MR-guided proton therapy: Impact of magnetic fields on the detector response

Online MR-guided proton and ion beam radiotherapy: investigation of image quality

Objective.Magnetic resonance (MR) images free of artefacts are of pivotal importance for MR-guided ion radiotherapy. This study investigates MR image quality for simultaneous irradiation in an experimental setup using phantom imaging as well asin-vivoimaging. Observed artefacts are described within the study and their cause is investigated with the goal to find conclusions and solutions for potential future hybrid devices.Approach.An open MR scanner with a field strength of 0.25 T has been installed in front of an ion beamline. Simultaneous magnetic resonance imaging and irradiation using raster scanning were performed to analyze image quality in dedicated phantoms. Magnetic field measurements were performed to assist the explanation of observed artifacts. In addition,in-vivoimages were acquired by operating the magnets for beam scanning without transporting a beam.Main Results.The additional frequency component within the isocenter caused by the fringe field of the horizontal beam scanning magnet correlates with the amplitude and frequency of the scanning magnet steering and can cause ghosting artifacts in the images. These are amplified with high currents and fast operating of the scanning magnet. Applying a real-time capable pulse sequencein-vivorevealed no ghosting artifacts despite a continuously changing current pattern and a clinical treatment plan activation scheme, suggesting that the use of fast imaging is beneficial for the aim of creating high quality in-beam MR images. This result suggests, that the influence of the scanning magnets on the MR acquisition might be of negligible importance and does not need further measures like extensive magnetic shielding of the scanning magnets.Significance.Our study delimited artefacts observed in MR images acquired during simultaneous raster scanning ion beam irradiation. The application of a fast pulse sequence showed no image artefacts and holds the potential that online MR imaging in future hybrid devices might be feasible.

Fano cavity test and investigation of the response of the Roos chamber irradiated by proton beams in perpendicular magnetic fields up to 1 T

Objective. The aim of this work is to investigate the response of the Roos chamber (type 34001) irradiated by clinical proton beams in magnetic fields.Approach. At first, a Fano test was implemented in Monte Carlo software package GATE version 9.2 (based on Geant4 version 11.0.2) using a cylindrical slab geometry in a magnetic field up to 1 T. In accordance to an experimental setup (Fuchset al2021), the magnetic field correction factorskQB⃗of the Roos chamber were determined at different energies up to 252 MeV and magnetic field strengths up to 1 T, by separately simulating the ratios of chamber signalsMQ/MQB⃗,without and with magnetic field, and the dose-conversion factorsDw,QB⃗/Dw,Qin a small cylinder of water, with and without magnetic field. Additionally, detailed simulations were carried out to understand the observed magnetic field dependence.Main results. The Fano test was passed with deviations smaller than 0.25% between 0 and 1 T. The ratios of the chamber signals show both energy and magnetic field dependence. The maximum deviation of the dose-conversion factors from unity of 0.22% was observed at the lowest investigated proton energy of 97.4 MeV andB⃗= 1 T. The resultingkQB⃗factors increase initially with the applied magnetic field and decrease again after reaching a maximum at around 0.5 T; except for the lowest 97.4 MeV beam that show no observable magnetic field dependence. The deviation from unity of the factors is also larger for higher proton energies, where the maximum lies at 1.0035(5), 1.0054(7) and 1.0069(7) for initial energies ofE0= 152, 223.4 and 252 MeV, respectively.Significance. Detailed Monte Carlo studies showed that the observed effect can be mainly attributed to the differences in the transport of electrons produced both outside and inside of the air cavity in the presence of a magnetic field.

Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane-parallel ionization chamber

BACKGROUND

The combination of magnetic resonance imaging and proton therapy offers the potential to improve cancer treatment. The magnetic field (MF)-dependent change in the dosage of ionization chambers in magnetic resonance imaging-integrated proton therapy (MRiPT) is considered by the correction factork B ⃗ , M , Q $k_{\vec{B},M,Q}$ , which needs to be determined experimentally or computed via Monte Carlo (MC) simulations.

PURPOSE

In this study,k B ⃗ , M , Q $k_{\vec{B},M,Q}$ was both measured and simulated with high accuracy for a plane-parallel ionization chamber at different clinical relevant proton energies and MF strengths.

MATERIAL AND METHODS

The dose-response of the Advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10 × $\times$ 10 cm2 $^2$ quasi mono-energetic fields, using 103.3, 128.4, 153.1, 223.1, and 252.7 MeV proton beams was measured in a water phantom placed in the MF of an electromagnet with MF strengths of 0.32, 0.5, and 1 T. The detector was positioned at a depth of 2 g/cm2 $^2$ in water, with chamber electrodes parallel to the MF lines and perpendicular to the proton beam incidence direction. The measurements were compared with TOPAS MC simulations utilizing COMSOL-calculated 0.32, 0.5, and 1 T MF maps of the electromagnet.k B ⃗ , M , Q $k_{\vec{B},M,Q}$ was calculated for the measurements for all energies and MF strengths based on the equation:k B ⃗ , M , Q = M Q M Q B ⃗ $k_{\vec{B},M,Q}=\frac{M_\mathrm{Q}}{M_\mathrm{Q}^{\vec{B}}}$ , whereM Q B ⃗ $M_\mathrm{Q}^{\vec{B}}$ andM Q $M_\mathrm{Q}$ were the temperature and air-pressure corrected detector readings with and without the MF, respectively. MC-based correction factors were determined ask B ⃗ , M , Q = D det D det B ⃗ $k_{\vec{B},M,Q}=\frac{D_\mathrm{det}}{D_\mathrm{det}^{\vec{B}}}$ , whereD det B ⃗ $D_\mathrm{det}^{\vec{B}}$ andD det $D_\mathrm{det}$ were the doses deposited in the air cavity of the ionization chamber model with and without the MF, respectively. Furthermore, MF effects on the chamber dosimetry are studied using MC simulations, examining the impact on the absorbed dose-to-water (D W $D_{W}$ ) and the shift in depth of the Bragg peak.

RESULTS

The detector showed a reduced dose-response for all measured energies and MF strengths, resulting in experimentally determinedk B ⃗ , M , Q $k_{\vec{B},M,Q}$ values larger than unity. For all energies and MF strengths examined,k B ⃗ , M , Q $k_{\vec{B},M,Q}$ ranged between 1.0065 and 1.0205. The dependence on the energy and the MF strength was found to be non-linear with a maximum at 1 T and 252.7 MeV. The MC simulatedk B ⃗ , M , Q $k_{\vec{B},M,Q}$ values agreed with the experimentally determined correction factors within their standard deviations with a maximum difference of 0.6%. The MC calculated impact onD W $D_{W}$ was smaller 0.2 %.

CONCLUSION

For the first time, measurements and simulations were compared for proton dosimetry within MFs using an Advanced Markus chamber. Good agreement ofk B ⃗ , M , Q $k_{\vec{B},M,Q}$ was found between experimentally determined and MC calculated values. The performed benchmarking of the MC code allows for calculatingk B ⃗ , M , Q $k_{\vec{B},M,Q}$ for various ionization chamber models, MF strengths and proton energies to generate the data needed for a proton dosimetry protocol within MFs and is, therefore, a step towards MRiPT.

A review of the clinical introduction of 4D particle therapy research concepts

Background and purpose

Many 4D particle therapy research concepts have been recently translated into clinics, however, remaining substantial differences depend on the indication and institute-related aspects. This work aims to summarise current state-of-the-art 4D particle therapy technology and outline a roadmap for future research and developments.

Material and methods

This review focused on the clinical implementation of 4D approaches for imaging, treatment planning, delivery and evaluation based on the 2021 and 2022 4D Treatment Workshops for Particle Therapy as well as a review of the most recent surveys, guidelines and scientific papers dedicated to this topic.

Results

Available technological capabilities for motion surveillance and compensation determined the course of each 4D particle treatment. 4D motion management, delivery techniques and strategies including imaging were diverse and depended on many factors. These included aspects of motion amplitude, tumour location, as well as accelerator technology driving the necessity of centre-specific dosimetric validation. Novel methodologies for X-ray based image processing and MRI for real-time tumour tracking and motion management were shown to have a large potential for online and offline adaptation schemes compensating for potential anatomical changes over the treatment course. The latest research developments were dominated by particle imaging, artificial intelligence methods and FLASH adding another level of complexity but also opportunities in the context of 4D treatments.

Conclusion

This review showed that the rapid technological advances in radiation oncology together with the available intrafractional motion management and adaptive strategies paved the way towards clinical implementation.

Technical note: Experimental dosimetric characterization of proton pencil beam distortion in a perpendicular magnetic field of an in-beam MR scanner

BACKGROUND

As it promises more precise and conformal radiation treatments, magnetic resonance imaging-integrated proton therapy (MRiPT) is seen as a next step in image guidance for proton therapy. The Lorentz force, which affects the course of the proton pencil beams, presents a problem for beam delivery in the presence of a magnetic field.

PURPOSE

To investigate the influence of the 0.32-T perpendicular magnetic field of an MR scanner on the delivery of proton pencil beams inside an MRiPT prototype system.

METHODS

An MRiPT prototype comprising of a horizontal pencil beam scanning beam line and an open 0.32-T MR scanner was used to evaluate the impact of the vertical magnetic field on proton beam deflection and dose spot pattern deformation. Three different proton energies (100, 150, and 220 MeV) and two spot map sizes (15 × 15 and 30 × 20 cm2 ) at four locations along the beam path without and with magnetic field were measured. Pencil-beam dose spots were measured using EBT3 films and a 2D scintillation detector. To study the magnetic field effects, a 2D Gaussian fit was applied to each individual dose spot to determine the central position( X , Y ) $(X,Y)$ , minimum and maximum lateral standard deviation (σ m i n $\sigma _{min}$ andσ m a x $\sigma _{max}$ ), orientation (θ), and the eccentricity (ε).

RESULTS

The dose spots were subjected to three simultaneous effects: (a) lateral horizontal beam deflection, (b) asymmetric trapezoidal deformation of the dose spot pattern, and (c) deformation and rotation of individual dose spots. The strongest effects were observed at a proton energy of 100 MeV with a horizontal beam deflection of 14-186 mm along the beam path. Within the central imaging field of the MR scanner, the maximum relative dose spot sizeσ m a x $\sigma _{max}$ decreased by up to 3.66%, whileσ m i n $\sigma _{min}$ increased by a maximum of 2.15%. The largest decrease and increase in the eccentricity of the dose spots were 0.08 and 0.02, respectively. The spot orientation θ was rotated by a maximum of 5.39°. At the higher proton energies, the same effects were still seen, although to a lesser degree.

CONCLUSIONS

The effect of an MRiPT prototype's magnetic field on the proton beam path, dose spot pattern, and dose spot form has been measured for the first time. The findings show that the impact of the MF must be appropriately recognized in a future MRiPT treatment planning system. The results emphasize the need for additional research (e.g., effect of magnetic field on proton beams with range shifters and impact of MR imaging sequences) before MRiPT applications can be employed to treat patients.

MR-guided ion therapy: Detector response in magnetic fields during carbon ion irradiation

BACKGROUND

Combining carbon ion therapy with on-bed MR imaging has the potential to bring particle therapy to a new level of precision. However, the introduction of magnetic fields brings challenges for dosimetry and quality assurance. For protons, a small, but significant change in detector response was shown in the presence of magnetic fields previously. For carbon ion beams, so far no such experiments have been performed.

PURPOSE

To investigate the influence of external magnetic fields on the response of air-filled ionization chambers.

METHODS

Four commercially available ionization chambers, three thimble type (Farmer, Semiflex, and PinPoint), and a plane parallel (Bragg peak) detector were investigated. Detectors were aligned in water such that their effective point of measurement was located at 2 cm depth. Irradiations were performed using10 × 10 cm 2 $10\times 10\nobreakspace \mathrm{cm}^2$ square fields for carbon ions of 186.1, 272.5, and 402.8 MeV/u employing magnetic field strengths of 0, 0.25, 0.5, and 1 T. In addition, the detector response for protons and carbon ions was compared taking into account the secondary electron spectra and employing protons of 252.7 MeV for comparison.

RESULTS

For all four detectors, a statistically significant change in detector response, dependent on the magnetic field strength, was found. The effect was more pronounced for higher energies. The highest effects were found at 0.5 T for the PinPoint detector with a change in detector response of 1.1%. The response of different detector types appeared to be related to the cavity diameter. For proton and carbon ion irradiation with similar secondary electron spectra, the change in detector response was larger for carbon ions compared to protons.

CONCLUSION

A small, but significant dependence of the detector response was found for carbon ion irradiation in a magnetic field. The effect was found to be larger for smaller cavity diameters and at medium magnetic field strengths. Changes in detector response were more pronounced for carbon ions compared to protons.

Monte Carlo simulation for proton dosimetry in magnetic fields: Fano test and magnetic field correction factorskBfor Farmer-type ionization chambers

Objective. In this contribution we present a special Fano test for charged particles in presence of magnetic fields in the MC code TOol for PArticle Simulation (TOPAS), as well as the determination of magnetic field correction factorskBfor Farmer-type ionization chambers using proton beams.Approach. Customized C++ extensions for TOPAS were implemented to model the special Fano tests in presence of magnetic fields for electrons and protons. The Geant4-specific transport parameters,DRoverRandfinalRange,were investigated to optimize passing rate and computation time. ThekBwas determined for the Farmer-type PTW 30013 ionization chamber, and 5 custom built ionization chambers with same geometry but varying inner radius, testing magnetic flux density ranging from 0 to 1.0 T and two proton beam energies of 157.43 and 221.05 MeV.Main results. Using the investigated parameters, TOPAS passed the Fano test within 0.39 ± 0.15% and 0.82 ± 0.42%, respectively for electrons and protons. The chamber response (kB,M,Q) gives a maximum at different magnetic flux densities depending of the chamber size, 1.0043 at 1.0 T for the smallest chamber and 1.0051 at 0.2 T for the largest chamber. The local dose differencecBremained ≤ 0.1% for both tested energies. The magnetic field correction factorkB, for the chamber PTW 30013, varied from 0.9946 to 1.0036 for both tested energies.Significance. The developed extension for the special Fano test in TOPAS MC code with the adjusted transport parameters, can accurately transport electron and proton particles in magnetic field. This makes TOPAS a valuable tool for the determination ofkB. The ionization chambers we tested showed thatkBremains small (≤0.72%). To the best of our knowledge, this is the first calculations ofkBfor proton beams. This work represents a significant step forward in the development of MRgPT and protocols for proton dosimetry in presence of magnetic field.

Commissioning a beam line for MR-guided particle therapy assisted by in silico methods

BACKGROUND

Radiation therapy is continuously moving towards more precise dose delivery. The combination of online MR imaging and particle therapy, for example, radiation therapy using protons or carbon ions, could enable the next level of precision in radiotherapy. In particle therapy, research towards a combination of MR and particle therapy is well underway, but still far from clinical systems. The combination of high magnetic fields with particle therapy delivery poses several challenges for treatment planning, treatment workflow, dose delivery, and dosimetry.

PURPOSE

To present a workflow for commissioning of a light ion beam line with an integrated dipole magnet to perform MR-guided particle therapy (MRgPT) research, producing not only basic beam data but also magnetic field maps for accurate dose calculation. Accurate dose calculation in magnetic field environments requires high-quality magnetic field maps to compensate for magnetic-field-dependent trajectory changes and dose perturbations.

METHODS

The research beam line at MedAustron was coupled with a resistive dipole magnet positioned at the isocenter. Beam data were measured for proton and carbon ions with and without an applied magnetic field of 1 T. Laterally integrated depth-dose curves (IDC) as well as beam profiles were measured in water while beam trajectories were measured in air. Based on manufacturer data, an in silico model of the magnet was created, allowing to extract high-quality 3D magnetic field data. An existing GATE/Geant4 Monte Carlo (MC) model of the beam line was extended with the generated magnetic field data and benchmarked against experimental data.

RESULTS

A 3D magnetic field volume covering fringe fields until 50 mT was found to be sufficient for an accurate beam trajectory modeling. The effect on particle range retraction was found to be 2.3 and 0.3 mm for protons and carbon ions, respectively. Measured lateral beam offsets in water agreed within 0.4 and -0.5 mm with MC simulations for protons and carbon ions, respectively. Experimentally determined in-air beam trajectories agreed within 0.4 mm in the homogeneous magnetic field area.

CONCLUSION

The presented approach based on in silico modeling and measurements allows to commission a beam line for MRgPT while providing benchmarking data for the magnetic field modeling, required for state-of-the art dose calculation methods.

An external perpendicular magnetic field does not influence survival and DNA damage after proton and carbon ion irradiation in human cancer cells

BACKGROUND AND PURPOSE

Magnetic field effects on the radiobiological effectiveness during treatment of magnetic resonance (MRI) guided particle therapy are being debated. This study aims at assessing the influence of a perpendicular magnetic field on the biological effects in two human cancer cell lines irradiated with proton or carbon ions.

METHODS AND MATERIALS

In vitro cell irradiations were performed in water inside a perpendicular magnetic field of 0 and 1T for both protons and carbon ions. Samples were located in the center of a spread-out Bragg peak at 8cm water equivalent depth with a dose averaged linear energy transfer (LETd) of 4.2 or 83.4keV/μm for protons and carbon ions, respectively. Physical dose levels of 0, 0.5, 1, 2, 4 and 6Gy were employed. The irradiation field was shifted and laterally enlarged, to compensate for the beam deflection due to the magnetic field and ensure consistent and homogenous irradiations of the flasks. The human cancer cell lines SKMel (Melanoma) and SW1353 (chondrosarcoma) were selected which represent a high and a low (α/β)x ratio cell type. Cell survival curves were generated applying a linear-quadratic curve fit. DNA damage and DNA damage clearance were assessed via γH2AX foci quantification at 1 and 24h post radiation treatment.

RESULTS

Without a magnetic field, RBE10 values of 1.04±0.03 (SW1353) and 1.51±0.06 (SKMel) as well as RBE80 values of 0.93±0.15 (SW1353) and 2.28±0.40 (SKMel) were calculated for protons. Carbon treatments yielded RBE10 values of 1.68±0.04 (SW1353) and 2.30±0.07 (SKMel) and RBE80 values of 2.19±0.24 (SW1353) and 4.06±0.33 (SKMel). For a field strength of B=1T, RBE10 values of 1.06±0.03 (SW1353) and 1.47±0.06 (SKMel) resulted from protons, while RBE10 values of 1.70±0.05 (SW1353) and 2.37±0.08 (SKMel) were obtained for carbon ions. RBE80 values were calculated to be 1.06±0.12 (SW1353) and 2.33±0.40 (SKMel) following protons and 2.13±0.25 (SW1353) and 4.29±0.35 (SKMel) following carbon treatments. Substantially increased γH2AX foci per nucleus were found in both cell lines 1h after radiation with both ion species. At the 24h time point only carbon treated samples of both cell lines showed increased γH2AX levels. The presence of the magnetic field did neither influence the survival parameters of either cell line, nor initial DNA damage and DNA damage clearance.

CONCLUSIONS

Applying a perpendicular magnetic field did not influence the cell survival, DNA repair, nor the biological effectiveness of protons or carbon ions in two human cancer cell lines.

Management of Motion and Anatomical Variations in Charged Particle Therapy: Past, Present, and Into the Future

The major aim of radiation therapy is to provide curative or palliative treatment to cancerous malignancies while minimizing damage to healthy tissues. Charged particle radiotherapy utilizing carbon ions or protons is uniquely suited for this task due to its ability to achieve highly conformal dose distributions around the tumor volume. For these treatment modalities, uncertainties in the localization of patient anatomy due to inter- and intra-fractional motion present a heightened risk of undesired dose delivery. A diverse range of mitigation strategies have been developed and clinically implemented in various disease sites to monitor and correct for patient motion, but much work remains. This review provides an overview of current clinical practices for inter and intra-fractional motion management in charged particle therapy, including motion control, current imaging and motion tracking modalities, as well as treatment planning and delivery techniques. We also cover progress to date on emerging technologies including particle-based radiography imaging, novel treatment delivery methods such as tumor tracking and FLASH, and artificial intelligence and discuss their potential impact towards improving or increasing the challenge of motion mitigation in charged particle therapy.

Integrated MRI-guided proton therapy planning: accounting for the full MRI field in a perpendicular system

PURPOSE

To present a first study on the treatment planning feasibility in perpendicular field MRI-integrated proton therapy which considers the full transport of protons from the pencil beam scanning assembly to the patient inside the MRI scanner.

METHODS

A generic proton pencil beam scanning (PBS) gantry was modelled as being integrated with a realistic split-bore MRI system in the perpendicular orientation. MRI field strengths were modeled as 0.5 T, 1 T, and 1.5 T. The PBS beam delivery and dose calculation was modeled using the TOPAS Monte Carlo toolkit coupled with matRad as the optimizer engine. A water phantom, liver and prostate plans were evaluated and optimized in the presence of the full MRI field distribution. A simple combination of gantry angle offset and small PBS nozzle skew was used to direct the proton beams along a path that closely follows the reference planning scenario, i.e. without magnetic field.

RESULTS

All planning metrics could be successfully achieved with the inclusion of gantry angle offsets in the range of 8°-29° when coupled with a PBS nozzle skew of 1.6°-4.4°. These two hardware based corrections were selected to minimize the average Euclidean distance (AED) in the beam path enabling the proton beams to travel inside the patient in a path that is close to the original path (AED smaller than 3 mm at 1.5 T). Final dose optimization, performed through further changes in the pencil beam scanning delivery, was then shown to be feasible for our selection of plans studied yielding comparable plan quality metrics to reference conditions.

CONCLUSIONS

For the first time, we have shown a robust method to account for the full proton beam deflection in a perpendicular orientation MRI-integrated proton therapy. These results support the ongoing development of the current prototype systems. This article is protected by copyright. All rights reserved.