Advancing proton minibeam radiation therapy: magnetically focussed proton minibeams at a clinical centre

Dosimetric study for breathing-induced motion effects in an abdominal pancreas phantom for carbon ion mini-beam radiotherapy

BACKGROUND

Particle mini-beam therapy exhibits promise in sparing healthy tissue through spatial fractionation, particularly notable for heavy ions, further enhancing the already favorable differential biological effectiveness at both target and entrance regions. However, breathing-induced organ motion affects particle mini-beam irradiation schemes since the organ displacements exceed the mini-beam structure dimensions, decreasing the advantages of spatial fractionation.

PURPOSE

In this study, the impact of breathing-induced organ motion on the dose distribution was examined at the target and organs at risk(OARs) during carbon ion mini-beam irradiation for pancreatic cancer.

METHODS

As a first step, the carbon ion mini-beam pattern was characterized with Monte Carlo simulations. To analyze the impact of breathing-induced organ motion on the dose distribution of a virtual pancreas tumor as target and related OARs, the anthropomorphic Pancreas Phantom for Ion beam Therapy (PPIeT) was irradiated with carbon ions. A mini-beam collimator was used to deliver a spatially fractionated dose distribution. During irradiation, varying breathing motion amplitudes were induced, ranging from 5 to 15 mm. Post-irradiation, the 2D dose pattern was analyzed, focusing on the full width at half maximum (FWHM), center-to-center distance (ctc), and the peak-to-valley dose ratio (PVDR).

RESULTS

The mini-beam pattern was visible within OARs, while in the virtual pancreas tumor a more homogeneous dose distribution was achieved. Applied motion affected the mini-beam pattern within the kidney, one of the OARs, reducing the PVDR from 3.78  ± $\pm$  0.12 to 1.478  ± $\pm$  0.070 for the 15 mm motion amplitude. In the immobile OARs including the spine and the skin at the back, the PVDR did not change within 3.4% comparing reference and motion conditions.

CONCLUSIONS

This study provides an initial understanding of how breathing-induced organ motion affects spatial fractionation during carbon ion irradiation, using an anthropomorphic phantom. A decrease in the PVDR was observed in the right kidney when breathing-induced motion was applied, potentially increasing the risk of damage to OARs. Therefore, further studies are needed to explore the clinical viability of mini-beam radiotherapy with carbon ions when irradiating abdominal regions.

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

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

Tuning spatially fractionated radiotherapy dose profiles using the moiré effect

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

Sharp dose profiles for high precision proton therapy using strongly focused proton beams

The main objective of radiotherapy is to exploit the curative potential of ionizing radiation while inflicting minimal radiation-induced damage to healthy tissue and sensitive organs. Proton beam therapy has been developed to irradiate the tumor with higher precision and dose conformity compared to conventional X-ray irradiation. The dose conformity of this treatment modality may be further improved if narrower proton beams are used. Still, this is limited by multiple Coulomb scattering of protons through tissue. The primary aim of this work was to develop techniques to produce narrow proton beams and investigate the resulting dose profiles. We introduced and assessed three different proton beam shaping techniques: (1) metal collimators (100/150 MeV), (2) focusing of conventional- (100/150 MeV), and (3) focusing of high-energy (350 MeV, shoot-through) proton beams. Focusing was governed by the initial value of the Twiss parameter [Formula: see text] ([Formula: see text]), and can be implemented with magnetic particle accelerator optics. The dose distributions in water were calculated by Monte Carlo simulations using Geant4, and evaluated by target to surface dose ratio (TSDR) in addition to the transverse beam size ([Formula: see text]) at the target. The target was defined as the location of the Bragg peak or the focal point. The different techniques showed greatly differing dose profiles, where focusing gave pronouncedly higher relative target dose and efficient use of primary protons. Metal collimators with radii [Formula: see text] gave low TSDRs ([Formula: see text]) and large [Formula: see text]([Formula: see text]). In contrast, a focused beam of conventional ([Formula: see text]) energy produced a very high TSDR ([Formula: see text]) with similar [Formula: see text] as a collimated beam. High-energy focused beams were able to produce TSDRs [Formula: see text] and [Formula: see text] around 1.5 mm. From this study, it appears very attractive to implement magnetically focused proton beams in radiotherapy of small lesions or tumors in close vicinity to healthy organs at risk. This can also lead to a paradigm change in spatially fractionated radiotherapy. Magnetic focusing would facilitate FLASH irradiation due to low losses of primary protons.

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.

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.

Converging Proton Minibeams with Magnetic Fields for Optimized Radiation Therapy: A Proof of Concept

Proton MiniBeam Radiation Therapy (pMBRT) is a novel strategy that combines the benefits of minibeam radiation therapy with the more precise ballistics of protons to further optimize the dose distribution and reduce radiation side effects. The aim of this study is to investigate possible strategies to couple pMBRT with dipole magnetic fields to generate a converging minibeam pattern and increase the center-to-center distance between minibeams. Magnetic field optimization was performed so as to obtain the same transverse dose profile at the Bragg peak position as in a reference configuration with no magnetic field. Monte Carlo simulations reproducing realistic pencil beam scanning settings were used to compute the dose in a water phantom. We analyzed different minibeam generation techniques, such as the use of a static multislit collimator or a dynamic aperture, and different magnetic field positions, i.e., before or within the water phantom. The best results were obtained using a dynamic aperture coupled with a magnetic field within the water phantom. For a center-to-center distance increase from 4 mm to 6 mm, we obtained an increase of peak-to-valley dose ratio and decrease of valley dose above 50%. The results indicate that magnetic fields can be effectively used to improve the spatial modulation at shallow depth for enhanced healthy tissue sparing.

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.

Future technological developments in proton therapy - A predicted technological breakthrough

In the current spectrum of cancer treatments, despite high costs, a lack of robust evidence based on clinical outcomes or technical and radiobiological uncertainties, particle therapy and in particular proton therapy (PT) is rapidly growing. Despite proton therapy being more than fifty years old (first proposed by Wilson in 1946) and more than 220,000 patients having been treated with in 2020, many technological challenges remain and numerous new technical developments that must be integrated into existing systems. This article presents an overview of on-going technical developments and innovations that we felt were most important today, as well as those that have the potential to significantly shape the future of proton therapy. Indeed, efforts have been done continuously to improve the efficiency of a PT system, in terms of cost, technology and delivery technics, and a number of different developments pursued in the accelerator field will first be presented. Significant developments are also underway in terms of transport and spatial resolution achievable with pencil beam scanning, or conformation of the dose to the target: we will therefore discuss beam focusing and collimation issues which are important parameters for the development of these techniques, as well as proton arc therapy. State of the art and alternative approaches to adaptive PT and the future of adaptive PT will finally be reviewed. Through these overviews, we will finally see how advances in these different areas will allow the potential for robust dose shaping in proton therapy to be maximised, probably foreshadowing a future era of maturity for the PT technique.

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.

A Monte Carlo Determination of Dose and Range Uncertainties for Preclinical Studies with a Proton Beam

Proton therapy (PRT) is an irradiation technique that aims at limiting normal tissue damage while maintaining the tumor response. To study its specificities, the ARRONAX cyclotron is currently developing a preclinical structure compatible with biological experiments. A prerequisite is to identify and control uncertainties on the ARRONAX beamline, which can lead to significant biases in the observed biological results and dose-response relationships, as for any facility. This paper summarizes and quantifies the impact of uncertainty on proton range, absorbed dose, and dose homogeneity in a preclinical context of cell or small animal irradiation on the Bragg curve, using Monte Carlo simulations. All possible sources of uncertainty were investigated and discussed independently. Those with a significant impact were identified, and protocols were established to reduce their consequences. Overall, the uncertainties evaluated were similar to those from clinical practice and are considered compatible with the performance of radiobiological experiments, as well as the study of dose-response relationships on this proton beam. Another conclusion of this study is that Monte Carlo simulations can be used to help build preclinical lines in other setups.

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.

Current delivery limitations of proton PBS for FLASH

PURPOSE

Proton Pencil Beam Scanning (PBS) is an attractive solution to realize the advantageous normal tissue sparing elucidated from FLASH high dose rates. The mechanics of PBS spot delivery will impose limitations on the effective field dose rate for PBS.

METHODS

This study incorporates measurements from clinical and FLASH research beams on uniform single energy and the spread-out Bragg Peak PBS fields to extrapolate the PBS dose rate to high cyclotron beam currents 350, 500, and 800 nA. The impact of the effective field dose rate from cyclotron current, spot spacing, slew time and field size were studied.

RESULTS

When scanning magnet slew time and energy switching time are not considered, single energy effective field FLASH dose rate (≥40 Gy/s) can only be achieved with less than 4 × 4 cm² fields when the cyclotron output current is above 500 nA. Slew time and energy switching time remain the limiting factors for achieving high effective dose rate of the field. The dose rate-time structures were obtained. The amount of the total dose delivered at the FLASH dose rate in single energy layer and volumetric field was also studied.

CONCLUSION

It is demonstrated that while it is difficult to achieve FLASH dose rate for a large field or in a volume, local FLASH delivery to certain percentage of the total dose is possible. With further understanding of the FLASH radiobiological mechanism, this study could provide guidance to adapt current clinical multi-field proton PBS delivery practice for FLASH proton radiotherapy.