes a set of generic and premade collimators with joint dose and PVDR optimization algorithm to optimize OAR-specific PVDR and target dose uniformity simultaneously.

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.

A novel treatment planning method via scissor beams for uniform-target-dose proton GRID with peak-valley-dose-ratio optimization

BACKGROUND

Proton spatially fractionated RT (SFRT) can potentially synergize the unique advantages of using proton Bragg peak and SFRT peak-valley dose ratio (PVDR) to reduce the radiation-induced damage for normal tissues. Uniform-target-dose (UTD) proton GRID is a proton SFRT modality that can be clinically desirable and conveniently adopted since its UTD resembles target dose distribution in conventional proton RT (CONV). However, UTD proton GRID is not used clinically, which is likely due to the lack of an effective treatment planning method.

PURPOSE

This work will develop a novel treatment planning method using scissor beams (SB) for UTD proton GRID, with the joint optimization of PVDR and dose objectives.

METHODS

The SB method for spatial dose modulation in normal tissues with UTD has two steps: (1) a primary beam (PB) is halved with interleaved beamlets, to generate spatial dose modulation in normal tissues; (2) a complementary beam (CB) is added to fill in previously valley-dose positions in the target to generate UTD, while the CB is angled slightly from the PB, to maintain spatial dose modulation in normal tissues. A treatment planning method with PVDR optimization via the joint total variation and L1 (TVL1) regularization is developed to jointly optimize PVDR and dose objectives. The plan optimization solution is obtained using an iterative convex relaxation algorithm.

RESULTS

The new methods SB and SB-TVL1 were validated in comparison with CONV. Compared to CONV of relatively homogeneous dose distribution, SB had modulated spatial dose pattern in normal tissues with UTD and comparable plan quality. Compared to SB, SB-TVL1 further maximized PVDR, with comparable dose-volume parameters.

CONCLUSIONS

A novel SB method is proposed that can generate modulated spatial dose pattern in normal tissues to achieve UTD proton GRID. A treatment planning method with PVDR optimization capability via TVL1 regularization is developed that can jointly optimize PVDR and dose objectives for proton GRID.

Transverse emittance growth of proton sources from laser-irradiated sub-μm-thin planar targets

Proton bunches with maximum energies between 12 and 22 MeV were emitted from submicrometer-thin plastic foils upon irradiation by laser pulses with peak intensity of 4×10^{20}W/cm^{2}. The images of the protons by a magnetic quadrupole doublet on a screen remained consistently larger by a factor of 10 compared to expectations drawn from the ultralow transverse emittance values reported for thick foil targets. Analytic estimates and particle-in-cell simulations attribute this drastically increased emittance to formerly excluded Coulomb collisions between charged particles. The presence of carbon ions and significant transparency likely play a decisive role. This observation is highly relevant because such thin, partially transparent foils are considered ideal for optimizing maximum proton energies.

Single-Cell Radiation Response Scoring with the Deep Learning Algorithm CeCILE 2.0

External stressors, such as ionizing radiation, have massive effects on life, survival, and the ability of mammalian cells to divide. Different types of radiation have different effects. In order to understand these in detail and the underlying mechanisms, it is essential to study the radiation response of each cell. This allows abnormalities to be characterized and laws to be derived. Tracking individual cells over several generations of division generates large amounts of data that can no longer be meaningfully analyzed by hand. In this study, we present a deep-learning-based algorithm, CeCILE (Cell classification and in vitro lifecycle evaluation) 2.0, that can localize, classify, and track cells in live cell phase-contrast videos. This allows conclusions to be drawn about the viability of the cells, the cell cycle, cell survival, and the influence of X-ray radiation on these. Furthermore, radiation-specific abnormalities during division could be characterized. In summary, CeCILE 2.0 is a powerful tool to characterize and quantify the cellular response to external stressors such as radiation and to put individual responses into a larger context. To the authors knowledge, this is the first algorithm with a fully integrated workflow that is able to do comprehensive single-cell and cell composite analysis, allowing them to draw conclusions on cellular radiation response.

Proton Microbeam Targeted Irradiation of the Gonad Primordium Region Induces Developmental Alterations Associated with Heat Shock Responses and Cuticle Defense in Caenorhabditis elegans

We describe a methodology to manipulate Caenorhabditis elegans (C. elegans) and irradiate the stem progenitor gonad region using three MeV protons at a specific developmental stage (L1). The consequences of the targeted irradiation were first investigated by considering the organogenesis of the vulva and gonad, two well-defined and characterized developmental systems in C. elegans. In addition, we adapted high-throughput analysis protocols, using cell-sorting assays (COPAS) and whole transcriptome analysis, to the limited number of worms (>300) imposed by the selective irradiation approach. Here, the presented status report validated protocols to (i) deliver a controlled dose in specific regions of the worms; (ii) immobilize synchronized worm populations (>300); (iii) specifically target dedicated cells; (iv) study the radiation-induced developmental alterations and gene induction involved in cellular stress (heat shock protein) and cuticle injury responses that were found.

TVL1-IMPT: Optimization of Peak-to-Valley Dose Ratio Via Joint Total-Variation and L1 Dose Regularization for Spatially Fractionated Pencil-Beam-Scanning Proton Therapy

PURPOSE

Proton minibeam radiation therapy (pMBRT) is a novel proton modality of spatially fractionated RT. pMBRT can reduce the radiation damage to normal tissues via biological dose sparing of high peak-to-valley dose ratio (PVDR). This work will develop a new spatially fractionated IMPT treatment planning method for pMBRT that jointly optimizes the plan quality and maximizes the PVDR.

METHODS

The new optimization method simultaneously maximizes the normal-tissue PVDR and optimizes the dose distribution at tumor targets and organs at risk. The PVDR maximization is through the joint total variation (TV) and L1 regularization with respect to the normal-tissue dose. That is, the beam-eye view projects dose slices of several depths for each beam angle; the TV of dose is maximized, corresponding to the PVDR maximization; and the L1 of dose is minimized, corresponding to the minimization of the organs-at-risk dose and maximization of survival fraction (SF).

RESULTS

The new IMPT method with TV and L1 regularization was validated in comparison with the conventional IMPT method for pMBRT in several clinical cases. The results show that TVL1 provided larger PVDR and SF than the conventional IMPT method for biological sparing of normal tissues, with preserved plan quality in terms of physical dose distribution.

CONCLUSIONS

A new spatially fractionated IMPT treatment planning method was developed for pMBRT that can optimize and improve normal-tissue PVDR and SF by incorporating TV and L1 dose regularization with properly chosen regularization parameters into IMPT.

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.

Small beams, fast predictions: a comparison of machine learning dose prediction models for proton minibeam therapy

BACKGROUND

Dose calculations for novel radiotherapy cancer treatments such as proton minibeam radiation therapy is often done using full Monte Carlo (MC) simulations. As MC simulations can be very time consuming for this kind of application, deep learning models have been considered to accelerate dose estimation in cancer patients.

PURPOSE

This work systematically evaluates the dose prediction accuracy, speed and generalization performance of three selected state-of-the-art deep learning models for dose prediction applied to the proton minibeam therapy. The strengths and weaknesses of those models are thoroughly investigated, helping other researchers to decide on a viable algorithm for their own application.

METHODS

The following recently published models are compared: first, a 3D U-Net model trained as a regression network, second, a 3D U-Net trained as a generator of a generative adversarial network (GAN) and third, a dose transformer model which interprets the dose prediction as a sequence translation task. These models are trained to emulate the result of MC simulations. The dose depositions of a proton minibeam with a diameter of 800μm and an energy of 20-100 MeV inside a simple head phantom calculated by full Geant4 MC simulations are used as a case study for this comparison. The spatial resolution is 0.5 mm. Special attention is put on the evaluation of the generalization performance of the investigated models.

RESULTS

Dose predictions with all models are produced in the order of a second on a GPU, the 3D U-Net models being fastest with an average of 130 ms. An investigated 3D U-Net regression model is found to show the strongest performance with overall 61.0 % ± $\%\pm$ 0.5% of all voxels exhibiting a deviation in energy deposition prediction of less than 3% compared to full MC simulations with no spatial deviation allowed. The 3D U-Net models are observed to show better generalization performance for target geometry variations, while the transformer-based model shows better generalization with regard to the proton energy.

CONCLUSIONS

This paper reveals that (1) all studied deep learning models are significantly faster than non-machine learning approaches predicting the dose in the order of seconds compared to hours for MC, (2) all models provide reasonable accuracy, and (3) the regression-trained 3D U-Net provides the most accurate predictions.

Longitudinally Heterogeneous Tumor Dose Optimizes Proton Broadbeam, Interlaced Minibeam, and FLASH Therapy

Simple Summary

The aim of any kind of external radiation therapy is to control a tumor with the highest possible probability of the lowest possible side effects. Here, we study further opportunities of reducing the side effects of proton therapy by applying longitudinally heterogeneous dose distributions in the tumor respecting the delivery of a minimum prescribed dose. In our simulations, the longitudinally heterogeneous dose distributions show a reduced dose in the healthy tissue already in the case of proton broadbeam irradiations, but a much higher (calculated) mean cell survival in the case of proton minibeam irradiation. This demonstrates its potential to substantially reduce side effects at a simultaneously higher tumor control probability, opening new opportunities of easier application when striving for high dose-rate applications of proton beams (>~10 Gy/s), in order to additionally profit from the so-called FLASH effects.

Abstract

The prerequisite of any radiation therapy modality (X-ray, electron, proton, and heavy ion) is meant to meet at least a minimum prescribed dose at any location in the tumor for the best tumor control. In addition, there is also an upper dose limit within the tumor according to the International Commission on Radiation Units (ICRU) recommendations in order to spare healthy tissue as well as possible. However, healthy tissue may profit from the lower side effects when waving this upper dose limit and allowing a larger heterogeneous dose deposition in the tumor, but maintaining the prescribed minimum dose level, particularly in proton minibeam therapy. Methods: Three different longitudinally heterogeneous proton irradiation modes and a standard spread-out Bragg peak (SOBP) irradiation mode are simulated for their depth-dose curves under the constraint of maintaining a minimum prescribed dose anywhere in the tumor region. Symmetric dose distributions of two opposing directions are overlaid in a 25 cm-thick water phantom containing a 5 cm-thick tumor region. Interlaced planar minibeam dose distributions are compared to those of a broadbeam using the same longitudinal dose profiles. Results and Conclusion: All longitudinally heterogeneous proton irradiation modes show a dose reduction in the healthy tissue compared to the common SOBP mode in the case of broad proton beams. The proton minibeam cases show eventually a much larger mean cell survival and thus a further reduced equivalent uniform dose (EUD) in the healthy tissue than any broadbeam case. In fact, the irradiation mode using only one proton energy from each side shows better sparing capabilities in the healthy tissue than the common spread-out Bragg peak irradiation mode with the option of a better dose fall-off at the tumor edges and an easier technical realization, particularly in view of proton minibeam irradiation at ultra-high dose rates larger than ~10 Gy/s (so-called FLASH irradiation modes).

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.

Technical aspects of proton minibeam radiation therapy: Minibeam generation and delivery

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the normal tissue sparing of sub-millimetric, spatially fractionated beams with the improved ballistics of protons. This may allow a safe dose escalation in the tumour and has already proven to provide a remarkable increase of the therapeutic index for high-grade gliomas in animal experiments. One of the main challenges in pMBRT concerns the generation of minibeams and the implementation in a clinical environment. This article reviews the different approaches for generating minibeams, using mechanical collimators and focussing magnets, and discusses the technical aspects of the implementation and delivery of pMBRT.

Dose Profile Modulation of Proton Minibeam for Clinical Application

Simple Summary

Proton minibeam radiation therapy (pMBRT) using multislit collimator (MSC) and scatterers has been proposed to spare healthy tissues and organs on the beam path and beyond the Bragg peak. An MSC that was much thicker than the maximum range of the proton beam could provide a sufficiently high peak-to-valley dose ratio at the patient’s skin, and the scatterers could actively convert the spatially fractionated proton beam to a uniform and broad beam in tumors by changing their thickness. The combination of the MSC and the scatterers can be a good solution for implementing pMBRT in clinical proton therapy facilities.

Abstract

The feasibility of proton minibeam radiation therapy (pMBRT) using a multislit collimator (MSC) and a scattering device was evaluated for clinical use at a clinical proton therapy facility. We fabricated, through Monte Carlo (MC) simulations, not only an MSC with a high peak-to-valley dose ratio (PVDR) at the entrance of the proton beam, to prevent radiation toxicity, but also a scattering device to modulate the PVDR in depth. The slit width and center-to-center distance of the diverging MSC were 2.5 mm and 5.0 mm at the large end, respectively, and its thickness and available field size were 100 mm and 76 × 77.5 mm², respectively. Spatially fractionated dose distributions were measured at various depths using radiochromic EBT3 films and also tested on bacterial cells. MC simulation showed that the thicker the MSC, the higher the PVDR at the phantom surface. Dosimetric evaluations showed that lateral dose profiles varied according to the scatterer’s thickness, and the depths satisfying PVDR = 1.1 moved toward the surface as their thickness increased. The response of the bacterial cells to the proton minibeams’ depth was also established, in a manner similar to the dosimetric pattern. Conclusively, these results strongly suggest that pMBRT can be implemented in clinical centers by using MSC and scatterers.

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.

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.

Planar Proton Minibeam Irradiation Elicits Spatially Confined DNA Damage in a Human Epidermis Model

Purpose: High doses of ionizing radiation in radiotherapy can elicit undesirable side effects to the skin. Proton minibeam radiotherapy (pMBRT) may circumvent such limitations due to tissue-sparing effects observed at the macro scale. Here, we mapped DNA damage dynamics in a 3D tissue context at the sub-cellular level. Methods: Epidermis models were irradiated with planar proton minibeams of 66 µm, 408 µm and 920 µm widths and inter-beam-distances of 2.5 mm at an average dose of 2 Gy using the scanning-ion-microscope SNAKE in Garching, GER. γ-H2AX + 53BP1 and cleaved-caspase-3 immunostaining revealed dsDNA damage and cell death, respectively, in time courses from 0.5 to 72 h after irradiation. Results: Focused 66 µm pMBRT induced sharply localized severe DNA damage (pan-γ-H2AX) in cells at the dose peaks, while damage in the dose valleys was similar to sham control. pMBRT with 408 µm and 920 µm minibeams induced DSB foci in all cells. At 72 h after irradiation, DNA damage had reached sham levels, indicating successful DNA repair. Increased frequencies of active-caspase-3 and pan-γ-H2AX-positive cells revealed incipient cell death at late time points. Conclusions: The spatially confined distribution of DNA damage appears to underlie the tissue-sparing effect after focused pMBRT. Thus, pMBRT may be the method of choice in radiotherapy to reduce side effects to the skin.

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.

Proton Minibeam Radiation Therapy and Arc Therapy: Proof of Concept of a Winning Alliance

Simple Summary

Normal tissue’s morbidity continues to limit the increase in the therapeutic index in radiation therapy. This study explores the potential advantages of combining proton arc therapy and proton minibeam radiation therapy, which have already individually shown a significant normal tissue’s sparing. This alliance aims to integrate the benefits of those techniques in a single approach.

Abstract

(1) Background: Proton Arc Therapy and Proton Minibeam Radiation Therapy are two novel therapeutic approaches with the potential to lower the normal tissue complication probability, widening the therapeutic window for radioresistant tumors. While the benefits of both modalities have been individually evaluated, their combination and its potential advantages are being assessed in this proof-of-concept study for the first time. (2) Methods: Monte Carlo simulations were employed to evaluate the dose and LET distributions in brain tumor irradiations. (3) Results: a net reduction in the dose to normal tissues (up to 90%), and the preservation of the spatial fractionation of the dose were achieved for all configurations evaluated. Additionally, Proton Minibeam Arc Therapy (pMBAT) reduces the volumes exposed to high-dose and high-LET values at expense of increased low-dose and intermediate-LET values. (4) Conclusions: pMBAT enhances the individual benefits of proton minibeams while keeping those of conventional proton arc therapy. These results might facilitate the path towards patients’ treatments since lower peak doses in normal tissues would be needed than in the case of a single array of proton minibeams.

Heavy Ion Minibeam Therapy: Side Effects in Normal Brain

The purpose of this work was to investigate whether minibeam therapy with heavy ions might offer improvements of the therapeutic ratio for the treatment of human brain cancers. To assess neurotoxicity, we irradiated normal juvenile rats using 120 MeV lithium-7 ions at an absorbed integral dose of 20 Gy. Beams were configured either as a solid parallel circular beam or as an array of planar parallel minibeams having 300-micron width and 1-mm center-to-center spacing within a circular array. We followed animals for 6 months after treatment and utilized behavioral testing and immunohistochemical studies to investigate the resulting cognitive impairment and chronic pathologic changes. We found both solid-beam therapy and minibeam therapy to result in cognitive impairment compared with sham controls, with no apparent reduction in neurotoxicity using heavy ion minibeams instead of solid beams under the conditions of this study.

Concept and performance evaluation of two 3 GHz buncher units optimizing the dose rate of a novel preclinical proton minibeam irradiation facility

To demonstrate the large potential of proton minibeam radiotherapy (pMBRT) as a new method to treat tumor diseases, a preclinical proton minibeam radiation facility was designed. It is based on a tandem Van-de-Graaff accelerator providing a 16 MeV proton beam and a 3 GHz linac post-accelerator (designs: AVO-ADAM S.A, Geneva, Switzerland and ENEA, Frascati, Italy). To enhance the transmission of the tandem beam through the post-accelerator by a factor of 3, two drift tube buncher units were designed and constructed: A brazed 5-gap structure (adapted SCDTL tank of the TOP-IMPLART project (ENEA)) and a non-brazed low budget 4-gap structure. Both are made of copper. The performance of the two differently manufactured units was evaluated using a 16 MeV tandem accelerator beam and a Q3D magnetic spectrograph. Both buncher units achieve the required summed voltage amplitude of 42 kV and amplitude stability at a power feed of less than 800 W.

First Evaluation of Temporal and Spatial Fractionation in Proton Minibeam Radiation Therapy of Glioma-Bearing Rats

Simple Summary

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic approach based on a distinct dose delivery method: the dose distributions follow a pattern with regions of peaks (high doses) and valleys (low doses). pMBRT was shown to be able to widen the therapeutic window in glioma-bearing rats. In previous studies the irradiation was performed in one single fraction. The work reported in this manuscript is the first evaluation detailing the response of glioma-bearing rats to a temporal fractionation in proton minibeam radiation therapy, delivered under a crossfire geometry. A significant increase of the median survival time was obtained when the dose was delivered over two sessions as opposed to in a single fraction. This result could facilitate the path towards pMBRT treatments.

Abstract

(1) Background: Proton minibeam radiation therapy (pMBRT) is a new radiotherapy technique using spatially modulated narrow proton beams. pMBRT results in a significantly reduced local tissue toxicity while maintaining or even increasing the tumor control efficacy as compared to conventional radiotherapy in small animal experiments. In all the experiments performed up to date in tumor bearing animals, the dose was delivered in one single fraction. This is the first assessment on the impact of a temporal fractionation scheme on the response of glioma-bearing animals to pMBRT. (2) Methods: glioma-bearing rats were irradiated with pMBRT using a crossfire geometry. The response of the irradiated animals in one and two fractions was compared. An additional group of animals was also treated with conventional broad beam irradiations. (3) Results: pMBRT delivered in two fractions at the biological equivalent dose corresponding to one fraction resulted in the highest median survival time, with 80% long-term survivors free of tumors. No increase in local toxicity was noted in this group with respect to the other pMBRT irradiated groups. Conventional broad beam irradiations resulted in the most severe local toxicity. (4) Conclusion: Temporal fractionation increases the therapeutic index in pMBRT and could ease the path towards clinical trials.

Conceptual Design of a Novel Nozzle Combined with a Clinical Proton Linac for Magnetically Focussed Minibeams

Simple Summary

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the tissue sparing potential of submillimetric, spatially fractionated beams (minibeams) with the improved ballistics of protons to enhance the tolerance of normal tissue and allow a dose escalation in the tumour. This approach could allow a more effective treatment of radioresistant tumours and has already shown excellent results for rat gliomas. To exploit the full potential of pMBRT, it should be delivered using magnetically focussed and scanned minibeams. However, such an implementation has not yet been demonstrated at clinically relevant beam energies. In this work, we therefore present a new design combining our recently developed minibeam nozzle with the first clinical proton linear accelerator. We show the suitability of this combination for the generation of magnetically focussed and scanned minibeams with clinically relevant parameters as well as for the delivery of conventional pencil beam scanning techniques.

Abstract

(1) Background: Proton minibeam radiation therapy (pMBRT) is a novel therapeutic approach with the potential to significantly increase normal tissue sparing while providing tumour control equivalent or superior to standard proton therapy. For reasons of efficiency, flexibility and minibeam quality, the optimal implementation of pMBRT should use magnetically focussed minibeams which, however, could not yet be generated in a clinical environment. In this study, we evaluated our recently proposed minibeam nozzle together with a new clinical proton linac as a potential implementation. (2) Methods: Monte Carlo simulations were performed to determine under which conditions minibeams can be generated and to evaluate the robustness against focussing magnet errors. Moreover, an example of conventional pencil beam scanning irradiation was simulated. (3) Results: Excellent minibeam sizes between 0.6 and 0.9 mm full width at half maximum could be obtained and a good tolerance to errors was observed. Furthermore, the delivery of a 10 cm × 10 cm field with pencil beams was demonstrated. (4) Conclusion: The combination of the new proton linac and minibeam nozzle could represent an optimal implementation of pMBRT by allowing the generation of magnetically focussed minibeams with clinically relevant parameters. It could furthermore be used for conventional pencil beam scanning.

A scanning dynamic collimator for spot-scanning proton minibeam production

In proton minibeam radiation therapy, proton minibeams are typically produced by modulating a uniform field using a multislit collimator. Multislit collimators produce minibeams of fixed length and width, and a new collimator has to be manufactured each time a new minibeam array is required, limiting its flexibility. In this work, we propose a scanning dynamic collimator for the generation of proton minibeams arrays. The new collimator system proposed is able to produce any minibeam required on an on-line basis by modulating the pencil beam spots of modern proton therapy machines, rather than a uniform field. The new collimator is evaluated through Monte Carlo simulations and the produced proton minibeams are compared with that of a multislit collimator. Furthermore, a proof of concept experiment is conducted to demonstrate the feasibility of producing a minibeam array by repositioning (i.e. scanning) a collimator. It is concluded that besides the technical challenges, the new collimator design is producing equivalent minibeam arrays to the multislit collimator, whilst is flexible to produce any minibeam array desired.

Models for Translational Proton Radiobiology-From Bench to Bedside and Back

The number of proton therapy centers worldwide are increasing steadily, with more than two million cancer patients treated so far. Despite this development, pending questions on proton radiobiology still call for basic and translational preclinical research. Open issues are the on-going discussion on an energy-dependent varying proton RBE (relative biological effectiveness), a better characterization of normal tissue side effects and combination treatments with drugs originally developed for photon therapy. At the same time, novel possibilities arise, such as radioimmunotherapy, and new proton therapy schemata, such as FLASH irradiation and proton mini-beams. The study of those aspects demands for radiobiological models at different stages along the translational chain, allowing the investigation of mechanisms from the molecular level to whole organisms. Focusing on the challenges and specifics of proton research, this review summarizes the different available models, ranging from in vitro systems to animal studies of increasing complexity as well as complementing in silico approaches.

Magnetically focused 70 MeV proton minibeams for preclinical experiments combining a tandem accelerator and a 3 GHz linear post-accelerator

PURPOSE

Radiotherapy plays an important role for the treatment of tumor diseases in two-thirds of all cases, but it is limited by side effects in the surrounding healthy tissue. Proton minibeam radiotherapy (pMBRT) is a promising option to widen the therapeutic window for tumor control at reduced side effects. An accelerator concept based on an existing tandem Van de Graaff accelerator and a linac enables the focusing of 70 MeV protons to form minibeams with a size of only 0.1 mm for a preclinical small animal irradiation facility, while avoiding the cost of an RFQ injector.

METHODS

The tandem accelerator provides a 16 MeV proton beam with a beam brightness of B = 4 nA mm 2 mrad 2 as averaged from 5 µs long pulses with a flat top current of 17 µA at 200 Hz repetition rate. Subsequently, the protons are accelerated to 70 MeV by a 3 GHz linear post-accelerator consisting of two Side Coupled Drift Tube Linac (SCDTL) structures and four Coupled Cavity Linac (CCL) structures [design: AVO-ADAM S.A (Geneva, Switzerland)]. A 3 GHz buncher and four magnetic quadrupole lenses are placed between the tandem and the post-accelerator to maximize the transmission through the linac. A quadrupole triplet situated downstream of the linac structure focuses the protons into an area of (0.1 × 0.1) mm² . The beam dynamics of the facility is optimized using the particle optics code TRACE three-dimensional (3D). Proton transmission through the facility is elaborated using the particle tracking code TRAVEL.

RESULTS

A study about buncher amplitude and phase shift between buncher and linac is showing that 49% of all protons available from the tandem can be transported through the post-accelerator. A mean beam current up to 19 nA is expected within an area of (0.1 × 0.1) mm² at the beam focus.

CONCLUSION

An extension of existing tandem accelerators by commercially available 3 GHz structures is able to deliver a proton minibeam that serves all requirements to obtain proton minibeams to perform preclinical minibeam irradiations as it would be the case for a complete commercial 3 GHz injector-RFQ-linac combination. Due to the modularity of the linac structure, the irradiation facility can be extended to clinically relevant proton energies up to or above 200 MeV.

Optimizing proton minibeam radiotherapy by interlacing and heterogeneous tumor dose on the basis of calculated clonogenic cell survival

Proton minibeam radiotherapy (pMBRT) is a spatial fractionation method using sub-millimeter beams at center-to-center (ctc) distances of a few millimeters to widen the therapeutic index by reduction of side effects in normal tissues. Interlaced minibeams from two opposing or four orthogonal directions are calculated to minimize side effects. In particular, heterogeneous dose distributions applied to the tumor are investigated to evaluate optimized sparing capabilities of normal tissues at the close tumor surrounding. A 5 cm thick tumor is considered at 10 cm depth within a 25 cm thick water phantom. Pencil and planar minibeams are interlaced from two (opposing) directions as well as planar beams from four directions. An initial beam size of σ0 = 0.2 mm (standard deviation) is assumed in all cases. Tissue sparing potential is evaluated by calculating mean clonogenic cell survival using a linear-quadratic model on the calculated dose distributions. Interlacing proton minibeams for homogeneous irradiation of the tumor has only minor benefits for the mean clonogenic cell survival compared to unidirectional minibeam irradiation modes. Enhanced mean cell survival, however, is obtained when a heterogeneous dose distribution within the tumor is permitted. The benefits hold true even for an elevated mean tumor dose, which is necessary to avoid cold spots within the tumor in concerns of a prescribed dose. The heterogeneous irradiation of the tumor allows for larger ctc distances. Thus, a high mean cell survival of up to 47% is maintained even close to the tumor edges for single fraction doses in the tumor of at least 10 Gy. Similar benefits would result for heavy ion minibeams with the advantage of smaller minibeams in deep tissue potentially offering even increased tissue sparing. The enhanced mean clonogenic cell survival through large ctc distances for interlaced pMBRT with heterogeneous tumor dose distribution results in optimum tissue sparing potential. The calculations show the largest enhancement of the mean cell survival in normal tissue for high-dose fractions. Thus, hypo-fractionation or even single dose fractions become possible for tumor irradiation. A widened therapeutic index at big cost reductions is offered by interlaced proton or heavy ion minibeam therapy.

Normal Tissue Response of Combined Temporal and Spatial Fractionation in Proton Minibeam Radiation Therapy

PURPOSE

Proton minibeam radiation therapy, a spatial fractionation concept, widens the therapeutic window. By reducing normal tissue toxicities, it allows a temporally fractionated regime with high daily doses. However, an array shift between daily fractions can affect the tissue-sparing effect by decreasing the total peak-to-valley dose ratio. Therefore, combining temporal fractions with spatial fractionation raises questions about the impact of daily applied dose modulations, reirradiation accuracies, and total dose modulations.

METHODS AND MATERIALS

Healthy mouse ear pinnae were irradiated with 4 daily fractions of 30 Gy mean dose, applying proton pencil minibeams (pMB) of Gaussian σ = 222 μm in 3 different schemes: a 16 pMB array with a center-to-center distance of 1.8 mm irradiated the same position in all sessions (FS1) or was shifted by 0.9 mm to never hit the previously irradiated tissue in each session (FS2), or a 64 pMB array with a center-to-center distance of 0.9 mm irradiated the same position in all sessions (FS3), resulting in the same total dose distribution as FS2. Reirradiation positioning and its accuracy were obtained from image guidance using the unique vessel structure of ears. Acute toxicities (swelling, erythema, and desquamation) were evaluated for 153 days after the first fraction. Late toxicities (fibrous tissue, inflammation) were analyzed on day 153.

RESULTS

Reirradiation of highly dose-modulated arrays at a positioning accuracy of 110 ± 52 μm induced the least severe acute and late toxicities. A shift of the same array in FS2 led to significantly inducted acute toxicities, a higher otitis score, and a slight increase in fibrous tissue. FS3 led to the strongest increase in acute and late toxicities.

CONCLUSIONS

The highest normal-tissue sparing is achieved after accurate reirradiation of a highly dose modulated pMB array, although high positioning accuracies are challenging in a clinical environment. Nevertheless, the same integral dose applied in highly dose-modulated fractions is superior to low daily dose-modulated fractions.

First proton minibeam radiation therapy treatment plan evaluation

Proton minibeam radiation therapy (pMBRT) is a novel dose delivery method based on spatial dose fractionation. pMBRT has been shown to be promising in terms of reduced side effects and superior tumour control in high-grade glioma-bearing rats compared to standard irradiation. These findings, together with the recent optimized implementation of pMBRT in a clinical pencil beam scanning system, have triggered reflection on the possible application to patient treatments. In this context, the present study was designed to conduct a first theoretical investigation of the clinical potential of this technique. For this purpose, a dedicated dose engine was developed and used to evaluate two clinically relevant patient treatment plans (high-grade glioma and meningioma). Treatment plans were compared with standard proton therapy plans assessed by means of a commercial treatment planning system (ECLIPSE-Varian Medical systems) and Monte Carlo simulations. A multislit brass collimator consisting of 0.4 mm wide slits separated by a centre-to-centre distance of 4 or 6 mm was placed between the nozzle and the patient to shape the planar minibeams. For each plan, spread-out Bragg peaks and homogeneous dose distributions (±7% dose variations) can be obtained in target volumes. The Peak-to-Valley Dose Ratios (PVDR) were evaluated between 9.2 and 12.8 at a depth of 20 mm for meningioma and glioma, respectively. Dose volume histograms (DVHs) for target volumes and organs at risk were quantitatively compared, resulting in a slightly better target homogeneity with standard PT than with pMBRT plans, but similar DVHs for deep-seated organs-at-risk and lower average dose for shallow organs. The proposed delivery method evaluated in this work opens the way to an effective treatment for radioresistant tumours and will support the design of future clinical research.

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.

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.

Proton minibeams-a springboard for physics, biology and clinical creativity

Proton minibeam therapy (PMBT) is a form of spatially fractionated radiotherapy wherein broad beam radiation is replaced with segmented minibeams-either parallel, planar minibeam arrays generated by a multislit collimator or scanned pencil beams that converge laterally at depth to create a uniform dose layer at the tumor. By doing so, the spatial pattern of entrance dose is considerably modified while still maintaining tumor dose and efficacy. Recent studies using computational modeling, phantom experiments, in vitro and in vivo preclinical models, and early clinical feasibility assessments suggest that unique physical and biological attributes of PMBT can be exploited for future clinical benefit. We outline some of the guiding principle of PMBT in this concise overview of this emerging area of preclinical and clinical research inquiry.

Proton pencil minibeam irradiation of an in-vivo mouse ear model spares healthy tissue dependent on beam size

Proton radiotherapy using minibeams of sub-millimeter dimensions reduces side effects in comparison to conventional proton therapy due to spatial fractionation. Since the proton minibeams widen with depth, the homogeneous irradiation of a tumor can be ensured by adjusting the beam distances to tumor size and depth to maintain tumor control as in conventional proton therapy. The inherent advantages of protons in comparison to photons like a limited range that prevents a dosage of distal tissues are maintained by proton minibeams and can even be exploited for interlacing from different beam directions. A first animal study was conducted to systematically investigate and quantify the tissue-sparing effects of proton pencil minibeams as a function of beam size and dose distributions, using beam widths between σ = 95, 199, 306, 411, 561 and 883 μm (standard deviation) at a defined center-to-center beam distance (ctc) of 1.8 mm. The average dose of 60 Gy was distributed in 4x4 minibeams using 20 MeV protons (LET ~ 2.7 keV/μm). The induced radiation toxicities were measured by visible skin reactions and ear swelling for 90 days after irradiation. The largest applied beam size to ctc ratio (σ/ctc = 0.49) is similar to a homogeneous irradiation and leads to a significant 3-fold ear thickness increase compared to the control group. Erythema and desquamation was also increased significantly 3–4 weeks after irradiation. With decreasing beam sizes and thus decreasing σ/ctc, the maximum skin reactions are strongly reduced until no ear swelling or other visible skin reactions should occur for σ/ctc < 0.032 (extrapolated from data). These results demonstrate that proton pencil minibeam radiotherapy has better tissue-sparing for smaller σ/ctc, corresponding to larger peak-to-valley dose ratios PVDR, with the best effect for σ/ctc < 0.032. However, even quite large σ/ctc (e.g. σ/ctc = 0.23 or 0.31, i.e. PVDR = 10 or 2.7) show less acute side effects than a homogeneous dose distribution. This suggests that proton minibeam therapy spares healthy tissue not only in the skin but even for dose distributions appearing in deeper layers close to the tumor enhancing its benefits for clinical proton therapy.

Beam size limit for pencil minibeam radiotherapy determined from side effects in an in-vivo mouse ear model

Side effects caused by radiation are a limiting factor to the amount of dose that can be applied to a tumor volume. A novel method to reduce side effects in radiotherapy is the use of spatial fractionation, in which a pattern of sub-millimeter beams (minibeams) is applied to spare healthy tissue. In order to determine the skin reactions in dependence of single beam sizes, which are relevant for spatially fractionated radiotherapy approaches, single pencil beams of submillimeter to 6 millimeter size were applied in BALB/c mice ears at a Small Animal Radiation Research Platform (SARRP) with a plateau dose of 60 Gy. Radiation toxicities in the ears were observed for 25 days after irradiation. Severe radiation responses were found for beams ≥ 3 mm diameter. The larger the beam diameter the stronger the observed reactions. No ear swelling and barely reddening or desquamation were found for the smallest beam sizes (0.5 and 1 mm). The findings were confirmed by histological sections. Submillimeter beams are preferred in minibeam therapy to obtain optimized tissue sparing. The gradual increase of radiation toxicity with beam size shows that also larger beams are capable of healthy tissue sparing in spatial fractionation.

Monte-Carlo dosimetry and real-time imaging of targeted irradiation consequences in 2-cell stage Caenorhabditis elegans embryo

Charged-particle microbeams (CPMs) provide a unique opportunity to investigate the effects of ionizing radiation on living biological specimens with a precise control of the delivered dose, i.e. the number of particles per cell. We describe a methodology to manipulate and micro-irradiate early stage C. elegans embryos at a specific phase of the cell division and with a controlled dose using a CPM. To validate this approach, we observe the radiation-induced damage, such as reduced cell mobility, incomplete cell division and the appearance of chromatin bridges during embryo development, in different strains expressing GFP-tagged proteins in situ after irradiation. In addition, as the dosimetry of such experiments cannot be extrapolated from random irradiations of cell populations, realistic three-dimensional models of 2 cell-stage embryo were imported into the Geant4 Monte-Carlo simulation toolkit. Using this method, we investigate the energy deposit in various chromatin condensation states during the cell division phases. The experimental approach coupled to Monte-Carlo simulations provides a way to selectively irradiate a single cell in a rapidly dividing multicellular model with a reproducible dose. This method opens the way to dose-effect investigations following targeted irradiation.

Spatially fractionated proton minibeams

Extraordinary normal tissue response to highly spatially fractionated X-ray beams has been explored for over 25 years. More recently, alternative radiation sources have been developed and utilized with the aim to evoke comparable effects. These include protons, which lend themselves well for this endeavour due to their physical depth dose characteristics as well as corresponding variable biological effectiveness. This paper addresses the motivation for using protons to generate spatially fractionated beams and reviews the technological implementations and experimental results to date. This includes simulation and feasibility studies, collimation and beam characteristics, dosimetry and biological considerations as well as the results of in vivo and in vitro studies. Experimental results are emerging indicating an extraordinary normal tissue sparing effect analogous to what has been observed for synchrotron generated X-ray microbeams. The potential for translational research and feasibility of spatially modulated proton beams in clinical settings is discussed.

Implementation of planar proton minibeam radiation therapy using a pencil beam scanning system: A proof of concept study

PURPOSE

Proton minibeam radiation therapy (pMBRT) is an innovative approach that combines the advantages of minibeam radiation therapy with the more precise ballistics of protons to further reduce the side effects of radiation. One of the main challenges of this approach is the generation of very narrow proton pencil beams with an adequate dose-rate to treat patients within a reasonable treatment time (several minutes) in existing clinical facilities. The aim of this study was to demonstrate the feasibility of implementing pMBRT by combining the pencil beam scanning (PBS) technique with the use of multislit collimators. This proof of concept study of pMBRT with a clinical system is intended to guide upcoming biological experiments.

METHODS

Monte Carlo simulations (TOPAS v3.1.p2) were used to design a suitable multislit collimator to implement planar pMBRT for conventional pencil beam scanning settings. Dose distributions (depth-dose curves, lateral profiles, Peak-to-Valley Dose Ratio (PVDR) and dose-rates) for different proton beam energies were assessed by means of Monte Carlo simulations and experimental measurements in a water tank using commercial ionization chambers and a new p-type silicon diode, the IBA RAZOR. An analytical intensity-modulated dose calculation algorithm designed to optimize the weight of individual Bragg peaks composing the field was also developed and validated.

RESULTS

Proton minibeams were then obtained using a brass multislit collimator with five slits measuring 2 cm × 400 μm in width with a center-to-center distance of 4 mm. The measured and calculated dose distributions (depth-dose curves and lateral profiles) showed a good agreement. Spread-out Bragg peaks (SOBP) and homogeneous dose distributions around the target were obtained by means of intensity modulation of Bragg peaks, while maintaining spatial fractionation at shallow depths. Mean dose-rates of 0.12 and 0.09 Gy/s were obtained for one iso-energy layer and a SOBP conditions in the presence of multislit collimator.

CONCLUSIONS

This study demonstrates the feasibility of implementing pMBRT on a PBS system. It also confirms the reliability of RAZOR detector for pMBRT dosimetry. This newly developed experimental methodology will support the design of future preclinical research with pMBRT.

Development, commissioning, and evaluation of a new intensity modulated minibeam proton therapy system

PURPOSE

To invent, design, construct, and commission an intensity modulated minibeam proton therapy system (IMMPT) without the need for physical collimation and to compare its resulting conformity to a conventional IMPT system.

METHODS

A proton therapy system (Hitachi, Ltd, Hitachi City, Japan; Model: Probeat-V) was specially modified to produce scanned minibeams without collimation. We performed integral depth dose acquisitions and calibrations using a large diameter parallel-plate ionization chamber in a scanning water phantom (PTW, Freiburg, Germany; Models: Bragg Peak ionization chamber, MP3-P). Spot size and shape was measured using radiochromic film (Ashland Advanced Materials, Bridgewater NJ; Type: EBT3), and a synthetic diamond diode type scanned point by point in air (PTW Models: MicroDiamond, MP3-P). The measured data were used as inputs to generate a Monte Carlo-based model for a commercial radiotherapy planning system (TPS) (Varian Medical Systems, Inc., Palo Alto, CA; Model: Eclipse v13.7.15). The regular ProBeat-V system (sigma ~2.5 mm) TPS model was available for comparison. A simulated base of skull case with small and medium targets proximal to brainstem was planned for both systems and compared.

RESULTS

The spot sigma is determined to be 1.4 mm at 221 MeV at the isocenter and below 1 mm at proximal distances. Integral depth doses were indistinguishable from the standard spot commissioning data. The TPS fit the spot profiles closely, giving a residual error maximum of 2.5% in the spot penumbra tails (below 5% of maximum) from the commissioned energies 69.4 to 221.3 MeV. The resulting IMMPT plans were more conformal than the IMPT plans due to a sharper dose gradient (90-10%) 1.5 mm smaller for the small target, and 1.3 mm for the large target, at a representative central axial water equivalent depth of 7 cm.

CONCLUSIONS

We developed, implemented, and tested a new IMMPT system. The initial results look promising in cases where treatments can benefit from additional dose sparing to neighboring sensitive structures.

Expanding the therapeutic index of radiation therapy by normal tissue protection

Normal tissue damages induced by radiation therapy remain dose-limiting factors in radiation oncology and this is still true despite recent advances in treatment planning and delivery of image-guided radiation therapy. Additionally, as the number of long-term cancer survivors increases, unacceptable complications emerge and dramatically reduce the patients' quality of life. This means that patients and clinicians expect discovery of new options for the therapeutic management of radiation-induced complications. Over the past four decades, research has enhanced our understanding of the pathophysiological, cellular and molecular processes governing normal tissue toxicity. Those processes are complex and involve the cross-talk between the various cells of a tissue, including fibroblasts, endothelial, immune and epithelial cells as well as soluble paracrine factors including growth factors and proteases. We will review the translatable pharmacological approaches that have been developed to prevent, mitigate, or reverse radiation injuries based upon the targeting of cellular and signalling pathways. We will summarize the different steps of the research strategy, from the definition of initial biological hypotheses to preclinical studies and clinical translation. We will also see how novel research and therapeutic hypotheses emerge along the way as well as briefly highlight innovative approaches based upon novel radiotherapy delivery procedures.

Biological and dosimetric characterisation of spatially fractionated proton minibeams

The biological effectiveness of proton beams varies with depth, spot size and lateral distance from the beam central axis. The aim of this work is to incorporate proton relative biological effectiveness (RBE) and equivalent uniform dose (EUD) considerations into comparisons of broad beam and highly modulated proton minibeams. A Monte Carlo model of a small animal proton beamline is presented. Dose and variable RBE is calculated on a per-voxel basis for a range of energies (30-109 MeV). For an open beam, the RBE values at the beam entrance ranged from 1.02-1.04, at the Bragg peak (BP) from 1.3 to 1.6, and at the distal end of the BP from 1.4 to 2.0. For a 50 MeV proton beam, a minibeam collimator designed to produce uniform dose at the depth of the BP peak, had minimal impact on the open beam RBE values at depth. RBE changes were observed near the surface when the collimator was placed flush with the irradiated object, due to a higher neutron contribution derived from proton interactions with the collimator. For proton minibeams, the relative mean RBE weighted entrance dose (RWD) was ~25% lower than the physical mean dose. A strong dependency of the EUD with fraction size was observed. For 20 Gy fractions, the EUD varied widely depending on the radiosensitivity of the cells. For radiosensitive cells, the difference was up to ~50% in mean dose and ~40% in mean RWD and the EUD trended towards the valley dose rather than the mean dose. For comparative studies of uniform dose with spatially fractionated proton minibeams, EUD derived from a per-voxel RWD distribution is recommended for biological assessments of reproductive cell survival and related endpoints.

Optimization of beam arrangements in proton minibeam radiotherapy by cell survival simulations

PURPOSE

Proton minibeam radiotherapy using submillimeter beam dimensions allows to enhance tissue sparing in the entrance channel by spatial fractionation additionally to advantageous proton depth dose distribution. In the entrance channel, spatial fractionation leads to reduced side effects compared to conventional proton therapy. The submillimeter sized beams widen with depth due to small angle scattering and enable therefore, in contrary to x-ray microbeam radiation therapy (MRT), the homogeneous irradiation of a tumor. Proton minibeams can either be applied as planar minibeams or pencil shaped with an additional possibility to vary between a quadratic and a hexagonal arrangement for pencil minibeams. The purpose of this work is to deduce interbeam distances to achieve a homogeneous dose distribution for different tumor depths and tumor thicknesses. Furthermore, we aim for a better understanding of the sparing effect on the basis of surviving cells calculated by the linear-quadratic model.

METHODS

Two-dimensional dose distributions are calculated for proton minibeams of different shapes and arrangements. For a tumor in 10-15 cm depth, treatment plans are calculated with initial beam size of σ₀  = 0.2 mm in a water phantom. Proton minibeam depth dose distributions are finally converted into cell survival using a linear-quadratic model.

RESULTS

Inter proton beam distances are maximized under the constraint of dose homogeneity in the tumor for tumor depths ranging from 4 to 15 cm and thickness ranging from 0.5 to 10 cm. Cell survival calculations for a 5 cm thick tumor covered by 10 cm healthy tissue show less cell death by up to 85%, especially in the superficial layers, while keeping the cell death in the tumor as in conventional therapy. In the entrance channel, the pencil minibeams result in higher cell survival in comparison to the planar minibeams while all proton minibeam irradiations show higher cell survival than conventional broadbeam irradiation.

CONCLUSION

The deduced constraints for interbeam distances simplify treatment planning for proton minibeam radiotherapy applications in future studies. The cell survival results indicate that proton minibeam radiotherapy reduces side effects but keeps tumor control as in conventional proton therapy. It makes proton minibeam, especially pencil minibeam radiotherapy a potentially attractive new approach for radiation therapy.

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.

Microirradiation techniques in radiobiological research

The aim of this work is to review the uses of laser microirradiation and ion microbeam techniques within the scope of radiobiological research. Laser microirradiation techniques can be used for many different purposes. In a specific condition, through the use of pulsed lasers, cell lysis can be produced for subsequent separation of different analytes. Microsurgery allows for the identification and isolation of tissue sections, single cells and subcellular components, using different types of lasers. The generation of different types of DNA damage, via this type of microirradiation, allows for the investigation of DNA dynamics. Ion microbeams are important tools in radiobiological research. There are only a limited number of facilities worldwide where radiobiological experiments can be performed. In the beginning, research was mostly focused on the bystander effect. Nowadays, with more sophisticated molecular and cellular biological techniques, ion microirradiation is used to unravel molecular processes in the field of radiobiology. These include DNA repair protein kinetics or chromatin modifications at the site of DNA damage. With the increasing relevance of charged particles in tumour therapy and new concepts on how to generate them, ion microbeam facilities are able to address unresolved questions concerning particle tumour therapy.

Proton Grid Therapy: A Proof-of-Concept Study

In this work, we studied the possibility of merging proton therapy with grid therapy. We hypothesized that patients with larger targets containing solid tumor growth could benefit from being treated with this method, proton grid therapy. We performed treatment planning for 2 patients with abdominal cancer with the suggested proton grid therapy technique. The proton beam arrays were cross-fired over the target volume. Circular or rectangular beam element shapes (building up the beam grids) were evaluated in the planning. An optimization was performed to calculate the fluence from each beam grid element. The optimization objectives were set to create a homogeneous dose inside the target volume with the constraint of maintaining the grid structure of the dose distribution in the surrounding tissue. The proton beam elements constituting the grid remained narrow and parallel down to large depths in the tissue. The calculation results showed that it is possible to produce target doses ranging between 100% and 130% of the prescribed dose by cross-firing beam grids, incident from 4 directions. A sensitivity test showed that a small rotation or translation of one of the used grids, due to setup errors, had only a limited influence on the dose distribution produced in the target, if 4 beam arrays were used for the irradiation. Proton grid therapy is technically feasible at proton therapy centers equipped with spot scanning systems using existing tools. By cross-firing the proton beam grids, a low tissue dose in between the paths of the elemental beams can be maintained down to the vicinity of a deep-seated target. With proton grid therapy, it is possible to produce a dose distribution inside the target volume of similar uniformity as can be created with current clinical methods.

Advances in microbeam technologies and applications to radiation biology

Charged-particle microbeams (CPMs) allow the targeting of sub-cellular compartments with a counted number of energetic ions. While initially developed in the late 1990s to overcome the statistical fluctuation on the number of traversals per cell inevitably associated with broad beam irradiations, CPMs have generated a growing interest and are now used in a wide range of radiation biology studies. Besides the study of the low-dose cellular response that has prevailed in the applications of these facilities for many years, several new topics have appeared recently. By combining their ability to generate highly clustered damages in a micrometric volume with immunostaining or live-cell GFP labelling, a huge potential for monitoring radiation-induced DNA damage and repair has been introduced. This type of studies has pushed end-stations towards advanced fluorescence microscopy techniques, and several microbeam lines are currently equipped with the state-of-the-art time-lapse fluorescence imaging microscopes. In addition, CPMs are nowadays also used to irradiate multicellular models in a highly controlled way. This review presents the latest developments and applications of charged-particle microbeams to radiation biology.