Proton Therapy Gantry Research Articles

Design of a large momentum acceptance proton therapy gantry utilizing AG-CCT magnets

Design of a large momentum acceptance proton therapy gantry utilizing AG-CCT magnets

Calibration assessment of the PSI proton therapy Gantry 2 scanning system after 10 years of operation

Sub-millimetre precision is a crucial criterion for beam delivery in Pencil Beam Scanning (PBS) proton therapy. Nowadays, most of the therapy systems use PBS technique where single beams with a regulated number of protons are delivered sequentially to different locations within the target. Beam energy defines the depth of the beam in the target and scanner magnets deflect the beam to the desired lateral position.The PSI Gantry 2 was among the first gantries to use the PBS technique. Available clinical proton beam energies ranging from 70 MeV to 230 MeV are achieved by de-grading a 250 MeV beam, initially provided by a super-conducting cyclotron. Beam scanning is performed just before the last bending magnet producing a nearly parallel beam at iso-centre with a well-focused beam over the full scan area of 20×12 cm2. We developed a calibration algorithm for the scanner magnets in order to achieve the desirable precision of the beam in the target. Measuring the beam position with a strip ion chamber in the gantry nozzle, we parametrized the propagation of the beam to the target calculating the beam angle at different scan positions as well as all gantry angles. During 10 years of operation we performed regular Quality Assurance (QA) tests to validate parameters obtained during the system commissioning. The QA data proved a stable system operation and no parametrization update was required so far. In this contribution, we describe the calibration pro-cess of scanning magnets with respect to beam position at the isocentre and show the stability of this implementation over the time.

From Alpha to Omicron: A Tertiary Care Hospital-Based Radiation Oncology Network's COVID-19 Response Experience

Purpose/Objective(s)The COVID-19 pandemic completely altered access to medical care, severely limiting or halting outpatient procedures over long periods of time. However, it was clear that radiation therapy needed to remain available to patients. We implemented asymptomatic testing algorithms and constructed negative air pressure CT simulator and treatment rooms within our tertiary care hospital-based Radiation Oncology network to accommodate the safe treatment of patients affected by the COVID-19 pandemic.Materials/MethodsOur Radiation Oncology department undertook dedicated construction to convert select CT simulator rooms and treatment vaults into negative pressure environments, including one CT simulator room and photon-based linear accelerator treatment vault at the main campus in 5/2020; and one CT simulator room, two photon-based linear accelerator vaults, and a single proton therapy gantry between two regional campuses in 12/2021. From 5/18/20 to 9/7/21, an asymptomatic screening protocol was implemented for all patients prior to initiation of radiation therapy. Patients with COVID-19 infection and Persons Under Investigation (PUIs) could undergo simulation and/or treatment in the newly constructed biocontainment environments at the discretion of the treating physician with guidance from a predefined priority scale. A biocontainment treatment protocol delineated specific roles within a treatment team of one Nursing Safety Officer, one Direct Contact Radiation Therapist, and one Indirect Contact Radiation Therapist. Safety drills were performed to establish and practice safe pathways for escorting patients directly from their car to the treatment room, and vice versa, for the last treatment slot(s) of the day. Patients who were treated under COVID-19 protocol from 6/2020 to 1/2022 were chart reviewed to assess utilization of our newly constructed and developed biocontainment resources.ResultsA total of 6,525 patients were treated from 6/2020 through 1/2022 across our 5 campuses. During this time, 42 patients were treated under the COVID-19 biocontainment protocol. All but 3 had documented personal COVID-19 positivity, while the others were designated as PUI, typically for COVID-positive household contacts. For 61.9% of patients, these safety measures mitigated the need for extended breaks during treatment courses that had already been initiated. The majority (64.3%) of patients were being treated with curative intent, while 10 patients (23.8%) were treated as a result of an inpatient palliative consult. A third (35.7%) were transferred from a regional campus to the main campus for management prior to expansion of biocontainment resources. The median number of biocontainment sessions required by each patient was 6 (range: 1-15).ConclusionThe construction of negative pressure environments and development of a COVID-19 biocontainment protocol have helped to mitigate the impact of the pandemic on our patients and to maximize efforts in protecting our staff.

Mechanical and Thermal Analysis of an HTS Superconducting Magnet for an Achromatic Gantry for Proton Therapy

The mechanical and thermal analysis of the superconducting magnets, forming an innovative fixed-field bending section for use in a proton therapy gantry, is presented here. The design concept has a large momentum acceptance, covering the full proton energy range of 70 to 220 MeV with fixed field in the superconducting magnets, and uses a simple magnet geometry based on double-pancake high temperature superconductor coils. The main planned steps for the assembly and fabrication of the magnets are discussed. The mechanical analyses of the magnet, with emphasis on the stress state of the conductor during the powering of the magnet, is here described. The thermal analysis investigating the adequacy of the copper structure surrounding the conductor to extract the heat deposited in the conductor during powering is also discussed in detail.

Increase of the transmission and emittance acceptance through a cyclotron-based proton therapy gantry.

PurposeIn proton therapy, the gantry, as the final part of the beamline, has a major effect on beam intensity and beam size at the isocenter. Most of the conventional beam optics of cyclotron‐based proton gantries have been designed with an imaging factor between 1 and 2 from the coupling point (CP) at the gantry entrance to the isocenter (patient location) meaning that to achieve a clinically desirable (small) beam size at isocenter, a small beam size is also required at the CP. Here we will show that such imaging factors are limiting the emittance which can be transported through the gantry. We, therefore, propose the use of large beam size and low divergence beam at the CP along with an imaging factor of 0.5 (2:1) in a new design of gantry beam optics to achieve substantial improvements in transmission and thus increase beam intensity at the isocenter.MethodsThe beam optics of our gantry have been re‐designed to transport higher emittance without the need of any mechanical modifications to the gantry beamline. The beam optics has been designed using TRANSPORT, with the resulting transmissions being calculated using Monte Carlo simulations (BDSIM code). Finally, the new beam optics have been tested with measurements performed on our Gantry 2 at PSI.ResultsWith the new beam optics, we could maximize transmission through the gantry for a fixed emittance value. Additionally, we could transport almost four times higher emittance through the gantry compared to conventional optics, whilst achieving good transmissions through the gantry (>50%) with no increased losses in the gantry. As such, the overall transmission (cyclotron to isocenter) can be increased by almost a factor of 6 for low energies. Additionally, the point‐to‐point imaging inherent to the optics allows adjustment of the beam size at the isocenter by simply changing the beam size at the CP.ConclusionWe have developed a new gantry beam optics which, by selecting a large beam size and low divergence at the gantry entrance and using an imaging factor of 0.5 (2:1), increases the emittance acceptance of the gantry, leading to a substantial increase in beam intensity at low energies. We expect that this approach could easily be adapted for most types of existing gantries.

Inverse Biot–Savart Optimization for Superconducting Accelerator Magnets

Superconducting (SC) magnets for accelerator concepts are often synthesized by numerically optimizing magnetic field waveforms, a process that requires a subsequent solution of a constrained inverse problem to identify suitable SC magnet windings. When the desired field distribution is intuitive, the inverse process is facilitated by seeding preconceived coil distributions into design optimization methods for refinement. With more complex magnetic field distributions, an initial design may be unknown, and topology optimization tools are required to synthesize current distributions without a priori guidance from a subject matter expert. In this work, we develop a constrained inverse Biot-Savart topology optimization methodology that synthesizes optimal distributions of current density in racetrack-like SC coils. The problem structure is exploited through a computationally efficient quadratic programming formulation, and the method is applied to recently published magnetic field waveforms for a recirculating proton phase shifter, a proton therapy gantry, and dipole magnets with sharp field transitions. The method and results herein identify novel winding configurations that can help magnet designers bring accelerator concepts to fruition.

Particle tracking and beam optics analysis on a toroidal gantry for proton therapy

GaToroid is a concept of toroidal gantry for hadron therapy under investigation at CERN It makes use of the toroidal magnetic field between each pair of coils to steer and focus the particle beams down to the patient. This peculiar concept requires detailed studies on particle tracking and beam optics to optimise the winding geometry and explore the properties of the system. The work presented in this manuscript is focused on the features of a GaToroid system for protons, specifically designed to minimise the footprint and weight of the gantry. Firstly, a two-dimensional single particle tracking was developed to optimise the coil geometry and the toroidal magnetic field, aiming to the maximisation of the energy acceptance of the magnet. Particles over the whole spectrum of treatment energy are directed at isocenter within 1 mm of precision. This procedure, restricted to the symmetry plane between each pair of coils, defines different beam orbits, function of the beam energy. Subsequently, a three-dimensional particle tracking was implemented to evaluate the interaction of a beam of finite dimensions with the complete magnetic field map in vacuum. The parameters of the simulated beam at the isocenter are coherent with the clinical requirements. The results of the three-dimensional tracking were then used to calculate the linear transfer matrix associated to each beam orbit. Finally, the option of performing the beam spot scanning at the isocenter by acting on the upstream steering magnet has been investigated, highlighting the potential of the concept, as well as the limitations related to the scanning field dimension and source-to-axis distance. In conclusion, the results described in this paper represent a crucial step toward the understanding of the beam optics properties of a GaToroid gantry.

A comprehensive Monte Carlo study of out-of-field secondary neutron spectra in a scanned-beam proton therapy gantry room

PurposeTo simulate secondary neutron radiation fields that had been measured at different relative positions during phantom irradiation inside a scanning proton therapy gantry treatment room. Further, to identify origin, energy distribution, and angular emission of the secondary neutrons as a function of proton beam energy. MethodsThe FLUKA Monte Carlo code was used to model the relevant parts of the treatment room in a scanned pencil beam proton therapy gantry including shielding walls, floor, major metallic gantry-components, patient table, and a homogeneous PMMA target. The proton beams were modeled based on experimental beam ranges in water and spot shapes in air. Neutron energy spectra were simulated at 0°, 45°, 90° and 135° relative to the beam axis at 2m distance from isocenter for monoenergetic 11×11cm2 fields from 200MeV, 140MeV, 75MeV initial proton beams, as well as for 118MeV protons with a 5cm thick PMMA range shifter. The total neutron spectra were scored for these four positions and proton energies. FLUKA neutron spectra simulations were crosschecked with Geant4 simulations using initial proton beam properties from FLUKA-generated phase spaces. Additionally, the room-components generating secondary neutrons in the room and their contributions to the total spectrum were identified and quantified. ResultsFLUKA and Geant4 simulated neutron spectra showed good general agreement with published measurements in the whole simulated neutron energy range of 10−10 to 103MeV. As in previous studies, high-energy (E≥19.6MeV) neutrons from the phantom are most prevalent along 0°, while thermalized (1meV≤E<0.4eV) and fast (100keV≤E<19.4MeV) neutrons dominate the spectra in the lateral and backscatter direction. The iron of the large bending magnet and its counterweight mounted on the gantry were identified as the most determinant sources of secondary fast-neutrons, which have been lacking in simplified room simulations. ConclusionsThe results helped disentangle the origin of secondary neutrons and their dominant contributions and were strengthened by the fact that a cross comparison was made using two independent Monte Carlo codes. The complexity of such room model can in future be limited using the result. They may further be generalized in that they can be used for an assessment of neutron fields, possibly even at facilities where detailed neutron measurements and simulations cannot be performed. They may also help to design future proton therapy facilities and to reduce unwanted radiation doses from secondary neutrons to patients.

A proof-of-concept study of an in-situ partial-ring time-of-flight PET scanner for proton beam verification.

Development of a PET system capable of in-situ imaging requires a design that can accommodate the proton treatment beam nozzle. Among the several PET instrumentation approaches developed thus far, the dual-panel PET scanner is often used as it is simpler to develop and integrate within the proton therapy gantry. Partial-angle coverage of these systems can however lead to limited-angle artefacts in the reconstructed PET image. We have previously demonstrated via simulations that time-of-flight (TOF) reconstruction reduces the artifacts accompanying limited-angle data, and permits proton range measurement with 1-2 mm accuracy and precision. In this work we show measured results from a small proof-of-concept dual-panel PET system that uses TOF information to reconstruct PET data acquired after proton irradiation. The PET scanner comprises of two detector modules, each comprised of an array of 4×4×30 mm3 lanthanum bromide scintillator. Measurements are performed with an oxygen-rich gel-water, an adipose tissue equivalent material, and in vitro tissue phantoms. For each phantom measurement, 2 Gy dose was deposited using 54 - 100 MeV proton beams. For each phantom, a Monte Carlo simulation generating the expected distribution of PET isotope from the corresponding proton irradiation was also performed. Proton range was calculated by drawing multiple depth-profiles over a central region encompassing the proton dose deposition. For each profile, proton range was calculated using two techniques (a) 50% pick-off from the distal edge of the profile, and (b) comparing the measured and Monte Carlo profile to minimize the absolute sum of differences over the entire profile. A 10 min PET acquisition acquired with minimal delay post proton-irradiation is compared with a 10 min PET scan acquired after a 20 min delay. Measurements show that PET acquisition with minimal delay is necessary to collect 15O signal, and maximize 11C signal collection with a short PET acquisition. In comparison with the 50% pick-off technique, the shift technique is more robust and offers better precision in measuring the proton range for the different phantoms. Range measurements from PET images acquired with minimal delay, and the shift technique demonstrate the ability to achieve <1.5 mm accuracy and precision in estimating proton range.

Magnetic Design of an Alternating Gradient Canted-Cosine-Theta Superconducting Magnet for Proton Therapy

Canted-Cosine-Theta (CCT) magnets exhibit superior field quality and structural compactness in superconducting magnet schemes. When combined with an alternating gradient (AG) field, CCT magnets can be applied to proton therapy gantries to achieve a significantly increased momentum acceptance. This paper introduces design considerations for a 67.5 degree AG-CCT magnet for a proton therapy gantry, including the design and optimization of a magnetic field combined with an AG function. The calculation of the magnetic field used to simulate the CCT coil with multiple lines based on the Biot-Savart law is discussed in detail. Technical issues covering the choice of superconducting coils and layering scheme are also discussed.

An achromatic gantry for proton therapy with fixed-field superconducting magnets

A novel concept for a superconducting, fixed-field bending section is presented for use in a proton therapy gantry. The large momentum acceptance of this design allows for treatment over the full proton energy range of 70–220 MeV with fixed field in the superconducting magnets, eliminating the technical risks associated with fast-field ramping to match beam energy changes during treatment. A combined study of beam dynamics and magnet design is shown for such a system in which a simple magnet geometry with straight Nb–Ti racetrack coils is used to produce the desired fields. Particle tracking through this design is compared with clinical requirements for beam spot shape and size at isocenter over the full range of proton energy.

Beam optics of a superconducting proton-therapy gantry with a large momentum acceptance

In proton therapy, the last part of the beam transport system is installed on a rotatable gantry, so that the beam can be aimed at the tumor from different angles. Since such a gantry system consists of many dipole and quadrupole magnets, it is typically a 100–200 tons device of more than 10 m in diameter. The use of superconducting (SC) magnets for proton therapy allows gantries to be significantly lighter and potentially smaller, which is attractive for this medical application. In addition to that, SC combined function magnets enable beam optics with a very large momentum acceptance. The latter can be advantageous for patient treatment, since the irradiation time can then be significantly reduced by avoiding magnet current changes. To design such an achromatic system, we performed precise high-order calculations. To reach the required accuracy and to check consistency of the obtained results, we have used different simulation tools in our iterative design approach. Here, we will describe our beam optics calculations in the code COSY Infinity and particle tracking using OPAL (open source software from PSI) in 3D field maps obtained from detailed magnet calculations performed in Opera. Our method has shown to be advantageous in a complicated beam optics study and it reduces the risk of systematic errors in a design.

Large energy acceptance gantry for proton therapy utilizing superconducting technology

When using superconducting (SC) magnets in a gantry for proton therapy, the gantry will benefit from some reduction in size and a large reduction in weight. In this contribution we show an important additional advantage of SC magnets in proton therapy treatments. We present the design of a gantry with a SC bending section and achromatic beam optics with a very large beam momentum acceptance of 15% (corresponding to about 30% in the energy domain). Due to the related very large energy acceptance, approximately 70% of the treatments can be performed without changing the magnetic field for synchronization with energy modulation. In our design this is combined with a 2D lateral scanning system and a fast degrader mounted in the gantry, so that this gantry will be able to perform pencil beam scanning with very rapid energy variations at the patient, allowing a significant reduction of the irradiation time.We describe the iterative process we have applied to design the magnets and the beam transport, for which we have used different codes. COSY Infinity and OPAL have been used to design the beam transport optics and to track the particles in the magnetic fields, which are produced by the magnets designed in Opera. With beam optics calculations we have derived an optimal achromatic beam transport with the large momentum acceptance of the proton pencil beam and we show the agreement with particle tracking calculations in the 3D magnetic field map.A new cyclotron based facility with this gantry will have a significantly smaller footprint, since one can refrain from the standard degrader and energy selection system behind the cyclotron. In the treatments, this gantry will enable a very fast proton beam delivery sequence, which may be of advantage for treatments in moving tissue.

Influence of fringing field and second-order aberrations on proton therapy gantry beamline

Abstract Huazhong University of Science and Technology has been developing a proton therapy facility based on a superconducting cyclotron. As an important section of the facility, the gantry beamline has been designed using TRANSPORT code with simplified fringing field model and linear transfer matrix. However, realistic magnetic fringing field and second-order terms of particle motion cannot be precisely described in this approach, which may deviate the actual result from the design value and thereby increase uncertainties for radiation therapy during beam delivery. For this consideration, the specific effects of fringing field and second-order aberrations have been investigated using OPERA code and ODE solvers. For the potential problems of the designed gantry beamline, COSY INFINITY code, which can calculate transfer matrix of any order under the realistic fringing field represented by Enge function, was used to optimize the optics of present gantry beamline. The final results suggest the beam analysis and optimization methods mentioned in this paper are effective and valuable for relevant applications.

A predictive algorithm for spot position corrections after fast energy switching in proton pencil beam scanning

Fast energy switching is of fundamental importance to implement motion mitigation techniques in pencil beam scanning proton therapy, allowing efficient irradiation and high patient throughput. However, depending on magnet design, when switching between different energy layers, eddy currents arise in the bending magnets' yoke, damping the speed of the magnetic field change and lengthening the settling time of the magnetic field. In a proton therapy gantry, this can cause a temporary displacement of the beam trajectory and consequently an incorrect beam position in the bending direction, resulting in an unacceptable loss of position precision at isocenter. The precision can be recovered by either increasing the beam off time after an energy change (waiting until the magnetic field is fully settled) or by actively correcting for the misplacement. We studied the transient magnetic field effects at PSI Gantry 2 in order to develop a correction strategy for this beam position misplacement. We used position and proton range sensitive detectors (segmented strip chamber and multilayer ionization chambers respectively) to measure the difference between expected and actual proton beam position and range as a function of time. The detectors are automatically triggered, read out, and analyzed by the treatment control system. We studied the effects due to the magnets on the gantry and those upstream of the gantry separately, in order to identify which elements contribute the most to the beam position instability. We then designed a spot position algorithm to be applied with the gantry scanning magnets, to correct for the displacement observed as a function of time and achieve the PSI Gantry 2 clinical target of 1mm precision at isocenter at all times, even after an energy change. When switching energy layers in a field, we observed an exponentially decaying spot position displacement at isocenter. The effect increases with increasing energy difference between energy layers (ΔE). The initial residuals between expected and measured position are higher than 1mm for most of the clinical cases at Gantry 2 and fall below 1mm within about 1s or more (depending on ΔE). We found no time dependence for the proton range, thus confirming that the displacement is purely due to a beam trajectory displacement resulting from the longer settling time of the magnetic field. A double exponential model, with two time constants and amplitudes depending on ΔE, fits the data and provides an easy model for the correction function. We implemented this correction as a spot position correction, applied by the scanning magnets during field application. After correction, the residuals were below 0.5mm right after the energy change. We developed a spot position correction for PSI Gantry 2 which reduces the beam off time needed in current state-of-the-art gantries to settle the magnetic fields in the bending magnets. Thanks to this correction, the spot position is stable within 100ms of an energy change at Gantry 2. This is low enough to make possible efficient use of motion mitigation techniques.

P132] Robust proton CSI planning in proteusone

Purpose This work studies the craniospinal irradiation (CSI) with pencil beam scanning proton therapy in an IBA Proteus® ONE gantry. Robust optimization can be used to create gradient-shaped field-junctions if robustness against setup uncertainties is considered in all dimensions. However, the number of scenarios to compute increases exponentially with the number of beams. In the case of Proteus® ONE, where the maximum field size is 20 × 24 cm2; at least four beams are typically needed to cover the whole craniospinal axis. This results in too many scenarios to be computed (7203 if 5 beams are used) and the optimization is not viable. In this work we investigated two possible solutions: (i) we limited the robust optimization to setup uncertainties in the craniocaudal direction only and (ii) we implemented the ancillary beam technique (ABT) proposed by Farace et al. (Acta Oncol 2015:54:1075–8). Methods Plans were generated with RayStation 6 on computed tomography images of two patients in supine position. Two lateral beams were used for brain irradiation, whereas three posterior beams were used to cover the spine. Approach 1 was optimized on the clinical target volume (CTV), considering robustness against setup uncertainties in the craniocaudal direction only (3 mm) and range uncertainties (3%). These settings reduced the number of scenarios to be computed to 729. For Approach 2, a planning target volume (PTV) was created as a 3-mm isotropic expansion of the CTV, and the ABT was implemented. The robustness of all plans was evaluated by simulating setup and range errors. Results Both approaches yielded an optimal nominal plan. For approach 1 a dose gradient was obtained through the junction, but the dose distribution for each individual beam was highly inhomogeneous in the anterior-posterior and left–right directions. Therefore, the plan was not robust to setup uncertainties in those directions. On the other hand, the ancillary beam method, together with the PTV, offered a homogeneous and robust dose distribution throughout the whole craniospinal axis. Conclusions The ancillary beam technique offered a superior plan quality and it is therefore the recommended solution to plan CSI irradiations in a Proteus® ONE proton therapy gantry.

An isocenter estimation tool for proton gantry alignment.

A novel tool has been developed to automate the process of locating the isocenter, center of rotation, and sphere of confusion of a proton therapy gantry. The tool uses a Radian laser tracker to estimate how the coordinate frame of the front-end beam-line components changes as the gantry rotates. The coordinate frames serve as an empirical model of gantry flexing. Using this model, the alignment of the front and back-end beam-line components can be chosen to minimize the sphere of confusion, improving the overall beam positioning accuracy of the gantry. This alignment can be performed without the beam active, improving the efficiency of installing new systems at customer sites.

Simulations and measurements of proton beam energy spectrum after energy degradation

At the proton therapy facility PROSCAN of the Paul Scherrer Institute the energy modulation of the cyclotron generated proton beam is performed via material insertion into the beam trajectory. The energy spectrum of the particles propagating forwards after such procedure has been simulated and measured. The current paper summarizes the results of these simulations and measurements and illustrates their significance for the future developments of a gantry for proton therapy at the Paul Scherrer Institute.

Neutron doses due to beam losses in a novel concept of a proton therapy gantry

A novel design of a gantry for proton therapy is investigated in which a degrader and emittance limiting collimators are mounted on the gantry. Due to the interactions of protons in these components there will be an additional neutron dose at the location where a patient is positioned during a proton therapy. The results of numerical study of this additional dose are presented. Neutron prompt dose at the patient position is estimated through the Monte Carlo simulation using the MCNPX 2.7.0 particle transport code. Secondary neutron and photon fluxes from the distinct beam loss points are taken into consideration and the resulting dose is calculated using realistic estimates of beam losses. The dependence of the dose on the beam energy and individual impacts of each loss point on the total dose at the patient position as well as on critical beam line components are estimated and potential design constraints are discussed. It has been found that compared with a conventional gantry the expected additional dose is higher but the optimization of the beam line configuration and additional shielding shall help to reduce the dose to an acceptable value.

Design of an Achromatic Superconducting Magnet for a Proton Therapy Gantry

Recent studies have shown that strong, alternating focusing magnets can be used to greatly increase the momentum acceptance of hadron therapy gantries. With the high gradients achievable with superconducting magnets a level of momentum acceptance can be reached which may have significant implications to medical gantries and to the introduction of superconducting technology in this area. The design of such a superconducting magnet system for a proton therapy gantry will be presented. The Canted-Cosine-Theta concept is extended to a curved magnet system generating the desired bending and alternating focusing fields for the achromatic optics. Magnetic, structural, and thermal analysis of this design is presented along with preliminary efforts towards fabrication and assembly of the curved magnet.