Background/Objectives: High linear energy transfer (LET) carbon ions achieved more effective and biologically robust tumor control than photons in preclinical prostate cancer models; however, the longitudinal development of histopathological parameters remains insufficiently characterized, limiting the selection of the optimal treatment modality in patients. This study analyzed the temporal histological patterns after isoeffective photon, proton, and carbon ion irradiations. Methods: Two Dunning R3327 prostate carcinoma sublines (H, HI) grown subcutaneously in male Copenhagen rats received single-fraction isoeffective curative photon or carbon ion doses. For HI-tumors, the effectiveness of isoeffective curative proton doses and isoeffective subcurative photon and carbon ion doses was additionally investigated. Tumors were collected prior and up to 3 weeks after irradiation and processed for quantitative histology of proliferation (BrdU), DNA damage (γH2AX), hypoxia (pimonidazole), vascular (CD31), and immune cell (CD3, CD68) markers. Results: All modalities induced an early peak in γH2AX+ tumor cells and a pronounced suppression of BrdU+ cells, with more sustained effects after isoeffective carbon ions doses, particularly in the HI-tumors. These findings, however, differed strongly between hypoxic and oxic micro-environments. Vascular parameters, diffusion distances, and global and compartment-specific hypoxic fractions showed distinct temporal dynamics between photons and carbon ions in HI-tumors, whereas H-tumors exhibited more moderate and reversible changes. At curative carbon ion doses, there was a late rebound of BrdU-positive tumor cells and increased CD68+ macrophage accumulation in chronically hypoxic regions. CD3+ T cells showed a biphasic decrease-recovery pattern in HI-tumors largely independent of radiation quality and oxygenation. Conclusions: Longitudinal histology revealed modality- and tumor-line-specific trajectories of tumor, vascular, hypoxic, and immune responses after isoeffective photon, proton, and carbon ion irradiations in prostate carcinoma. The more persistent tumor cell damage and distinct vascular response, together with late proliferative and macrophage rebounds under chronic hypoxia after carbon ions, provide mechanistic support for the increased biological effectiveness and highlight hypoxia-driven repopulation and inflammation as key processes.

PURPOSE:We conducted a multicenter study on single-isocenter multitarget stereotactic radiosurgery to dosimetrically assess end-to-end test results and identify approaches and techniques influencing spatial accuracy and treatment plan quality. METHODS AND MATERIALS:An anthropomorphic head phantom with radiochromic film and polymer gel inserts was used with a reference structure set of 5 brain metastases. End-to-end tests were performed on-site at 23 centers in Germany, Austria, and Switzerland, each following its own single-isocenter multitarget stereotactic radiosurgery protocol. Spatial accuracy was quantified by comparing planned and measured prescription isodose-volume centroids. Plan quality was assessed from treatment planning system calculations using the Paddick gradient index (GI) and Paddick conformity index. Statistical analyses, including a generalized linear model, correlated results with protocol parameters to identify favorable systems and techniques. RESULTS:The mean spatial offset between measured and calculated prescription isodose centroids across all centers and targets was 0.9 ± 0.4(1σ) mm. Offsets above 1 mm were observed in 33% of centers. Better imaging-to-radiation isocenter consistency (ICC) yielded significantly higher accuracy (P = .002): dICC<median = 0.6 ± 0.2 mm versus 1.1 ± 0.3 mm. Mean GI was 6.7 ± 3.3. Automated planning (AP) tools achieved significantly lower GI (4.9 ± 0.7) than conventional planning (8.1 ± 3.8, P = .015). Mean Paddick conformity index was 0.75 ± 0.17, with AP significantly improving conformity (0.83 ± 0.07 vs 0.68 ± 0.19; P = .028) and reducing variability in both indices. Target-to-isocenter distance had no significant influence on spatial accuracy, GI, or Paddick conformity index. CONCLUSIONS:Spatial accuracy in a static phantom was primarily determined by ICC and less by specific delivery infrastructure or techniques, emphasizing the importance of a precise imaging isocenter calibration. AP tools significantly improved and standardized treatment plan quality across centers.

The exploitation of the high relative biological effectiveness (RBE) of carbon ions is one of the major rationales for their use as a radiation therapy modality. As the RBE depends on many physical and biological factors, biophysical models are used to compute it for the complex radiation fields used in clinical settings. However, the models currently applied in clinics or used to interpret clinical results make different RBE predictions. This creates difficulties for direct comparability of RBE-weighted doses delivered and reported within different approaches. Additional conventions on how these models are applied also differ and further complicate the comparison. Consequently, it is crucial to understand the impact of RBE modeling on the delivered absorbed doses and the reported RBE-weighted doses. Translation concepts between dose prescription systems, that is, the models and the context in which they are used, are needed to exchange treatment protocols between centers with different planning methods and to establish joint clinical studies or meta-studies. Although many of these problems are solved for specific cases, a broad perspective is lacking on how to transparently proceed with multiple RBE models and corresponding concepts of RBE-weighted dose. The present publication is a product of an initiative within the subcommittee on Guidelines in Carbon Ion Radiation Therapy of the Particle Therapy Co-operative group (PTCOG). It aims to (1) raise awareness of the problem; (2) demonstrate the impact of different models used for RBE predictions; (3) provide information on how RBE is currently accounted for; and (4) give an overview of approaches toward the translation of doses. Along this route, we provide several expert consensus statements agreed on by all authors, which provide insights into the complexity of understanding and comparing different dose prescription systems. Despite this complexity, transforming treatment plans between any 2 systems is feasible, opening up novel planning strategies that consider multiple models and paving the way for multi-institutional clinical studies .

BACKGROUND:To ensure accurate, safe, and reproducible patient treatments, it is essential to have precise knowledge and a solid understanding of patient-specific quality assurance (PSQA). For many years, the delivery of doses to all patients has been verified using dosimetric measurements. However, these measurements require substantial work, and the reasons for the occasional deviations are unclear. For these reasons, alternative methods such as independent dose calculations (IDCs) and analysis of beam-monitor log files are increasingly discussed in the particle therapy community. Nevertheless, before replacing dose-verification measurements with other methods, existing measurement data should be thoroughly analyzed to determine what can be learned from them and how they compare with potential alternatives. These alternative methods are mentioned in this work only to provide context and to outline possible directions for future studies. PURPOSE:To evaluate the dosimetric accuracy and efficiency of PSQA using a water phantom (WP) over a 10-year period at the Heidelberg Ion Beam Therapy Center (HIT). METHODS:Between 2016 and 2025, 23014 treatment fields with protons, carbon, or helium ions were verified using a WP equipped with 24 pinpoint ionization chambers. The patient treatment plans were recalculated in the water phantom geometry and compared to measured absolute doses. The data were categorized by treatment room, ion species, treatment planning systems (TPS), range shifter (RaShi) use, indication, depth, and target volume, excluding measurements with human errors. Statistical analysis compared measured and calculated doses, focusing on mean, maximum, and minimum dose deviations. Furthermore, the workflow efficiency was assessed based on the beam time required for dosimetric verification, as well as the total time needed for preparation and analysis. RESULTS:Mean dose deviations were in general slightly negative (t-test, p < 0.01), within ±1 % across all categories (total mean ± SD  =  -0.50 ± 0.90 %), with 91 % of fields passing institutional ±5 % tolerances. Further, significant differences (p < 0.01) were also observed between treatment rooms, ion species, TPS platforms, and RaShi settings. Additionally, the RayStation TPS showed lower deviations than the Syngo TPS, and helium ions had the smallest deviations. Moreover, repeated verifications reduced variability but without significant improvement. Correlations with target depth or volume were statistically significant but clinically negligible. Less than 1 % of maximum and minimum dose measurements exceeded ±7 % annually. Finally, over 4308 h of beam time, preparation, and analysis were spent on PSQA during the 10-year period. CONCLUSIONS:PSQA at HIT demonstrated high dosimetric accuracy and delivery stability. Integration of IDCs and log file analysis may improve efficiency and allow to omit verification measurements in well-established cases without compromising patient safety and treatment quality, if the extensive machine QA program is maintained.

PURPOSE:Proton therapy applies a constant relative biological effectiveness (RBE) of 1.1, despite increasing evidence that RBE varies with physical and biological factors such as linear energy transfer (LET), dose, and tissue type. Variable RBE models from in vitro data may not reflect in vivo radiation responses. This study aimed to compile and analyze a comprehensive database of published in vivo RBE data from proton irradiations. METHODS AND MATERIALS:In vivo proton RBE studies were identified through a literature review. RBE values, doses, biological endpoints were extracted from 25 studies. Dose-averaged LET (LETd) was estimated by reconstructing experimental setups in a treatment planning system. Data were stratified into three endpoint groups: early jejunal crypt regeneration, other early endpoints, and late endpoints. Regression fits were obtained for RBE as a function of LET and fraction dose (Dp). RESULTS:174 RBE data points were compiled. Across endpoints, RBE increased with LET, with statistically significant slopes in most groups and a more pronounced dependency for late endpoints. A negative correlation between RBE and Dp was observed for crypt-, other early-, and pooled data, and was also retained when excluding experiments using kV X-rays as reference. CONCLUSIONS:The present in vivo analysis supports proton RBE dependencies on LET, dose, and endpoint previously observed in vitro and highlights the need for standardized methodologies and more extensive in vivo data to enable reliable endpoint-specific analyses. The compiled database, with consistently derived LET estimates across studies, provides a solid foundation for continued in vivo RBE research.

PURPOSE:To describe performance measurements, adaptations and time stability over 20 months of a diagnostic MR scanner for integration into MR-guided photon and particle radiotherapy. MATERIAL AND METHODS:For realization of MR-guided photon and particle therapy (MRgRT/MRgPT), a 1.5 T MR scanner was installed at the Heidelberg Ion Beam Therapy Center. To integrate MRI into the treatment process, a flat tabletop and dedicated coil holders for flex coils were used, which prevent deformation of the patient external contour and allow for the use of immobilization tools for reproducible positioning. The signal-to-noise ratio (SNR) was compared for the diagnostic and therapy-specific setup using the flat couch top and flexible coils for the a) head & neck and b) abdominal region as well as for different bandwidths and clinical pulse sequences. Additionally, a quality assurance (QA) protocol with monthly measurements of the ACR phantom and measurement of geometric distortions for a large field-of-view (FOV) was implemented to assess the imaging quality parameters of the device over the course of 20 months. RESULTS:The SNR measurements showed a decreased SNR for the RT-specific as compared to the diagnostic setup of (a) 26% to 34% and (b) 11% to 33%. No significant bandwidth dependency for this ratio was found. The longitudinal assessment of the image quality parameters with the ACR and distortion phantom confirmed the long-term stability of the MRI device. CONCLUSION:A diagnostic MRI was commissioned for use in MR-guided particle therapy. Using a radiotherapy specific setup, a high geometric accuracy and signal homogeneity was obtained after some adaptions and the measured parameters were shown to be stable over a period of 20 months.

Purpose To determine the relative biological effectiveness (RBE) in the rat spinal cord after 6 fractions of protons as a function of linear energy transfer (LET) and dose. Methods and Materials The rat spinal cord was irradiated at 4 different positions of a 6 cm spread-out Bragg peak using 6 fractions of protons (LET: 1.4, 2.7, 3.9, and 5.5 keV/µm). Dose-response curves were established for the endpoint paresis grade 2, and the RBE was calculated based on the dose at 50% effect probability. Including data with single and split doses, the measured RBE values were compared with predictions from 4 mechanistic, 3 phenomenological, and 2 patient-derived variable RBE models. Results With increasing LET, the dose at 50% effect probability decreased from 51.3 Gy to 43.3 Gy, resulting in a rise in the RBE from 1.11 to 1.32. The biologically equivalent dose decreased markedly between the 2 proximal and 2 distal spinal cord positions, resulting in extrapolated maximum RBE values of up to 1.87 in the limit of zero dose per fraction. The α/β values ranged between 1.5 Gy and 4.2 Gy. At 3.9 and 5.5 keV/µm, the RBE increased with decreasing dose, and at 1.8 Gy per fraction, the RBE was extrapolated to 1.40 and 1.42, respectively. The agreement between predicted and measured RBE varied between the different models. Conclusions A fixed RBE of 1.1 provides a good approximation up to the center of the spread-out Bragg peak; however, at 3 mm from the distal end, the RBE increases markedly and may reach values above 1.4 at clinical fraction schedules. Using predictions from a variable RBE model may, therefore, be reasonable; however, the model and model parameters should be carefully selected, ideally as a consensus among the proton therapy centers.

Background and purpose:Hypoxia in tumors significantly contributes to radiation resistance, often leading to poor treatment outcomes. In vitro studies demonstrated that proton irradiation not only exhibits an increased relative biological effectiveness (RBE), but also a lower oxygen enhancement ratio (OER). This study explored the impact of hypoxic and oxic conditions on local tumor control after proton irradiation in a rat prostate carcinoma. Material and methods:Subcutaneously transplanted Dunning R3327-HI rat prostate carcinomas were irradiated with varying single doses of protons under oxic or hypoxic conditions. Hypoxia was induced by clamping the tumor-supplying vessels. The biological endpoint was local tumor control assessed 300 days after irradiation. Dose-response curves were determined and based on the doses required for 50 % tumor control probability (TCD50), the RBE and OER were calculated. Results:The TCD50 value was significantly higher under hypoxic than under oxic conditions (73.4 ± 1.9 Gy vs. 50.5 ± 1.6 Gy), resulting in an OER of 1.45 ± 0.06 for proton irradiation. Compared to photon irradiation, the RBE for protons was of 1.23 ± 0.07 under oxic and 1.30 ± 0.04 under hypoxic conditions. Conclusion:Proton irradiations showed a 5 % reduction in OER compared to the previously measured photon value of 1.53 ± 0.08, suggesting a slightly higher effectiveness of protons in hypoxic tumors as compared to photons.

Single-isocenter multitarget stereotactic radiosurgery (SIMT SRS) offers enhanced clinical efficiency for treating multiple brain metastases. However, it introduces additional uncertainties, such as off-center dose and beam profile inaccuracies, as well as quality assurance (QA) challenges, complicating its implementation. This study aims to evaluate different SIMT SRS approaches. We collected and analyzed SIMT SRS protocol and infrastructure parameters from 23 radiotherapy centers across Germany, Austria, and Switzerland, encompassing immobilization systems, computed tomography (CT) protocols, linear accelerators, treatment planning systems, beam configurations, imaging techniques, and QA practices. Consensus, deviations, and compliance with current guidelines were assessed. Subsequent studies will include on-site measurements, evaluation of treatment plan quality and delivery accuracy, and correlation of these findings with the analyzed protocols to identify potential links between protocol parameters and clinical outcomes. There is consensus (at least 80

BACKGROUND:PAGAT (PolyAcrylamide Gelatine gel fabricated at ATmospheric conditions) gel dosimetry provides a valuable three-dimensional (3D) measurement tool for photon therapy, particularly in the context of advanced radiotherapy techniques such as volumetric modulated arc therapy or intensity-modulated radiation therapy. However, the production of PAGAT gel is highly sensitive to variations in the preparation process, particularly in open environments. METHODS:To ensure reliable production of PAGAT gel and a simplified workflow for inexperienced users, a gel preparation device was designed. This device enables (i) single-component preparation, (ii) transfer of gel components, and (iii) filling of gel in 3D-printed vials. The gel batches prepared with this device were analyzed for stability and reproducibility. Furthermore, sensitivity to oxygen exposure was tested with and without Tetrakis(hydroxymethyl)phosphonium chloride (THPC). Additionally, vials used for gel storage and irradiation were fabricated using two different PolyJet 3D printers, with either internal support material (opaque finish) or no internal support material (glossy finish). Post-processing of the printed vials was carried out using either sodium hydroxide (NaOH) cleaning or water rinsing. RESULTS:The PAGAT gel produced with this device demonstrated high reproducibility, with a standard error of the mean below 1.8%. The gel remained stable for up to 14 days post-preparation. However, THPC was necessary to remove the residual oxygen from the solution. While the choice of 3D printer did not influence gel performance, the cleaning method played a critical role, with NaOH-cleaned opaque prints showing reduced gel response. CONCLUSIONS:The gel preparation device enabled low-cost PAGAT gel production. The system provided a stable and closed environment to ensure consistent gel properties, reducing variability in dosimetry applications. Its design also offers potential for use with other polymer gel formulations requiring controlled conditions.

To determine the relative biological effectiveness (RBE) in the rat spinal cord after 6 fractions of protons as a function of linear energy transfer (LET) and dose. The rat spinal cord was irradiated at 4 different positions of a 6 cm spread-out Bragg peak using 6 fractions of protons (LET: 1.4, 2.7, 3.9, and 5.5 keV/µm). Dose-response curves were established for the endpoint paresis grade 2, and the RBE was calculated based on the dose at 50% effect probability. Including data with single and split doses, the measured RBE values were compared with predictions from 4 mechanistic, 3 phenomenological, and 2 patient-derived variable RBE models. With increasing LET, the dose at 50% effect probability decreased from 51.3 Gy to 43.3 Gy, resulting in a rise in the RBE from 1.11 to 1.32. The biologically equivalent dose decreased markedly between the 2 proximal and 2 distal spinal cord positions, resulting in extrapolated maximum RBE values of up to 1.87 in the limit of zero dose per fraction. The α/β values ranged between 1.5 Gy and 4.2 Gy. At 3.9 and 5.5 keV/µm, the RBE increased with decreasing dose, and at 1.8 Gy per fraction, the RBE was extrapolated to 1.40 and 1.42, respectively. The agreement between predicted and measured RBE varied between the different models. A fixed RBE of 1.1 provides a good approximation up to the center of the spread-out Bragg peak; however, at 3 mm from the distal end, the RBE increases markedly and may reach values above 1.4 at clinical fraction schedules. Using predictions from a variable RBE model may, therefore, be reasonable; however, the model and model parameters should be carefully selected, ideally as a consensus among the proton therapy centers.

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 as in-vivo imaging. 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-vivo images 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 sequence in-vivo revealed 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.

Background and purpose: Ion beams exhibit an increased relative biological effectiveness (RBE) with respect to photons. This study determined the RBE of oxygen ion beams as a function of linear energy transfer (LET) and dose in the rat spinal cord. Materials and methods: The spinal cord of rats was irradiated at four different positions of a 6 cm spread-out Bragg-peak (LET: 26, 66, 98 and 141 keV/µm) using increasing levels of single and split oxygen ion doses. Dose-response curves were established for the endpoint paresis grade II and based on ED50 (dose at 50 % effect probability), the RBE was determined and compared to model predictions. Results: When LET increased from 26 to 98 keV/µm, ED50 decreased from 17.2 ± 0.3 Gy to 13.5 ± 0.4 Gy for single and from 21.7 ± 0.4 Gy to 15.5 ± 0.5 Gy for split doses, however, at 141 keV/µm, ED50 rose again to 15.8 ± 0.4 Gy and 17.2 ± 0.4 Gy, respectively. As a result, the RBE increased from 1.43 ± 0.05 to 1.82 ± 0.08 (single dose) and from 1.58 ± 0.04 to 2.21 ± 0.08 (split dose), respectively, before declining again to 1.56 ± 0.06 for single and 1.99 ± 0.06 for split doses at the highest LET. Deviations from RBE-predictions were model-dependent. Conclusion: This study established first RBE data for the late reacting central nervous system after single and split doses of oxygen ions. The data was used to validate the RBE-dependence on LET and dose of three RBE-models. This study extends the existing data base for protons, helium and carbon ions and provides important information for future patient treatments with oxygen ions.

This study investigates the LET-dependence of dose measurements performed with a nanocomposite Fricke gel (NC-FG) upon irradiation with carbon ion beams to address the problem of quenching of gel dosimeters in high-LET particle therapy. The preparation of the NC-FG in a simple gel preparation device was performed for atmospheric and anoxic environmental conditions and two different container materials. Irradiation was performed with carbon ion beams at energies of 133.39MeV/u or 194.87MeV/u, complemented by a modulated 10 mm carbon ion spread-out Bragg peak (SOBP) irradiation. The R1 profiles obtained from the MRI readout were compared to the treatment planning system and a 2D-ionisation chamber array measurement performed for the mono-energetic and SOBP irradiations, respectively. For different dose levels, no difference in the gel response was obtained for the 3D-printed plastic containers, while the glass vials exhibited a linear dose response. Additional use of the glove box changes produced similar results. For both the mono-energetic and the SOBP irradiation, a linear dose response without quenching was observed. For depths ranging up to the Bragg peak position, the mono-energetic profiles deviated by less than ± 15% from the planned dose distribution, excluding the lowest entrance surface dose (ESD). For the SOBP profiles, the deviation from the measured dose distribution was less than ± 10% for depths between 20 mm and 33 mm. This study presents a LET-independent measurement of mono-energetic carbon ion Bragg peaks and, for the first time, a LET-independent measurement of a carbon ion SOBP with the NC-FG prepared in a simple gel preparation device under atmospheric conditions. The NC-FG provides a possible solution for 3D dosimetry in carbon ion radiotherapy.

Carbon-ion irradiation is increasingly used at the skull base and spine near the radiation-sensitive spinal cord. To better characterize the in vivo radiation response of the cervical spinal cord, radiogenic changes in the high-dose area were measured in rats using magnetic resonance imaging (MRI) diffusion measurements in comparison to conventional photon irradiations. In this longitudinal MRI study, we examined the gray matter (GM) of the cervical spinal cord in 16 female Sprague-Dawley rats after high-dose photon (n = 8) or carbon-ion (12C) irradiation (n = 8) and in 6 sham-exposed rats until myelopathy occurred. The differences in the diffusion pattern of the GM of the cervical spinal cord were examined until the endpoint of the study, occurrence of paresis grade II of both forelimbs was reached. In both radiation techniques, the same order of the occurrence of MR-morphological pathologies was observed – from edema formation to a blood spinal cord barrier (BSCB) disruption to paresis grade II of both forelimbs. However, carbon-ion irradiation showed a significant increase of the mean apparent diffusion coefficient (ADC; P = 0.031) with development of a BSCB disruption in the GM. Animals with paresis grade II as a late radiation response had a highly significant increase in mean ADC (P = 0.0001) after carbon-ion irradiation. At this time, a tendency was observed for higher mean ADC values in the GM after 12C irradiation as compared to photon irradiation (P = 0.059). These findings demonstrated that carbon-ion irradiation leads to greater structural damage to the GM of the rat cervical spinal cord than photon irradiation due to its higher linear energy transfer (LET) value.

Objective.To investigate magnetic field effects on the dose distribution and ionization chambers response in carbon ion reference fields and determine magnetic field correction factors for chambers of different volumes.Approach.The response of six Farmer-type chambers with varying radii (1-6 mm, termed as R1-R6) was measured in magnetic fields up to 1 T in 0.1 T increments using an experimental electromagnet and compared with Monte Carlo simulations. Chamber readings were measured in the entrance region of a monoenergetic carbon ion beam of 390.75 MeV u-1. A lower energy of 200.28 MeV u-1was applied to chamber R3 for comparison. Polarity and recombination corrections were investigated for the R3 chamber. The local dose change induced by the magnetic field was calculated by Monte Carlo, which together with change of the chamber's response, was used to calculate the final magnetic field correction factors.Main results.The dependence of the chamber response on the magnetic field was non-linear and volume-dependent. Maximum changes ranged from 0.30% (R4) to 0.62% (R5) at 0.2 T. For R3, the response for the lower energy was systematically decreased by 0.2% in the range of 0.2 T to 0.7 T. No significant effect of the magnetic field on polarity and ion recombination correction was found. The maximum variation of the local dose was found to be (0.03 ± 0.08) % at 0.2 T for beam energy of 390.75 MeV u-1. Magnetic field correction factors for the different chambers ranged from 0.28% (R4) to 0.60% (R5).Significance.This study provides the first detailed analysis of chambers' response to magnetic flux densities of up to 1 T using chambers of different radii and comparison with simulations. By combining the chamber response alterations with local dose changes magnetic field correction factors were calculated for all six chambers, including the commercial Farmer-type chamber.

The advantages of proton and light ion beam therapy compared to conventional photon radiation therapy are well-known, mainly thanks to the characteristic depth dose distribution of ions and their radio-biological effectiveness. Nevertheless, the use of ions implies different nuclear reactions that generate secondary particles, with neutrons among them. These secondary neutrons can travel far away from the treatment volume, their measurement is a challenging complex task, and their biological effects are particularly high for neutrons with energies in the MeV range. In this review, a comprehensive description of secondary neutron dosimetry in proton and light ion beam therapy is given. Many studies have been conducted on the quantification of the secondary neutron dose, most of them have been performed for proton beams, whereas for other ions like carbon, the available information is scarce. The main measurement campaigns are summarised, focusing on the type of detectors used. In line with the detectors’ advantages and limitations, measurements performed inside and outside anthropomorphic phantoms are considered. The role of Monte Carlo radiation transport simulations is discussed since many experimental detection techniques need additional simulations to provide dose estimates. A focus on the current challenges for the measurements of neutrons with energies above 20 MeV is given, as this is one of the main components of secondary neutrons produced by therapeutic ion beams. Finally, the potential clinical relevance of the available and needed secondary neutron dose data is discussed, in terms of its impact on the treatment of patients. For this, the relative biological effectiveness of neutrons and the potential risk of cancer induction re-incidence or secondary cancer due to secondary neutron doses play a key role.

Accurate Magnetic Resonance Imaging (MRI) simulation is fundamental for high-precision stereotactic radiosurgery and fractionated stereotactic radiotherapy, collectively referred to as stereotactic radiotherapy (SRT), to deliver doses of high biological effectiveness to well-defined cranial targets. Multiple MRI hardware related factors as well as scanner configuration and sequence protocol parameters can affect the imaging accuracy and need to be optimized for the special purpose of radiotherapy treatment planning. MRI simulation for SRT is possible for different organizational environments including patient referral for imaging as well as dedicated MRI simulation in the radiotherapy department but require radiotherapy-optimized MRI protocols and defined quality standards to ensure geometrically accurate images that form an impeccable foundation for treatment planning. For this guideline, an interdisciplinary panel including experts from the working group for radiosurgery and stereotactic radiotherapy of the German Society for Radiation Oncology (DEGRO), the working group for physics and technology in stereotactic radiotherapy of the German Society for Medical Physics (DGMP), the German Society of Neurosurgery (DGNC), the German Society of Neuroradiology (DGNR) and the German Chapter of the International Society for Magnetic Resonance in Medicine (DS-ISMRM) have defined minimum MRI quality requirements as well as advanced MRI simulation options for cranial SRT.