Beam commissioning of the first compact proton therapy system with spot scanning and dynamic field collimation

Characterization of commercial detectors for absolute proton UHDR dosimetry on a compact clinical proton synchrocyclotron

BACKGROUND

Modern compact proton synchrocyclotrons can achieve ultra-high dose rates ( ≥ $ \ge $ 40 Gy/s) to support ultra-high-dose-rate (UHDR) preclinical experiments utilizing pencil beam scanning (PBS) protons. Unique to synchrocyclotrons is a pulsed proton time structure as compared to the quasi-continuous nature of other proton accelerators like isochronous cyclotrons. Thus, high instantaneous proton currents in the order of several µA must be generated to achieve UHDRs. This will lead to high doses-per-pulse (DPP), which may cause significant charge recombination for ionization chambers, which must be characterized for accurate UHDR dosimetry programs.

PURPOSE

In this work, we investigate the suitability of various commercial radiation detectors for accurate proton UHDR dosimetry using PBS proton beams from a compact proton synchrocyclotron (IBA ProteusONE). This is achieved by cross-calibrating them with conventional dose rates, measuring UHDR recombination (Pion) and polarity correction factors (Ppol) for ionization chambers, and determining the absorbed proton UHDR dose delivered for all detectors.

METHODS

An IBA ProteusONE synchrocyclotron was initially tuned to achieve UHDRs with 228 MeV protons at 0° gantry angle. Various detectors, including Razor Chamber, Razor Nano Chamber, Razor Diode, and microDiamond, were cross-calibrated against a PPC05 plane-parallel ionization chamber (PPIC) that had an ADCL calibration coefficient of 59.23 cGy/nC. Then, all ionization chambers were exposed to UHDR protons with the Ppol and Pion subsequently calculated. Pion was calculated using two methods: TRS-398 methods and Niatel's model. Finally, the absolute UHDR proton doses delivered were determined for all detectors and cross-compared.

RESULTS

Faraday cup measurements were performed for a single spot proton UHDR beam, and the nozzle current at the isocenter was determined to be 129.5 nA during UHDR irradiations at 98.61% of the maximum theoretical dose rate. Repeated Faraday cup measurements of the UHDR beam yielded a percentage standard deviation of 0.8%, which was higher than 0.120% when similar repeated measurements were performed with conventional proton beams. Ppol was found to be relatively dose-rate independent for all ionization chambers investigated. Pion was found to be the lowest for the PPC05 ionization chamber (1.0097) compared to corresponding values of 1.0214 and 1.0294 for the Razor and Razor Nano detectors, respectively, for UHDRs. Pion values calculated using Niatel's model closely matched values from TRS-398 if the VH/VL ratio were kept at 2.5 for the PPC05 and Razor detectors and 2.0 for the Razor Nano detector. Absolute proton UHDR doses determined using cross-calibration factors were generally within ± 1% of PPC05 measurements. However, Razor Diode was found to over-respond by up to 3.79% within UHDR proton beams, rendering them unsuitable for proton UHDR dosimetry.

CONCLUSION

In this work, we comprehensively evaluated the suitability of various commercial detectors for absolute dosimetry with a pulsed UHDR beam structure from a proton synchrocyclotron. PPC05 had the lowest ionic recombination correction compared to Razor and Razor Nano ion chambers. Other than the diode detector, all other investigated detectors (PPC05, Razor, Razor Nano, microDiamond) were within ± 1% of one another and can be used for accurate absolute proton UHDR dosimetry.

Proton therapy of lung cancer patients - Treatment strategies and clinical experience from a medical physicist's perspective

PURPOSE

Proton therapy of moving targets is considered a challenge. At Maastro, we started treating lung cancer patients with proton therapy in October 2019. In this work, we summarise the developed treatment strategies and gained clinical experience from a physics point of view.

METHODS

We report on our clinical approaches to treat lung cancer patients with the Mevion Hyperscan S250i proton machine. We classify lung cancer patients as small movers (tumour movement ≤ 5 mm) or large movers (tumour movement > 5 mm). The preferred beam configuration has evolved over the years of clinical treatment, and currently mostly two or three beam directions are used. All patients are treated with robustly optimised plans (5 mm setup and 3% range uncertainty). Small movers are planned based on a clinical target volume (CTV) with a 3 mm isotropic margin expansion to account for motion, while large movers are planned based on an internal target volume (ITV). All patients are treated in free-breathing.

RESULTS

Between October 2019 and December 2023, 379 lung cancer patients have been treated, of which 130 were large movers. The adaptation rate was 28%. The median treatment time has been reduced from 30 to 23 min. The mean dose to the heart, oesophagus, and lungs was on average 4.3, 15.4, and 11.0 Gy, respectively.

CONCLUSIONS

Several treatment planning and workflow improvements have been introduced over the years, resulting in an increase of treatment quality and number of treated patients, as well as reduction of planning and treatment time.

Out-of-field dose assessment for pencil beam scanning proton radiotherapy versus photon radiotherapy for breast cancer in pregnant women

Background and purpose

Cancer affects 1 in 1000-2000 pregnancies annually worldwide, creating challenges in balancing cancer treatment and fetal safety. This study compares out-of-field radiation doses between two treatment modalities: 6MV external photon radiotherapy (XRT) and pencil beam scanning proton-therapy (PBS-PRT) for breast cancer, including imaging, to evaluate PBS-PRT as a potential new treatment option.

Materials and methods

For breast cancer involving lymph node levels 1-4 and the intramammary lymph nodes, treatment plans were created for XRT (with Flattening Filter (FF) and FF-Free (FFF)) and PBS-PRT, prescribing 15 × 2.67 Gy(RBE). Measurements were conducted using an adapted anthropomorphic phantom representing 20- and 30-week pregnancy. Bubble detectors placed in the phantom's abdomen assessed neutron dose from PBS-PRT, while a Farmer ion chamber was used for imaging and XRT dose.

Results

At 20 weeks, PBS-PRT including imaging delivered 22.4 mSv, reducing dose 3.4-fold versus 6FF XRT and 2.5-fold versus 6FFF XRT. At 30 weeks, the PBS-PRT dose was 25.4 mSv, resulting in 7.6-fold and 6.3-fold reductions compared to 6FF and 6FFF XRT, respectively.

Conclusions

This study presents the first one-by-one comparison between PBS-PRT and different XRT modalities for pregnant breast cancer patients with an adapted anthropomorphic phantom. PBS-PRT measurements showed that the total equivalent dose was below the 100 mSv threshold outlined in AAPM Task Group Report No. 36 for a 30-week pregnancy, even under a worst-case scenario, maintaining treatment goals. These findings support the adoption of PBS-PRT as the preferred approach for treating pregnant breast cancer patients, should radiotherapy be required.

Robustness of intensity modulated proton treatment of esophageal cancer for anatomical changes and breathing motion

BACKGROUND AND PURPOSE

In this study, we assessed the robustness of intensity modulated proton therapy (IMPT) in esophageal cancer for anatomical variations during treatment.

METHODS

The first sixty esophageal cancer patients, treated clinically with chemoradiotherapy were included. The treatment planning strategy was based on an internal target volume (ITV) approach, where the ITV was created from the clinical target volumes (CTVs) delineated on all phases of a 4DCT. For optimization, a 3 mm isotropic margin was added to the ITV, combined with robust optimization using 5 mm setup and 3 % range uncertainty. Each patient received weekly repeat CTs (reCTs). Robust plan re-evaluation on all reCTs, and a robust dose summation was performed. To assess the factors influencing ITV coverage, a multivariate linear regression analysis was performed. Additionally, clinical adaptations were evaluated.

RESULTS

The target coverage was adequate (ITV V94%>98 % on the robust voxel-wise minimum dose) on most reCTs (91 %), and on the summed dose in 92 % of patients. Significant predictors for ITV coverage in the multivariate analysis were diaphragm baseline shift and water equivalent depth (WED) of the ITV in the beam direction. Underdosage of the ITV mainly occurred in week 1 and 4, leading to treatment adaptation of eight patients, all on the first reCT.

CONCLUSION

Our IMPT treatment of esophageal cancer is robust for anatomical variations. Adaptation appears to be most effective in the first week of treatment. Diaphragm baseline shifts and WED are predictive factors for ITV underdosage, and should be incorporated in an adaptation protocol.

Navigating the straits: realizing the potential of proton FLASH through physics advances and further pre-clinical characterization

Ultra-high dose-rate 'FLASH' radiotherapy may be a pivotal step forward for cancer treatment, widening the therapeutic window between radiation tumour killing and damage to neighbouring normal tissues. The extent of normal tissue sparing reported in pre-clinical FLASH studies typically corresponds to an increase in isotoxic dose-levels of 5-20%, though gains are larger at higher doses. Conditions currently thought necessary for FLASH normal tissue sparing are a dose-rate ≥40 Gy s-1, dose-per-fraction ≥5-10 Gy and irradiation duration ≤0.2-0.5 s. Cyclotron proton accelerators are the first clinical systems to be adapted to irradiate deep-seated tumours at FLASH dose-rates, but even using these machines it is challenging to meet the FLASH conditions. In this review we describe the challenges for delivering FLASH proton beam therapy, the compromises that ensue if these challenges are not addressed, and resulting dosimetric losses. Some of these losses are on the same scale as the gains from FLASH found pre-clinically. We therefore conclude that for FLASH to succeed clinically the challenges must be systematically overcome rather than accommodated, and we survey physical and pre-clinical routes for achieving this.

ESTRO-EPTN radiation dosimetry guidelines for the acquisition of proton pencil beam modelling data

Proton therapy (PT) is an advancing radiotherapy modality increasingly integrated into clinical settings, transitioning from research facilities to hospital environments. A critical aspect of the commissioning of a proton pencil beam scanning delivery system is the acquisition of experimental beam data for accurate beam modelling within the treatment planning system (TPS). These guidelines describe in detail the acquisition of proton pencil beam modelling data. First, it outlines the intrinsic characteristics of a proton pencil beam-energy distribution, angular-spatial distribution and particle number. Then, it lists the input data typically requested by TPSs. Finally, it describes in detail the set of experimental measurements recommended for the acquisition of proton pencil beam modelling data-integrated depth-dose curves, spot maps in air, and reference dosimetry. The rigorous characterization of these beam parameters is essential for ensuring the safe and precise delivery of proton therapy treatments.

Utilizing collimated aperture with proton pencil beam scanning (PBS) for stereotactic radiotherapy

PURPOSE

Proton stereotactic radiosurgery (PSRS) has emerged as an innovative proton therapy modality aimed at achieving precise dose delivery with minimal impact on healthy tissues. This study explores the dosimetric outcomes of PSRS in comparison to traditional intensity-modulated proton therapy (IMPT) by focusing on cases with small target volumes. A custom-made aperture system designed for proton therapy, specifically tailored to small target volumes, was developed and implemented for this investigation.

METHODS

A prerequisite mechanical validation through an isocentricity test precedes dosimetric assessments, ensuring the seamless integration of mechanical and dosimetry analyses. Five patients were enrolled in the study, including two with choroid melanoma and three with arteriovenous malformations (AVM). Two treatment plans were meticulously executed for each patient, one utilizing a collimated aperture and the other without. Both plans were subjected to robust optimization, maintaining identical beam arrangements and consistent optimization parameters to account for setup errors of 2 mm and range uncertainties of 3.5%. Plan evaluation metrics encompassing the Heterogeneity Index (HI), Paddick Conformity Index (CIPaddick), Gradient Index (GI), and the R50% index to evaluate alterations in low-dose volume distribution.

RESULTS

The comparative analysis between PSRS and traditional PBS treatment revealed no significant differences in plan outcomes, with both modalities demonstrating comparable target coverage. However, collimated apertures resulted in discernible improvements in dose conformity, dose fall-off, and reduced low-dose volume.

CONCLUSIONS

This study underscores the advantageous impact of the aperture system on proton therapy, particularly in cases involving small target volumes.

Experimental comparison of cylindrical and plane parallel ionization chambers for reference dosimetry in continuous and pulsed scanned proton beams

Objective. In this experimental work we compared the determination of absorbed dose to water using four ionization chambers (ICs), a PTW-34045 Advanced Markus, a PTW-34001 Roos, an IBA-PPC05 and a PTW-30012 Farmer, irradiated under the same conditions in one continuous- and in two pulsed-scanned proton beams.Approach. The ICs were positioned at 2 cm depth in a water phantom in four square-field single-energy scanned-proton beams with nominal energies between 80 and 220 MeV and in the middle of 10 × 10 × 10 cm3dose cubes centered at 10 cm or 12.5 cm depth in water. The water-equivalent thickness (WET) of the entrance window and the effective point of measurement was considered when positioning the plane parallel (PP) ICs and the cylindrical ICs, respectively. To reduce uncertainties, all ICs were calibrated at the same primary standards laboratory. We used the beam quality (kQ) correction factors for the ICs under investigation from IAEA TRS-398, the newly calculated Monte Carlo (MC) values and the anticipated IAEA TRS-398 updated recommendations.Main results. Dose differences among the four ICs ranged between 1.5% and 3.7% using both the TRS-398 and the newly recommendedkQvalues. The spread among the chambers is reduced with the newlykQvalues. The largest differences were observed between the rest of the ICs and the IBA-PPC05 IC, obtaining lower dose with the IBA-PPC05.Significance. We provide experimental data comparing different types of chambers in different proton beam qualities. The observed dose differences between the ICs appear to be related to inconsistencies in the determination of thekQvalues. For PP ICs, MC studies account for the physical thickness of the entrance window rather than the WET. The additional energy loss that the wall material invokes is not negligible for the IBA-PPC05 and might partially explain the lowkQvalues determined for this IC. To resolve this inconsistency and to benchmark MC values,kQvalues measured using calorimetry are needed.

Beam monitor chamber calibration of a synchro-cyclotron high dose rate per pulse pulsed scanned proton beam

Objective. Ionization chambers, mostly used for beam calibration and for reference dosimetry, can show high recombination effects in pulsed high dose rate proton beams. The aims of this paper are: first, to characterize the linearity response of newly designed asymmetrical beam monitor chambers (ABMC) in a 100-226 MeV pulsed high dose rate per pulse scanned proton beam; and secondly, to calibrate the ABMC with a PPC05 (IBA Dosimetry) plane parallel ionization chamber and compare to calibration with a home-made Faraday cup (FC).Approach. The ABMC response linearity was evaluated with both the FC and a PTW 60019 microDiamond detector. Regarding ionometry-based ABMC calibration, recombination factors were evaluated theoretically, then numerically, and finally experimentally measured in water for a plane parallel ionization chamber PPC05 (IBA Dosimetry) throughkssaturation curves. Finally, ABMC calibration was also achieved with FC and compared to the ionometry method for 7 energies.Main results. Linearity measurements showed that recombination losses in the new ABMC design were well taken into account for the whole range of the machine dose rates. The two-voltage-method was not suitable for recombination correction, but Jaffé's plots analysis was needed, emphasizing the current IAEA TRS-398 reference protocol limitations. Concerning ABMC calibration, FC based absorbed dose estimation and PPC05-based absorbed dose estimation differ by less than 6.3% for the investigated energies.Significance.So far, no update on reference dosimetry protocols is available to estimate the absorbed dose in ionization chambers for clinical high dose rate per pulse pulsed scanned proton beams. This work proposes a validation of the new ABMC design, a method to take into account the recombination effect for ionometry-based ABMC calibration and a comparison with FC dose estimation in this type of proton beams.

Dose-volume comparisons of proton therapy for pencil beam scanning with and without multi-leaf collimator and passive scattering in patients with lung cancer

This study evaluated the dose distributions of proton pencil beam scanning (PBS) with/without a multileaf collimator (MLC) compared to passive scattering (PS) for stage I/II lung cancers. Collimated/uncollimated (PBS+/PBS-) and PS plans were created for 20 patients. Internal-clinical-target-volumes (ICTVs) and planning-target-volumes (PTVs) with a 5 mm margin were defined on the gated CTs. Organs-at-risk (OARs) are defined as the normal lungs, spinal cord, esophagus, and heart. The prescribed dose was 66 Gy relative-biological-effectiveness (RBE) in 10 fractions at the isocenter and 50% volume of the ICTVs for the PS and PBS, respectively. We compared the target and OAR dose statistics from the dose volume histograms. The PBS+ group had a significantly better mean PTV conformity index than the PBS- and PS groups. The mean dose sparing for PBS+ was better than those for PBS- and PS. Only the normal lung doses of PBS- were worse than those of PS. The overall performance of the OAR sparing was in the order of PBS+, PBS-, and PS. The PBS+ plan showed significantly better target homogeneity and OAR sparing than the PBS- and PS plans. PBS requires collimating systems to treat lung cancers with the most OAR sparing while maintaining the target coverage.

Requirements for dose calculation on an active scanned proton beamline for small, shallow fields

INTRODUCTION

A growing interest in using proton pencil beam scanning in combination with collimators for the treatment of small, shallow targets, such as ocular melanoma or pre-clinical research emerged recently. This study aims at demonstrating that the dose of a synchrotron-based PBS system with a dedicated small, shallow field nozzle can be accurately predicted by a commercial treatment planning system (TPS) following appropriate tuning of both, nozzle and TPS.

MATERIALS

A removable extension to the clinical nozzle was developed to modify the beam shape passively. Five circular apertures with diameters between 5 to 34mm, mounted 72cm downstream of a range shifter were used. For each collimator treatment plans with spread-out Bragg peaks (SOBP) with a modulation of 3 to 30mm were measured and calculated with GATE/Geant4 and the research TPS RayStation (RS11B-R). The dose grid, multiple coulomb scattering and block discretization resolution were varied to find the optimal balance between accuracy and performance.

RESULTS

For SOBPs deeper than 10mm, the dose in the target agreed within 1% between RS11B-R, GATE/Geant4 and measurements for aperture diameters between 8 to 34mm, but deviated up to 5% for smaller apertures. A plastic taper was introduced reducing scatter contributions to the patient (from the pipe) and improving the dose calculation accuracy of the TPS to a 5% level in the entrance region for large apertures.

CONCLUSION

The commercial TPS and GATE/Geant4 can accurately calculate the dose for shallow, small proton fields using a collimator and pencil beam scanning.

Clinical benefit of range uncertainty reduction in proton treatment planning based on dual-energy CT for neuro-oncological patients

OBJECTIVE

Several studies have shown that dual-energy CT (DECT) can lead to improved accuracy for proton range estimation. This study investigated the clinical benefit of reduced range uncertainty, enabled by DECT, in robust optimisation for neuro-oncological patients.

METHODS

DECT scans for 27 neuro-oncological patients were included. Commercial software was applied to create stopping-power ratio (SPR) maps based on the DECT scan. Two plans were robustly optimised on the SPR map, keeping the beam and plan settings identical to the clinical plan. One plan was robustly optimised and evaluated with a range uncertainty of 3% (as used clinically; denoted 3%-plan); the second plan applied a range uncertainty of 2% (2%-plan). Both plans were clinical acceptable and optimal. The dose-volume histogram parameters were compared between the two plans. Two experienced neuro-radiation oncologists determined the relevant dose difference for each organ-at-risk (OAR). Moreover, the OAR toxicity levels were assessed.

RESULTS

For 24 patients, a dose reduction >0.5/1 Gy (relevant dose difference depending on the OAR) was seen in one or more OARs for the 2%-plan; e.g. for brainstem D0.03cc in 10 patients, and hippocampus D40% in 6 patients. Furthermore, 12 patients had a reduction in toxicity level for one or two OARs, showing a clear benefit for the patient.

CONCLUSION

Robust optimisation with reduced range uncertainty allows for reduction of OAR toxicity, providing a rationale for clinical implementation. Based on these results, we have clinically introduced DECT-based proton treatment planning for neuro-oncological patients, accompanied with a reduced range uncertainty of 2%.

ADVANCES IN KNOWLEDGE

This study shows the clinical benefit of range uncertainty reduction from 3% to 2% in robustly optimised proton plans. A dose reduction to one or more OARs was seen for 89% of the patients, and 44% of the patients had an expected toxicity level decrease.

Global democratisation of proton radiotherapy

Proton radiotherapy is an advanced treatment option compared with conventional x-ray treatment, delivering much lower doses of radiation to healthy tissues surrounding the tumour. However, proton therapy is currently not widely available. In this Review, we summarise the evolution of proton therapy to date, together with the benefits to patients and society. These developments have led to an exponential growth in the number of hospitals using proton radiotherapy worldwide. However, the gap between the number of patients who should be treated with proton radiotherapy and those who have access to it remains large. We summarise the ongoing research and development that is contributing to closing this gap, including the improvement of treatment efficiency and efficacy, and advances in fixed-beam treatments that do not require an enormously large, heavy, and costly gantry. The ultimate goal of decreasing the size of proton therapy machines to fit into standard treatment rooms appears to be within reach, and we discuss future research and development opportunities to achieve this goal.

Dosimetric delivery validation of dynamically collimated pencil beam scanning proton therapy

Objective. Pencil beam scanning (PBS) proton therapy target dose conformity can be improved with energy layer-specific collimation. One such collimator is the dynamic collimation system (DCS), which consists of four nickel trimmer blades that intercept the scanning beam as it approaches the lateral extent of the target. While the dosimetric benefits of the DCS have been demonstrated through computational treatment planning studies, there has yet to be experimental verification of these benefits for composite multi-energy layer fields. The objective of this work is to dosimetrically characterize and experimentally validate the delivery of dynamically collimated proton therapy with the DCS equipped to a clinical PBS system.Approach. Optimized single field, uniform dose treatment plans for 3 × 3 × 3 cm³target volumes were generated using Monte Carlo dose calculations with depths ranging from 5 to 15 cm, trimmer-to-surface distances ranging from 5 to 18.15 cm, with and without a 4 cm thick polyethylene range shifter. Treatment plans were then delivered to a water phantom using a prototype DCS and an IBA dedicated nozzle system and measured with a Zebra multilayer ionization chamber, a MatriXX PT ionization chamber array, and Gafchromic™ EBT3 film.Main results. For measurements made within the SOBPs, average 2D gamma pass rates exceeded 98.5% for the MatriXX PT and 96.5% for film at the 2%/2 mm criterion across all measured uncollimated and collimated plans, respectively. For verification of the penumbra width reduction with collimation, film agreed with Monte Carlo with differences within 0.3 mm on average compared to 0.9 mm for the MatriXX PT.Significance. We have experimentally verified the delivery of DCS-collimated fields using a clinical PBS system and commonly available dosimeters and have also identified potential weaknesses for dosimeters subject to steep dose gradients.

Optimization of proton pencil beam positioning in collimated fields

BACKGROUND

The addition of static or dynamic collimator systems to the pencil beam scanning delivery technique increases the number of options for lateral field shaping. The collimator shape needs to be optimized together with the intensity modulation of spots.

PURPOSE

To minimize the proton field's lateral penumbra by investigating the fundamental relations between spot and collimating aperture edge position.

METHODS

Analytical approaches describing the effect of spot position on the resulting spot profile are presented. The theoretical description is then compared with Monte Carlo simulations in TOPAS and in the RayStation treatment planning system, as well as with radiochromic film measurements at a clinical proton therapy facility. In the model, one single spot profile is analyzed for various spot positions in air. Further, irradiation setups in water with different energies, the combination with a range shifter, and two-dimensional proton fields were investigated in silico.

RESULTS

The further the single spot is placed beyond the collimating aperture edge ('overscanning'), the sharper the relative lateral dose fall-off and thus the lateral penumbra. Overscanning up to 5 mm $5\,\text{mm}$ reduced the lateral penumbra by about 20% on average after a propagation of 13 cm $13\,\text{cm}$ in air. This benefit from overscanning is first predicted by the analytical proofs and later verified by simulations and measurements. Corresponding analyses in water confirm the benefit in lateral penumbra with spot position optimization as observed theoretically and in air. The combination of spot overscanning with fluence modulation facilitated an additional improvement.

CONCLUSIONS

The lateral penumbra of single spots in collimated scanned proton fields can be improved by the method of spot overscanning. This suggests a better sparing of proximal organs at risk in smaller water depths at higher energies, especially in the plateau of the depth dose distribution. All in all, spot overscanning in collimated scanned proton fields offers particular potential in combination with techniques such as fluence modulation or dynamic collimation for optimizing the lateral penumbra to spare normal tissue.

Quality assurance of scanned proton beams at different gantry angles using an ionization chamber array in a rotational phantom

PURPOSE

To implement a single set-up monthly QA procedure for 9 different beam parameters at different gantry angles and evaluate its clinical implementation over a 12 month period.

METHODS

We developed a QA procedure using an array detector (PTW Octavius 1500XDR) embedded in a rotational unit (PTW Octavius 4D) at our proton facility. With a single set-up we can monitor field central axis position, field symmetry, field size, flatness, penumbrae, output, spot size, spot position and range at different gantry angles (AAPM TG 224). The set-up is irradiated with homogenous 2D fields with dynamic aperture and spot patterns at five gantry angles. A modular top is used to check the range consistency. Absolute γ analysis were performed to compare measured dose distributions to calculated dose. All other parameters are directly extracted from the measurements. Additionally, the sensitivity of the set-up to small changes in beam parameters were compared to the Lynx detector (IBA).

RESULTS

Over a 12 month period, output, symmetry, and flatness were within ± 2 %; FWHM, spot positions, penumbra widths, and central axis fields were within ± 1 mm. Range differences were all within 1/2 of the energy spacing (±0.6 MeV) relative to baseline. Most (2 %, 2 mm) γ-analysis showed agreement scores higher than 90 %. The sensitivity is comparable to the Lynx detector and measurement time is reduced by 40 %.

CONCLUSION

The time-efficient monthly QA procedure that we developed can accurately be used to measure a large range of beam parameters at different gantry angles, within the TG 224 AAPM recommendations.

Validation of pencil beam scanning proton therapy with multi-leaf collimator calculated by a commercial Monte Carlo dose engine

This study aimed to evaluate the clinical beam commissioning results and lateral penumbra characteristics of our new pencil beam scanning (PBS) proton therapy using a multi-leaf collimator (MLC) calculated by use of a commercial Monte Carlo dose engine. Eighteen collimated uniform dose plans for cubic targets were optimized by the RayStation 9A treatment planning system (TPS), varying scan area, modulation widths, measurement depths, and collimator angles. To test the patient-specific measurements, we also created and verified five clinically realistic PBS plans with the MLC, such as the liver, prostate, base-of-skull, C-shape, and head-and-neck. The verification measurements consist of the depth dose (DD), lateral profile (LP), and absolute dose (AD). We compared the LPs and ADs between the calculation and measurements. For the cubic plans, the gamma index pass rates (γ-passing) were on average 96.5% ± 4.0% at 3%/3 mm for the DD and 95.2% ± 7.6% at 2%/2 mm for the LP. In several LP measurements less than 75 mm depths, the γ-passing deteriorated (increased the measured doses) by less than 90% with the scattering such as the MLC edge and range shifter. The deteriorated γ-passing was satisfied by more than 90% at 2%/2 mm using uncollimated beams instead of collimated beams except for three planes. The AD differences and the lateral penumbra width (80%-20% distance) were within ±1.9% and ± 1.1 mm, respectively. For the clinical plan measurements, the γ-passing of LP at 2%/2 mm and the AD differences were 97.7% ± 4.2% on average and within ±1.8%, respectively. The measurements were in good agreement with the calculations of both the cubic and clinical plans inserted in the MLC except for LPs less than 75 mm regions of some cubic and clinical plans. The calculation errors in collimated beams can be mitigated by substituting uncollimated beams.

EURADOS REM-COUNTER INTERCOMPARISON AT MAASTRO PROTON THERAPY CENTRE: COMPARISON WITH LITERATURE DATA

The Maastro Proton Therapy Centre is the first European facility housing the Mevion S250i Hyperscan synchrocyclotron. The proximity of the accelerator to the patient, the presence of an active pencil beam delivery system downstream of a passive energy degrader and the pulsed structure of the beam make the Mevion stray neutron field unique amongst proton therapy facilities. This paper reviews the results of a rem-counter intercomparison experiment promoted by the European Radiation Dosimetry Group at Maastro and compares them with those at other proton therapy facilities. The Maastro neutron H*(10) in the room (100-200 μSv/Gy at about 2 m from the isocentre) is in line with accelerators using purely passive or wobbling beam delivery modalities, even though Maastro shows a dose gradient peaked near the accelerator. Unlike synchrotron- and cyclotron-based facilities, the pulsed beam at Maastro requires the employment of rem-counters specifically designed to withstand pulsed neutron fields.

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.

Evaluation of the clinical value of automatic online dose restoration for adaptive proton therapy of head and neck cancer

INTRODUCTION

Intensity modulated proton therapy (IMPT) is highly sensitive to anatomical variations which can cause inadequate target coverage during treatment. This study compares not-adapted (NA) robust plans to two adaptive IMPT methods - a fully-offline adaptive (FOA) and a simplified automatic online adaptive strategy (dose restoration (DR)) to determine the benefit of DR, in head and neck cancer (HNC).

MATERIAL/METHODS

Robustly optimized clinical IMPT doses in planning-CTs (pCTs) were available for a cohort of 10 HNC patients. During robust re-optimization, DR used isodose contours, generated from the clinical dose on pCTs, and patient specific objectives to reproduce the clinical dose in every repeated-CT(rCT). For each rCT(n=50), NA, DR and FOA plans were robustly evaluated.

RESULTS

An improvement in DVH-metrics and robustness was seen for DR and FOA plans compared to NA plans. For NA plans, 74%(37/50) of rCTs did not fulfill the CTV coverage criteria (D98%>95%Dprescription). DR improved target coverage, target homogeneity and variability on critical risk organs such as the spinal cord. After DR, 52%(26/50) of rCTs met all clinical goals. Because of large anatomical changes and/or inaccurate patient repositioning, 48%(24/50) of rCTs still needed full offline adaptation to ensure an optimal treatment since dose restoration was not able to re-establish the initial plan quality.

CONCLUSION

Robust optimization together with fully-automatized DR avoided offline adaptation in 52% of the cases. Implementation of dose restoration in clinical routine could ensure treatment plan optimality while saving valuable human and material resources to radiotherapy departments.

Technical Note: Evaluation and 2nd check of a commercial Monte Carlo dose engine for small-field apertures in pencil beam scanning proton therapy

PURPOSE

To evaluate the accuracy of the RayStation Monte Carlo dose engine (RayStation MC) in modeling small-field block apertures in proton pencil beam scanning. Furthermore, we evaluate the suitability of MCsquare as a 2^nd check for RayStation MC.

METHODS

We have enhanced MCsquare to model block apertures. To test the accuracy of both RayStation MC and the newly enhanced MCsquare, we compare the dose predictions of each to in-water dose measurements obtained using diode detectors and radiochromic film. Nine brass apertures with openings of 1, 2, 3, 4, and 5 cm and either 2 cm or 4 cm thickness were used in the irradiation of a water phantom. Two measurement setups were used, one with a range shifter and 119.7 MeV proton beam energy and the other with no range shifter and 147 MeV proton beam energy. To further test the validity of RayStation MC and MCsquare in modeling block apertures and to evaluate MCsquare as a 2^nd check tool, ten small-field (average target volume 8.3 cm³ ) patient treatment plans were calculated by each dose engine followed by a statistical comparison.

RESULTS

Comparing to the absolute dose measurements in water, RayStation MC differed by 1.2% ± 1.0% while MCsquare differed by -1.8% ± 3.7% in the plateau region of a pristine Bragg peak. Compared to the in-water film measurements, RayStation MC and MCsquare both performed well with an average 2D-3D gamma passing rate of 99.4% and 99.7% (3%/3mm) respectively. A t-test comparing the agreement with the film measurements between RayStation MC and MCsquare suggested that the relative spatial dose distributions calculated by MCsquare and RayStation MC were statistically indistinguishable. Directly comparing the dose calculations between MCsquare and RayStation MC over ten patients resulted in an average 3D-3D gamma passing rates of 98.5% (3%/3mm) and 94.1% (2%/2mm) respectively.

CONCLUSION

The validity of RayStation MC algorithm for use with patient-specific apertures has been expanded to include small apertures. MCsquare has been enhanced to model apertures and was found to be an adequate 2^nd check of RayStation MC in this scenario. This article is protected by copyright. All rights reserved.

Joint EURADOS WG9-WG11 rem-counter intercomparison in a Mevion S250i proton therapy facility with Hyperscan pulsed synchrocyclotron

Objective Proton therapy is gaining popularity because of the improved dose delivery over conventional radiation therapy. The secondary dose to healthy tissues is dominated by secondary neutrons. Commercial rem-counters are valuable instruments for the on-line assessment of neutron ambient dose equivalent (H*(10)). In general, however, a priori knowledge of the type of facility and of the radiation field is required for the proper choice of any survey meter. The novel Mevion S250i Hyperscan synchrocyclotron mounts the accelerator directly on the gantry. It provides a scanned 227 MeV proton beam, delivered in pulses with a pulse width of 10 µs at 750 Hz frequency, which is afterwards degraded in energy by a range shifter modulator system. This environment is particularly challenging for commercial rem-counters; therefore, we tested the reliability of some of the most widespread rem-counters to understand their limits in the Mevion S250i stray neutron field. Approach This work, promoted by the European Radiation Dosimetry Group (EURADOS), describes a rem-counter intercomparison at the Maastro Proton Therapy centre in the Netherlands, which houses the novel Mevion S250i Hyperscan system. Several rem-counters were employed in the intercomparison (LUPIN, LINUS, WENDI-II, LB6411, NM2B-458, NM2B-495Pb), which included simulation of a patient treatment protocol employing a water tank phantom. The outcomes of the experiment were compared with models and data from the literature. Main results We found that only the LUPIN allowed for a correct assessment of H*(10) within a 20% uncertainty. All other rem-counters underestimated the reference H*(10) by factors from 2 to more than 10, depending on the detector model and on the neutron dose per pulse. In pulsed fields, the neutron dose per pulse is a fundamental parameter, while the average neutron dose rate is a secondary quantity. An average 150-200 µSv/GyRBE neutron H*(10) at various positions around the phantom and at distances between 186 cm and 300 cm from it was measured per unit therapeutic dose delivered to the target. Significance Our results are partially in line with results obtained at similar Mevion facilities employing passive energy modulation. Comparisons with facilities employing active energy modulation confirmed that the neutron H*(10) can increase up to more than a factor of 10 when passive energy modulation is employed. The challenging environment of the Mevion stray neutron field requires the use of specific rem-counters sensitive to high-energy neutrons (up to a few hundred MeV) and specifically designed to withstand pulsed neutron fields.

Implementation of MR-linac and proton therapy in two radiotherapy departments in The Netherlands: Recommendations based on lessons learned

Recently, two new treatment techniques, i.e. proton therapy and MR-linac based radiotherapy (RT), have been introduced in Dutch RT centres with major impact on daily practice. The content and context of these techniques are frequently described in scientific literature while little is reported about the implementation phase. This process is complex due to a variety of aspects, such as the involvement of multiple stakeholders, significant unpredictability in the start-up phase, the impact of the learning curve, standard operating procedures under development, new catchment areas, and extensive training programs. Insight about implementation in daily care is utterly important for clinics that are about to introduce these new technologies in order to prevent that every centre needs to reinvent the wheel. This position paper gives an overview of the implementation of proton therapy and MR-linac based RT in two large academic RT centres in the Netherlands, i.e. Maastro and Radboudumc respectively. With this paper we aim to report our lessons learned, in order to facilitate other RT centres that consider introducing these and other new techniques in their departments.

Future Developments in Charged Particle Therapy: Improving Beam Delivery for Efficiency and Efficacy

The physical and clinical benefits of charged particle therapy (CPT) are well recognized. However, the availability of CPT and complete exploitation of dosimetric advantages are still limited by high facility costs and technological challenges. There are extensive ongoing efforts to improve upon these, which will lead to greater accessibility, superior delivery, and therefore better treatment outcomes. Yet, the issue of cost remains a primary hurdle as utility of CPT is largely driven by the affordability, complexity and performance of current technology. Modern delivery techniques are necessary but limited by extended treatment times. Several of these aspects can be addressed by developments in the beam delivery system (BDS) which determines the overall shaping and timing capabilities enabling high quality treatments. The energy layer switching time (ELST) is a limiting constraint of the BDS and a determinant of the beam delivery time (BDT), along with the accelerator and other factors. This review evaluates the delivery process in detail, presenting the limitations and developments for the BDS and related accelerator technology, toward decreasing the BDT. As extended BDT impacts motion and has dosimetric implications for treatment, we discuss avenues to minimize the ELST and overview the clinical benefits and feasibility of a large energy acceptance BDS. These developments support the possibility of advanced modalities and faster delivery for a greater range of treatment indications which could also further reduce costs. Further work to realize methodologies such as volumetric rescanning, FLASH, arc, multi-ion and online image guided therapies are discussed. In this review we examine how increased treatment efficiency and efficacy could be achieved with improvements in beam delivery and how this could lead to faster and higher quality treatments for the future of CPT.

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.

Treatment planning and 4D robust evaluation strategy for proton therapy of lung tumors with large motion amplitude

Purpose

Intensity‐modulated proton therapy (IMPT) for lung tumors with a large tumor movement is challenging due to loss of robustness in the target coverage. Often an upper cut‐off at 5‐mm tumor movement is used for proton patient selection. In this study, we propose (1) a robust and easily implementable treatment planning strategy for lung tumors with a movement larger than 5 mm, and (2) a four‐dimensional computed tomography (4DCT) robust evaluation strategy for evaluating the dose distribution on the breathing phases.

Materials and methods

We created a treatment planning strategy based on the internal target volume (ITV) concept (aim 1). The ITV was created as a union of the clinical target volumes (CTVs) on the eight 4DCT phases. The ITV expanded by 2 mm was the target during robust optimization on the average CT (avgCT). The clinical plan acceptability was judged based on a robust evaluation, computing the voxel‐wise min and max (VWmin/max) doses over 28 error scenarios (range and setup errors) on the avgCT. The plans were created in RayStation (RaySearch Laboratories, Stockholm, Sweden) using a Monte Carlo dose engine, commissioned for our Mevion S250i Hyperscan system (Mevion Medical Systems, Littleton, MA, USA). We developed a new 4D robust evaluation approach (4DRobAvg; aim 2). The 28 scenario doses were computed on each individual 4DCT phase. For each scenario, the dose distributions on the individual phases were deformed to the reference phase and combined to a weighted sum, resulting in 28 weighted sum scenario dose distributions. From these 28 scenario doses, VWmin/max doses were computed. This new 4D robust evaluation was compared to two simpler 4D evaluation strategies: re‐computing the nominal plan on each individual 4DCT phase (4DNom) and computing the robust VWmin/max doses on each individual phase (4DRobInd). The treatment planning and dose evaluation strategies were evaluated for 16 lung cancer patients with tumor movement of 4–26 mm.

Results

The ratio of the ITV and CTV volumes increased linearly with the tumor amplitude, with an average ratio of 1.4. Despite large ITV volumes, a clinically acceptable plan fulfilling all target and organ at risk (OAR) constraints was feasible for all patients. The 4DNom and 4DRobInd evaluation strategies were found to under‐ or overestimate the dosimetric effect of the tumor movement, respectively. 4DRobInd showed target underdosage for five patients, not observed in the robust evaluation on the avgCT or in 4DRobAvg. The accuracy of dose deformation used in 4DRobAvg was quantified and found acceptable, with differences for the dose‐volume parameters below 1 Gy in most cases.

Conclusion

The proposed ITV‐based planning strategy on the avgCT was found to be a clinically feasible approach with adequate tumor coverage and no OAR overdosage even for large tumor movement. The new proposed 4D robust evaluation, 4DRobAvg, was shown to give an easily interpretable understanding of the effect of respiratory motion dose distribution, and to give an accurate estimate of the dose delivered in the different breathing phases.

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.

Innovations and the Use of Collimators in the Delivery of Pencil Beam Scanning Proton Therapy

Purpose

The development of collimating technologies has become a recent focus in pencil beam scanning (PBS) proton therapy to improve the target conformity and healthy tissue sparing through field-specific or energy-layer–specific collimation. Given the growing popularity of collimators for low-energy treatments, the purpose of this work was to summarize the recent literature that has focused on the efficacy of collimators for PBS and highlight the development of clinical and preclinical collimators.

Materials and Methods

The collimators presented in this work were organized into 3 categories: per-field apertures, multileaf collimators (MLCs), and sliding-bar collimators. For each case, the system design and planning methodologies are summarized and intercompared from their existing literature. Energy-specific collimation is still a new paradigm in PBS and the 2 specific collimators tailored toward PBS are presented including the dynamic collimation system (DCS) and the Mevion Adaptive Aperture.

Results

Collimation during PBS can improve the target conformity and associated healthy tissue and critical structure avoidance. Between energy-specific collimators and static apertures, static apertures have the poorest dose conformity owing to collimating only the largest projection of a target in the beam's eye view but still provide an improvement over uncollimated treatments. While an external collimator increases secondary neutron production, the benefit of collimating the primary beam appears to outweigh the risk. The greatest benefit has been observed for low- energy treatment sites.

Conclusion

The consensus from current literature supports the use of external collimators in PBS under certain conditions, namely low-energy treatments or where the nominal spot size is large. While many recent studies paint a supportive picture, it is also important to understand the limitations of collimation in PBS that are specific to each collimator type. The emergence and paradigm of energy-specific collimation holds many promises for PBS proton therapy.

Clinical Implementation of Proton Therapy Using Pencil-Beam Scanning Delivery Combined With Static Apertures

Proton therapy makes use of the favorable depth-dose distribution with its characteristic Bragg peak to spare normal tissue distal of the target volume. A steep dose gradient would be desired in lateral dimensions, too. The widespread spot scanning delivery technique is based, however, on pencil-beams with in-air spot full-widths-at-half-maximum of typically 1 cm or more. This hampers the sparing of organs-at-risk if small-scale structures adjacent to the target volume are concerned. The trimming of spot scanning fields with collimating apertures constitutes a simple measure to increase the transversal dose gradient. The current study describes the clinical implementation of brass apertures in conjunction with the pencil-beam scanning delivery mode at a horizontal, clinical treatment head based on commercial hardware and software components. Furthermore, clinical cases, which comprised craniopharyngiomas, re-irradiations and ocular tumors, were evaluated. The dosimetric benefits of 31 treatment plans using apertures were compared to the corresponding plans without aperture. Furthermore, an overview of the radiation protection aspects is given. Regarding the results, robust optimization considering range and setup uncertainties was combined with apertures. The treatment plan optimizations followed a single-field uniform dose or a restricted multi-field optimization approach. Robustness evaluation was expanded to account for possible deviations of the center of the pencil-beam delivery and the mechanical center of the aperture holder. Supplementary apertures improved the conformity index on average by 15.3%. The volume of the dose gradient surrounding the PTV (evaluated between 80 and 20% dose levels) was decreased on average by 17.6%. The mean dose of the hippocampi could be reduced on average by 2.9 GyRBE. In particular cases the apertures facilitated a sparing of an organ-at-risk, e.g. the eye lens or the brainstem. For six craniopharyngioma cases the inclusion of apertures led to a reduction of the mean dose of 1.5 GyRBE (13%) for the brain and 3.1 GyRBE (16%) for the hippocampi.

Characterization of penumbra sharpening and scattering by adaptive aperture for a compact pencil beam scanning proton therapy system

PURPOSE

To quantitatively access penumbra sharpening and scattering by adaptive aperture (AA) under various beam conditions and clinical cases for a Mevion S250i compact pencil beam scanning proton therapy system.

METHODS

First, in-air measurements were performed using a scintillation detector for single spot profile and lateral penumbra for five square field sizes (3 × 3 to 18 × 18 cm² ), three energies (33.04, 147.36, and 227.16 MeV), and three snout positions (5, 15, and 33.6 cm) with Open and AA field. Second, treatment plans were generated in RayStation treatment planning system (TPS) for various combination of target size (3- and 10-cm cube), target depth (5, 10, and 15 cm) and air gap (5-20 cm) for both Open and AA field. These plans were delivered to EDR2 films in the solid water and penumbra reduction by AA was quantified. Third, the effect of the AA scattered protons on the surface dose was studied at 5 mm depth by EDR2 film and the RayStation TPS computation. Finally, dosimetric advantage of AA over Open field was studied for five brain and five prostate cases using the TPS simulation.

RESULTS

The spot size changed dramatically from 3.8 mm at proton beam energy of 227.15 MeV to 29.4 mm at energy 33.04 MeV. In-air measurements showed that AA substantially reduced the lateral penumbra by 30% to 60%. The EDR2 film measurements in solid water presented the maximum penumbra reduction of 10 to 14 mm depending on the target size. The maximum increase of 25% in field edge dose at 5 mm depth as compared to central axis was observed. The substantial penumbra reduction by AA produced less dose to critical structures for all the prostate and brain cases.

CONCLUSIONS

Adaptive aperture sharpens the penumbra by factor of two to three depending upon the beam condition. The absolute penumbra reduction with AA was more noticeable for shallower target, smaller target, and larger air gap. The AA-scattered protons contributed to increase in surface dose. Clinically, AA reduced the doses to critical structures.

Considerations for shoot-through FLASH proton therapy

PURPOSE

To discuss several pertinent issues related to shoot-through FLASH proton therapy based on an illustrative case.

METHODS

We argue that with the advent of FLASH proton radiotherapy and due to the issues associated with conventional proton radiotherapy regarding the uncertainties of positioning of the Bragg peaks, the difficulties of in vivo verification of the dose distribution, the use of treatment margins and the uncertainties surrounding linear energy transfer (LET) and relative biological effectiveness (RBE), a special mode of shoot-through FLASH proton radiotherapy should be investigated. In shoot-through FLASH, the proton beams have sufficient energy to reach the distal exit side of the patient. Due to the FLASH sparing effect of normal tissues at both the proximal and distal side of tumors, radiotherapy plans can be developed that meet current planning constraints and issues regarding RBE can be avoided.

RESULTS

A preliminary proton plan for a neurological tumor in close proximity to various organs at risk (OAR) with strict dose constraints was studied. A plan with four beams mostly met the constraints for the OAR, using a treatment planning system that was not optimized for this novel treatment modality. When new treatment planning algorithms would be developed for shoot-through FLASH, constraints would be easier to meet. The shoot-through FLASH plan led to a significant effective dose reduction in large parts of the healthy tissue. The plan had no uncertainties associated to Bragg peak positioning, needed in principle no large proximal or distal margins and LET increases near the Bragg peak became irrelevant.

CONCLUSION

Shoot-through FLASH proton radiotherapy may be an interesting treatment modality to explore further. It would remove some of the current sources of uncertainty in proton radiotherapy. An additional advantage could be that portal dosimetry may be possible with beams penetrating the patient and impinging on a distally placed imaging detector, potentially leading to a practical treatment verification method. With current proton accelerator technology, trials could be conducted for neurological, head&neck and thoracic cancers. For abdominal and pelvic cancer a higher proton energy would be required.

Automating proton treatment planning with beam angle selection using Bayesian optimization

PURPOSE

To present a fully automated treatment planning process for proton therapy including beam angle selection using a novel Bayesian optimization approach and previously developed constrained hierarchical fluence optimization method.

METHODS

We adapted our in-house automated intensity modulated radiation therapy (IMRT) treatment planning system, which is based on constrained hierarchical optimization and referred to as ECHO (expedited constrained hierarchical optimization), for proton therapy. To couple this to beam angle selection, we propose using a novel Bayesian approach. By integrating ECHO with this Bayesian beam selection approach, we obtain a fully automated treatment planning framework including beam angle selection. Bayesian optimization is a global optimization technique which only needs to search a small fraction of the search space for slowly varying objective functions (i.e., smooth functions). Expedited constrained hierarchical optimization is run for some initial beam angle candidates and the resultant treatment plan for each beam configuration is rated using a clinically relevant treatment score function. Bayesian optimization iteratively predicts the treatment score for not-yet-evaluated candidates to find the best candidate to be optimized next with ECHO. We tested this technique on five head-and-neck (HN) patients with two coplanar beams. In addition, tests were performed with two noncoplanar and three coplanar beams for two patients.

RESULTS

For the two coplanar configurations, the Bayesian optimization found the optimal beam configuration after running ECHO for, at most, 4% of all potential configurations (23 iterations) for all patients (range: 2%-4%). Compared with the beam configurations chosen by the planner, the optimal configurations reduced the mandible maximum dose by 6.6 Gy and high dose to the unspecified normal tissues by 3.8 Gy, on average. For the two noncoplanar and three coplanar beam configurations, the algorithm converged after 45 iterations (examining <1% of all potential configurations).

CONCLUSIONS

A fully automated and efficient treatment planning process for proton therapy, including beam angle optimization was developed. The algorithm automatically generates high-quality plans with optimal beam angle configuration by combining Bayesian optimization and ECHO. As the Bayesian optimization is capable of handling complex nonconvex functions, the treatment score function which is used in the algorithm to evaluate the dose distribution corresponding to each beam configuration can contain any clinically relevant metric.