Maximum Field Size Research Articles

Feasibility of Synchrotron-Based Ultra-High Dose Rate (UHDR) Proton Irradiation with Pencil Beam Scanning for FLASH Research.

This study aims to present the feasibility of developing a synchrotron-based proton ultra-high dose rate (UHDR) pencil beam scanning (PBS) system. The RF extraction power in the synchrotron system was increased to generate 142.4 MeV pulsed proton beams for UHDR irradiation at ~100 nA beam current. The charge per spill was measured using a Faraday cup. The spill length and microscopic time structure of each spill was measured with a 2D strip transmission ion chamber. The measured UHDR beam fluence was used to derive the spot dwell time for pencil beam scanning. Absolute dose distributions at various depths and spot spacings were measured using Gafchromic films in a solid-water phantom. For proton UHDR beams at 142.4 MeV, the maximum charge per spill is 4.96 ± 0.10 nC with a maximum spill length of 50 ms. This translates to an average beam current of approximately 100 nA during each spill. Using a 2 × 2 spot delivery pattern, the delivered dose per spill at 5 cm and 13.5 cm depth is 36.3 Gy (726.3 Gy/s) and 56.2 Gy (1124.0 Gy/s), respectively. The synchrotron-based proton therapy system has the capability to deliver pulsed proton UHDR PBS beams. The maximum deliverable dose and field size per pulse are limited by the spill length and extraction charge.

Commissioning of a novel gantry-less proton therapy system.

The focus of this article is to describe the configuration, testing, and commissioning of a novel gantry-less synchrotron-based proton therapy (PT) facility. The described PT system delivers protons with a water equivalent range between 4 and 38 cm in 1800 energy layers. The fixed beam delivery permits a maximum field size of 28 × 30 cm2. The patient positioning and imaging system includes a six-degree-of-freedom robotic arm, a convertible patient chair, a vertical 4DCT, and an orthogonal 2D X-ray imaging system. The spot positioning reproducibility was consistent within ±1 mm. The width (σ) of the beam profile at the isocenter was energy dependent and ranged from 2.8 mm to 7.7 mm. Absolute dose reproducibility was measured and deviations were found to be <0.62% for all possible beam scenarios. The built-in dose monitoring system was successfully tested for its ability to generate interlocks under specific conditions (beam spot deviation ≥2 mm, individual spot dose ≥10% or ≥0.25 Gy, spot energy deviation ≥0.5 MeV). The robot positioning exhibited a consistent reproducibility within ±1 mm. All tested scenarios achieved laser-free initial 3D/3D image-guided positioning within ±5 mm. Subsequent 2D/3D positioning showed an accuracy of ±1 mm. A single 2D/3D image registration event corrected positions in all cases. Results of gamma analysis (3%, 3 mm) demonstrated pass rates greater than 95% for head and neck, thorax, abdomen treatment plans. We report on the performance of a novel single-room gantry-less PT system comprised of a compact synchrotron and an adjustable (from nearly horizontal to almost vertical) patient positioning system. The commissioning results show high accuracy and reproducibility of the main proton beam parameters and the patient positioning system. The new PT facility started patient treatments in March 2023, which were the first in Israel and the Middle Eastern region.

FLASH radiotherapy using high-energy X-rays: Current status of PARTER platform in FLASH research

PurposeRecent studies indicated that ultrahigh dose rate (FLASH) radiation can reduce damage to normal tissue while maintaining anti-tumour activity compared to conventional dose rate (CONV) radiation. This paper provides a comprehensive description of the current status of the Platform for Advanced Radiotherapy Research (PARTER), which serves as the first experimental FLASH platform utilizing megavoltage X-rays and has facilitated numerous experiments. Methods and MaterialsPARTER was established in 2019 based on a superconducting linac to support experimental FLASH studies using megavoltage X-rays. Continuous upgrades have been made to the accelerator, collimators, flattening filters, monitors, other auxiliary devices, and irradiation process in order to achieve optimal results. Passive and active dosimeters are employed for measuring dose distribution and to ensure traceability of radiation doses. ResultsThe dose monitors and dosimeters demonstrate reliable performance with acceptable stability. At PARTER, the maximum mean dose rate is approximately 400 Gy/s at a surface-source distance of 20 cm (over 1000 Gy/s at smaller distances), with an instantaneous dose rate of approximately 8E5 Gy/s. Both passive and active dosimeters exhibit good linearity and agreement during FLASH X-ray irradiation. The monitors show good linearity to dose rate, with short-term fluctuations within 1.5 % for the diamond monitor. The discrepancy between measured absorbed dose and dose protocol is typically less than 4 %. The X-ray energy spectra on PARTER are comparable to those for megavoltage CONV linacs operating in flattening filter-free mode. The maximum field size of the FLASH beam is 4.5 cm × 4.5 cm. The FLASH dose profile demonstrates satisfactory flatness (1.04) and similar penumbra compared to clinical CONV linac, while the percentage depth dose curve of FLASH X-rays is steeper than that of the clinical megavoltage CONV X-ray. ConclusionsPARTER represents a pioneering platform for conducting megavolts FLASH X-ray irradiation in biological experiments. It effectively fulfills the requirements of preclinical research on megavoltage X-ray FLASH and undergoes continuous upgrades to meet increasingly demanding performance criteria.

Fixed-Field IMRT for Cervix Carcinoma Patients on an MR-LINAC Platform: Dosimetric Feasibility and Challenges

To investigate the impact of MR-LINAC performance characteristics and inverse planning implementation on the feasibility of fixed-field IMRT for cervix carcinoma patients by benchmarking MR-LINAC plans against clinically used VMAT plans in a single institution study and multi-institutional treatment planning challenge. For 10 cervix cancer patients who had previously received Linac-based VMAT, new treatment plans were optimized for MR-LINAC IMRT using 6X FFF fixed fields with maximum available field size of 27.4 x 24.1 cm2. Dose optimization was performed on the clinically used planning CT and structure set. Prescribed dose was 48.6 Gy in 27 fractions for all patients with 6 patients receiving an additional integrated boost for a total of 58.05 Gy to involved nodes. Constraints were based on our institutional protocol as per Table 1. IMRT delivery time was limited to 20 min. Original clinically used VMAT plans were generated on Eclipse (Varian Medical System) using 3 to 4 arcs. For the multi-institutional planning challenge, the data set from a single patient was anonymized and shared to participants. Participants used a single MR-based Linac planning platform to generate a plan based on our institutional constraints, with maximum treatment time limited to 20 min. For all analyses, a paired samples t-test was used to compare the significance defined at p < 0.05. For MR-LINAC plans, the mean number of fields used was 23, mean number of segments 229, and the average estimated treatment delivery time was 17.3 minutes. MR-LINAC plans showed a significantly higher heterogeneity and dose to organs at risk compared to VMAT plans (Table 1). For the planning challenge, a total of 30 participants registered interest. Of this, seven plans were submitted to the challenge. On average, participants generated a plan that would be acceptable based on our institutional constraints (Table 1). However, the volumetric dose to bowel and pelvic bones were higher on MR-LINAC plans compared to the reference VMAT plan. MR-LINAC fixed-field IMRT for cervix cancer patients is feasible but system constraints and optimization implementation result in greater dose heterogeneity and worse organ-at-risk sparing compared to Linac based VMAT. Further research is needed to determine if potential reduction of treatment margins, allowed by better MRI soft-tissue visualization, will result in MR-LINAC IMRT superior to Linac VMAT.

Clinical Experience with Commissioning a GRID Collimator for Spatially Fractionated Radiation Therapy

In this study, we report our physics commissioning experience for clinically implementing a GRID collimator based Spatially Fractionated Radiation Therapy (SFRT). For SFRT commissioning, we utilized a commercially available brass GRID collimator (DotDecimal, Sanford, FL) that is mounted on a blocking tray for positioning into the accessory mount of the Linac. The brass GRID is 7.62 cm thick and weighs 15.8 kg. The GRID has a total of 149 divergent holes arranged in a hexagonal pattern (1.43 cm in diameter and hole-centers spaced at 2.11 cm when projected at the isocenter distance). The GRID encompasses a maximum field size of 25 × 25 cm at the isocenter. A commercial 48 × 48 × 48 cm water phantom scanning system was used to collect GRID output factors (GRID field to open field ratio), depth dose and beam profile data. Output measurements were performed using a 0.13 cm3 active volume ion-chamber and beam scans were obtained with a diode detector. Data was collected for both flattening and flattening-free beams of nominal energies 6 MV and 10 MV photons. The measurement depths were at dmax (1.5 cm for 6 MV and 2.5 cm for 10 MV), 5-cm and 10-cm respectively. For each energy and depth of measurement, collimator settings were varied from 5 × 5 cm to 28 × 28 cm. From scan profiles at different depths, the valley (lowest) to peak (highest) dose ratios (VPDR) were calculated. A commercial treatment planning system (TPS) was used to test the accuracy of dose calculations with GRID. This was accomplished by importing vendor generated DICOM RT file into the TPS. A block transmission factor of 7% for 6 MV and 10.2% for 10 MV energy beams were applied. All measured data were compared with corresponding TPS calculated data. Test patient treatment plans with GRID were created in TPS and planned distributions were verified using a commercial detector array with 2.5 mm detector spacing. The VPDR, expressed as %, are presented in Table 1. Measured and TPS calculated output factors agreed within 2% for 6 MV and within 3% for 10 MV photon beams. Percent depth doses were lower in magnitude for GRID field compared to open field for all energies studied (for e.g., 6 MV depth dose at 10 cm depth for 10 cm x10cm field 62% for GRID field vs. 66% for open field). Measured and calculated GRID beam profiles agreed within 5% dose difference and 1 mm distance-to agreement. For all test cases, the planned vs. measured dose distributions passed at an average gamma passing rate of 96.5% using a 3% dose difference/ 3 mm distance to agreement and 10% threshold criteria. The Dot Decimal GRID collimator provides a simple way of achieving SFRT in the clinic, albeit heavy to use and has an irradiation field size limitation of 25 cm × 25 cm.

Characterization of 250 MeV Protons from the Varian ProBeam PBS System for FLASH Radiation Therapy.

Shoot-through proton FLASH radiation therapy has been proposed where the highest energy is extracted from a cyclotron to maximize the dose rate (DR). Although our proton pencil beam scanning system can deliver 250 MeV (the highest energy), this energy is not used clinically, and as such, 250 MeV has yet to be characterized during clinical commissioning. We aim to characterize the 250-MeV proton beam from the Varian ProBeam system for FLASH and assess the usability of the clinical monitoring ionization chamber (MIC) for FLASH use. We measured the following data for beam commissioning: integral depth dose curve, spot sigma, and absolute dose. To evaluate the MIC, we measured output as a function of beam current. To characterize a 250 MeV FLASH beam, we measured (1) the central axis DR as a function of current and spot spacing and arrangement, (2) for a fixed spot spacing, the maximum field size that achieves FLASH DR (ie, > 40 Gy/s), and (3) DR reproducibility. All FLASH DR measurements were performed using an ion chamber for the absolute dose, and irradiation times were obtained from log files. We verified dose measurements using EBT-XD films and irradiation times using a fast, pixelated spectral detector. R90 and R80 from integral depth dose were 37.58 and 37.69 cm, and spot sigma at the isocenter were σx = 3.336 and σy = 3.332 mm, respectively. The absolute dose output was measured as 0.343 Gy*mm2/MU for the commissioning conditions. Output was stable for beam currents up to 15 nA and gradually increased to 12-fold for 115 nA. Dose and DR depended on beam current, spot spacing, and arrangement and could be reproduced with 6.4% and 4.2% variations, respectively. Although FLASH was achieved and the largest field size that delivers FLASH DR was determined as 35 × 35 mm2, the current MIC has DR dependence, and users should measure dose and DR independently each time for their FLASH applications.

Surface Dose and Skin Toxicity in Low-Magnetic Field MR-Linac

<h3>Purpose/Objective(s)</h3> Radiation induced skin toxicity is a common side effect that results in poor patient satisfaction and unplanned treatment breaks thus prolonging the overall treatment time with poor outcome. It is hypothesized that treatments on low-field MR-Linac will not produce higher surface dose which is investigated in this study. <h3>Materials/Methods</h3> Skin dose has always been an area of active clinical interest that is dependent on many parameters including beam energy, field size, material thickness and atomic number in the beam, material distance from surface, source to surface distance, beam angle (obliquity factor), concomitant chemotherapy etc. Along with these parameters, magnetic field is also a critical parameter that affect surface dose. With the development of the MRL system focus has shifted towards Lawrence force giving rise to electron return effect along with other treatment parameters affecting the skin dose. A Linac equipped with 6MV FFF photons with 90 cm source to axis distance is used. A Markus parallel plate ionization chamber is used in a solid water phantom with proper correction factor to investigate surface and buildup dose with six different gantry angles 15⁰ apart gantry (equivalent of standard Linac 0⁰, 15⁰, 30⁰, 45⁰, 60⁰, and 75⁰) for field sizes of 1.66 × 1.66 cm² (small), 9.96 × 9.9 cm² (reference) and 24.1 × 24.1 cm² (maximum). Measurements were made with increasing buildup thickness in steps of 1 mm to 20 mm of solid water in 0.35 T magnetic field. <h3>Results</h3> In traditional linear accelerators surface dose is 30-40% for a 6MV photon beam, without any modifier. This value increases significantly with beam angle. In MR-Linac, surface dose is relatively low, 12.6%, 14.9% and 16.7% for small, reference and maximum fields at zero-degree gantry angle, respectively. These values are 12% to 2% higher (at 0⁰ and 75⁰ respectively) for maximum field size compared to the reference field size. For lower field sizes minimum variation of 10% was observed at 75° for reference field size. The surface dose increases with increasing radiation field size but magnitude is still lower as the case with traditional lilacs. Table 1 shows the surface dose with field size and gantry angles. <h3>Conclusion</h3> Surface dose in low-field magnetic environment is relatively lower than traditional accelerators. It is concluded that low-field MR-Linac system may not add any additional skin toxicity compared to conventional Linac. However, the effect may be different for high-field MR-Linac system, and appropriate skin evaluation needs to be performed to differentiate between radiation induced toxicity specially for the oblique beams versus RF-thermal effect.

A multi-layer strip ionization chamber (MLSIC) device for proton pencil beam scan quality assurance

Objective. Proton pencil beam scanning (PBS) treatment fields needs to be verified before treatment deliveries to ensure patient safety. In current practice, treatment beam quality assurance (QA) is measured at a few selected depths using film or a 2D detector array, which is insensitive and time-consuming. A QA device that can measure all key dosimetric characteristics of treatment beams spot-by-spot within a single beam delivery is highly desired. Approach. We developed a multi-layer strip ionization chamber (MLSIC) prototype device that comprises of two layers of strip ionization chambers (IC) plates for spot position measurement and 64 layers of plate IC for beam energy measurement. The 768-channel strip ion chamber signals are integrated and sampled at a speed of 3.125 kHz. It has a 25.6 cm × 25.6 cm maximum measurement field size and 2 mm spatial resolution for spot position measurement. The depth resolution and maximum depth were 2.91 mm and 18.6 cm for 1.6 mm thick IC plate, respectively. The relative weight of each spot was determined from total charge by all IC detector channels. Main results. The MLSIC is able to measure ionization currents spot-by-spot. The depth dose measurement has a good agreement with the ground truth measured using a water tank and commercial one-dimensional (1D) multi-layer plate chamber. It can verify the spot position, energy, and relative weight of clinical PBS beams and compared with the treatment plans. Significance. The MLSIC is a highly efficient QA device that can measure the key dosimetric characteristics of proton treatment beams spot-by-spot with a single beam delivery. It may improve the quality and efficiency of clinical proton treatments.

Optimization of solar-coal hybridization for low solar augmentation

This work presents a process to determine the preliminary optimal configuration of a concentrating solar power-coal hybrid power plant with low solar augmentation, and is demonstrated on a regenerative steam Rankine cycle coal power plant in Castle Dale, UT, USA (average DNI of 542 W m−2). A representative plant model is developed and validated against published data for a coal power plant. The simplifications that lead to the representative model from a coal power plant model include combining multiple feedwater heaters, combining turbines, and using a mass-average calculation for extraction steam properties. Comparing net power generation and boiler heating estimates from the representative model to the benchmark power plant, the representative model predictions are accurate to within ±2.5% of the accepted value. Methods for quantifying solar resource based on geography and simulating a concentrating solar power field arrangement are provided and the solar contribution to electrical power output is estimated using an exergy balance. A financial model is also included to estimate the solar marginal levelized cost of electricity and payback time using a cash-flow analysis. A multi-objective optimization routine is then employed to determine the optimal configuration using the models described in this study.It is shown that a solar augmentation of >3% of boiler heating is required for a hybrid design to be considered thermodynamically feasible. However, as the augment fraction is increased, the financial benefit from fuel savings is insufficient, without a carbon tax, to offset the higher capital cost. Optimization results, constrained to a maximum solar field size of 20 ac, are also provided assuming a common carbon tax value (16 USD sh.tn.−1). The resulting optimal design for the Castle Dale Plant with a carbon tax and no premium indicates the use of parabolic trough collector technology at an augment fraction of k=9% to bypass feedwater heater 6. The resulting marginal solar levelized cost of electricity is 9.5 x 10-4 USD kWh−1 with an estimated payback time of 25.2 years. With a green energy premium price of 0.018 USD kWh−1 and no carbon tax, the payback time reduces to 3.9 years. This process can be applied to a subcritical Rankine cycle coal power plant for which operating data and meteorological data are available to evaluate preliminary hybridization feasibility.

Beam commissioning of the first clinical biology‐guided radiotherapy system

This study reports the beam commissioning results for the first clinical RefleXion Linac. Methods: The X1 produces a 6 MV photon beam and the maximum clinical field size is 40 × 2 cm2 at source‐to‐axis distance of 85 cm. Treatment fields are collimated by a binary multileaf collimator (MLC) system with 64 leaves with width of 0.625 cm and y‐jaw pairs to provide either a 1 or 2 cm opening. The mechanical alignment of the radiation source, the y‐jaw, and MLC were checked with film and ion chambers. The beam parameters were characterized using a diode detector in a compact water tank. In‐air lateral profiles and in‐water percentage depth dose (PDD) were measured for beam modeling of the treatment planning system (TPS). The lateral profiles, PDDs, and output factors were acquired for field sizes from 1.25 × 1 to 40 × 2 cm2 field to verify the beam modeling. The rotational output variation and synchronicity were tested to check the gantry angle, couch motion, and gantry rotation. Results: The source misalignments were 0.049 mm in y‐direction, 0.66% out‐of‐focus in x‐direction. The divergence of the beam axis was 0.36 mm with a y‐jaw twist of 0.03°. Clinical off‐axis treatment fields shared a common center in y‐direction were within 0.03 mm. The MLC misalignment and twist were 0.57 mm and 0.15°. For all measured fields ranging from the size from 1.25 × 1 to 40 × 2 cm2, the mean difference between measured and TPS modeled PDD at 10 cm depth was −0.3%. The mean transverse profile difference in the field core was −0.3% ± 1.1%. The full‐width half maximum (FWHM) modeling was within 0.5 mm. The measured output factors agreed with TPS within 0.8%. Conclusions: This study summarizes our specific experience commissioning the first novel RefleXion linac, which may assist future users of this technology when implementing it into their own clinics.

Monte Carlo Modeling and Verification of 6 MV Linear Accelerator

Purpose: To model the ELEKTA COMPACT accelerator head by using EGSnrc/BEAMnrc/DOSXYZnrc and to validatethe simulation according to the depth-dose and lateral profiles of different radiation fields measured by the water phantom. Methods: IBA Blue Water Phantom2 and CC13 Ionization Chamber were used to measure the depth-dose curves at 10 cm × 10 cm field and profile curves at 10 cm depth underwater. In BEAMnrc, the main components of accelerator head and the initial electron beam are established based on the specifications file, and the phase space file containing the photon beam information is generated. In DOXYZnrc, phase space files were used to irradiate a homogeneous water phantom of the same size as the IBA water phantom, and the simulated percentage depth dose curves and lateral profiles were outputted. The accuracy of the model was evaluated by mean square error (MSE) compared with the measured data. PDD curves are used to determine the energy of the initial electron beam. Dose profile curves are used to adjust the flattening filter. The penumbra on lateral profiles is used to adjust the full-width half-maximum (FWHM) of the electron source. Result: The electron energy of 5.8 MeV was considered the best match after comparing the PDD curves of 5.6 - 6.2 MeV electron beams. The flattening filter can only be adjusted by trial. In the final result, the maximum fluctuation of profile curve within 80% of the maximum field size is less than 3%, which meets the requirements of field flatness. The optimum FWHM for different fields is not consistent due to the Transmission penumbra. But a match can be approached by adjusting the FWHM every 10 cm field size.

Small-field measurement and Monte Carlo model validation of a novel image-guided radiotherapy system.

The RefleXion™ X1 is a novel radiotherapy system that is designed for image-guided radiotherapy, and eventually, biology-guided radiotherapy (BgRT). BgRT is a treatment paradigm that tracks tumor motion using real-time positron emission signals. This study reports the small-field measurement results and the validation of a Monte Carlo (MC) model of the first clinical RefleXion unit. The RefleXion linear accelerator (linac) produces a 6 MV flattening filter free (FFF) photon beam and consists of a binary multileaf collimator (MLC) system with 64 leaves and two pairs of y-jaws. The maximum clinical field size achievable is 400×20 mm2 . The y-jaws provide either a 10 or 20mm opening at source-to-axis distance (SAD) of 850mm. The width of each MLC leaf at SAD is 6.25mm. Percentage depth doses (PDDs) and relative beam profiles were acquired using an Edge diode detector in a water tank for field sizes from 12.5×10 to 100×20 mm2 . Beam profiles were also measured using films. Output factors of fields ranging from 6.25×10 to 100×20 mm2 were measured using W2 scintillator detector, Edge detector, and films. Output correction factors k of the Edge detector for RefleXion were calculated. An MC model of the linac including pre-MLC beam sources and detailed structures of MLC and lower y-jaws was validated against the measurements. Simulation codes BEAMnrc and GATE were utilized. The diode measured PDD at 10cm depth (PDD10) increases from 53.6% to 56.9% as the field opens from 12.5×10 to 100×20 mm2 . The W2-measured output factor increases from 0.706 to 1 as the field opens from 6.25×10 to 100×20 mm2 (reference field size). The output factors acquired by diode and film differ from the W2 results by 1.65% (std=1.49%) and 2.09% (std=1.41%) on average, respectively. The profile penumbra and full-width half-maximum (FWHM) measured by diode agree well with the film results with a deviation of 0.60mm and 0.73% on average, respectively. The averaged beam profile consistency calculated between the diode- and film-measured profiles among different depths is within 1.72%. By taking the W2 measurements as the ground truth, the output correction factors k for Edge detector ranging from 0.958 to 1 were reported. For the MC model validation, the simulated PDD10 agreed within 0.6% to the diode measurement. The MC-simulated output factor differed from the W2 results by 2.3% on average (std=3.7%), while the MC simulated beam penumbra differed from the diode results by 0.67mm on average (std=0.42mm). The MC FWHM agreed with the diode results to within 1.40% on average. The averaged beam profile consistency calculated between the diode and MC profiles among different depths is less than 1.29%. This study represents the first small-field dosimetry of a clinical RefleXion system. A complete and accurate MC model of the RefleXion linac has been validated.

Acceptance procedure for the linear accelerator component of the 1.5 T MRI-linac.

PurposeTo develop and implement an acceptance procedure for the new Elekta Unity 1.5 T MRI‐linac.MethodsTests were adopted and, where necessary adapted, from AAPM TG106 and TG142, IEC 60976 and NCS 9 and NCS 22 guidelines. Adaptations were necessary because of the atypical maximum field size (57.4 × 22 cm), FFF beam, the non‐rotating collimator, the absence of a light field, the presence of the 1.5 T magnetic field, restricted access to equipment within the bore, fixed vertical and lateral table position, and the need for MR image to MV treatment alignment. The performance specifications were set for stereotactic body radiotherapy (SBRT).ResultsThe new procedure was performed similarly to that of a conventional kilovoltage x‐ray (kV) image guided radiation therapy (IGRT) linac. Results were acquired for the first Unity system.ConclusionsA comprehensive set of tests was developed, described and implemented for the MRI‐linac. The MRI‐linac met safety requirements for patients and operators. The system delivered radiation very accurately with, for example a gantry rotation locus of isocenter of radius 0.38 mm and an average MLC absolute positional error of 0.29 mm, consistent with use for SBRT. Specifications for clinical introduction were met.

Characteristics of very high-energy electron beams for the irradiation of deep-seated targets.

Driven by advances in accelerator technology and the potential of exploiting the FLASH effect for the treatment of deep-seated targets (>5cm), there is an active interest in the construction of devices to deliver very high-energy electron (VHEE) beams for radiation therapy. The application of novel VHEE devices, however, requires an assessment of the tradeoffs between the different beam parameter choices including beam energies, beam divergences, and maximal field sizes. This study systematically examines the dosimetric beam properties of VHEE beams, determining their clinical usefulness while marking their limits of applications for different beam configurations. We performed Monte Carlo simulations of the dose distributions of electron beams for different energies (25-250MeV), source-to-surface distances (SSD) (50cm, 100cm, parallel), and field sizes (2cm2 ×2cm2 to 15cm2 ×15cm2 ) in water using a research version of the RayStation treatment planning system (RaySearch Labs 9A IONPG). The beam was simulated using a monoenergetic point source and perfect collimation. Central axis percentage depth dose (PDD) and transverse dose profiles at multiple depths were evaluated and compared to those of MV photon beams. Profile characteristics including therapeutic range (TR) at 90%, proximal fall-off (PFO) at 90%, lateral penumbra (LP) at 90%-10%, and field width (FW) at 90% were obtained. Very high-energy electrons beams with SSD 100cm and parallel beams (infinite SSD) exhibit a linear to near-linear increase of TR as a function of energy in the simulated energy range and reach values well beyond the typical depths of lesions encountered in clinics (<20cm). Their TR show a marked field size dependence only for field sizes <10cm2 ×10cm2 . For VHEE beams with SSD 50cm, TR are largely reduced (4-8cm). For beam energies >150MeV with large SSD (>100cm), for many configurations, there is no substantial difference in PDD when adding an opposed beam. This may potentially reduce the number of VHEE beams needed for treatment by a factor of two compared to a treatment using lower energies and lower SSD. In order to cover deep-seated targets homogeneously, VHEE devices with a parallel beam must provide a maximum field size up to several centimeters larger than the tumor size. For the investigated diverging beams, there is not such a significant field width reduction with depth for larger fields as it is compensated by divergence. Penumbrae of VHEE beams are smaller than those of clinical MV photon beams for lower depths (<5cm) but increase quickly for larger depths. There is only a relatively small dependence of penumbra on the SSD of the beam. The findings presented in this study assess the performance of VHEE beams and offer a first estimate of treatment indications and tradeoffs for a given design of a VHEE device. SSD >100cm results in clinically more favorable PDD. Beam energies of 100MeV and above are needed to cover common tumors (5-15cm in-depth) conformally. Higher energies provide an additional benefit specifically for small and deep-seated lesions due to their reduced lateral penumbrae.

Dosimetric accuracy of dual isocenter irradiation in low magnetic field resonance guided radiotherapy system for extended abdominal tumours.

Due to limited field size of Magnetic Resonance Linear Accelerators (MR-Linac), some treatments could require a dual-isocenter planning approach to achieve a complete target coverage and thus exploit the benefits of the online adaptation. This study evaluates the dosimetric accuracy of the dual-isocenter intensity modulated radiation therapy (IMRT) delivery technique for MR-Linac. Dual-isocenter multi leaf collimator (MLC) and couch accuracy tests have been performed to evaluate the delivery accuracy of the system. A mono-isocenter plan delivered in clinical practice has then been retrospectively re-planned with dual-isocenter technique. The dual-isocenter plan has been re-calculated and delivered on a 3-dimensional (3D) ArcCHECK phantom and 2-dimensional (2D) films to assess its dosimetric accuracy in terms of gamma analysis. Clinical and planning target volume (CTV and PTV respectively) coverage robustness was then investigated after the introduction of±2mm and±5mm positioning errors by shifting the couch. MLC and couch accuracy tests confirmed the system accuracy in delivering a dual-isocenter irradiation. 2D/3D gamma analysis results occurred always to be above 95% if considered a gamma criteria 1%/2 mm and 1%/1 mm respectively for the 2D and 3D analysis. The mean variations for CTV D98% and PTV V95% were 0.2% and 1.1% respectively when positioning error was introduced separately in each direction, while the maximum observed variations were 0.9% (CTV) and 3.7% (PTV). The dosimetric accuracy of dual-isocenter irradiation has been verified for MR-Linac, achieving accurate and robust treatment strategy and improving dose conformality also in presence of targets whose extension exceeds the nominal maximum field size.

Medical Applications of the GEMPix

The GEMPix is a small gaseous detector with a highly pixelated readout, consisting of a drift region, three Gas Electron Multipliers (GEMs) for signal amplification, and four Timepix ASICs with 55 µm pixel pitch and a total of 262,144 pixels. A continuous flow of a gas mixture such as Ar:CO2:CF4, Ar:CO2 or propane-based tissue equivalent gas is supplied externally at a rate of 5 L/h. This article reviews the medical applications of the GEMPix. These include relative dose measurements in conventional photon radiation therapy and in carbon ion beams, by which on-line 2D dose images provided a similar or better performance compared to gafchromic films. Depth scans in a water phantom with 12C ions allowed measuring the 3D energy deposition and reconstructing the Bragg curve of a pencil beam. Microdosimetric measurements performed in neutron and photon fields allowed comparing dose spectra with those from Tissue Equivalent Proportional Counters and, additionally, to obtain particle track images. Some preliminary measurements performed to check the capabilities as the detector in proton tomography are also illustrated. The most important on-going developments are: (1) a new, larger area readout to cover the typical maximum field size in radiation therapy of 20 × 20 cm2; (2) a sealed and low-pressure version to facilitate measurements and to increase the equivalent spatial resolution for microdosimetry; (3) 3D particle track reconstruction when operating the GEMPix as a Time Projection Chamber.

Development and assessment of a quality assurance device for radiation field-light field congruence testing in diagnostic radiology.

Purpose: Existing methods for checking the light field-radiation field congruence on x-ray equipment either do not fully meet the conditions of various quality control standards regarding inherent uncertainty requirements or contain subjective steps, further increasing the uncertainty of the end result. The aim of this work was to develop a method to check the light field-radiation field congruence on all x-ray equipment. The result should have a low uncertainty which is accomplished by eliminating most subjective user steps in the method. A secondary aim was to maintain the same level of usability as of comparable methods but still able to store the result. Approach: A new device has been developed where the light field and corresponding radiation field are monitored through measurements of the field edge locations (in total: ). The maximum field size location deviation between light field and radiation field in the new method is constrained by the physical limitations of the sensors used in various versions of the prototype: linear image sensors (LISs) of 25 to 29mm active sensor length. The LISs were sensitized to x-rays by applying a phosphor strip of covering the light sensor input area. Later prototypes of the completed LIS device also have the option of a Bluetooth (100-m range standard) connection, thus increasing the mobility. Results: The developed device has a special feature of localization a field edge without any prior, subjective, alignment procedure of the user, i.e., the signals produced were processed by software storing the associated field edge profiles, localizing the edges in them, and finally displaying the calculated deviation. The uncertainty in field edge location difference was estimated to be ( ). The calculated uncertainty is lower than for other, commercially available, methods for light field-radiation field congruence also presented in this work. Conclusions: A method to check the light field-radiation field congruence of x-ray systems was developed to improve the limitations found in existing methods, such as device detector resolution, subjective operator steps, or the lack of storing results for later analysis. The development work overcame several challenges including mathematically describing real-life edges of light and radiation fields, noise reduction of radiation edges, and mapping/quantification of the rarely observed phenomenon of focal spot wandering. The assessment of the method showed that the listed limitations were overcome, and the aims were accomplished. It is therefore believed that the device can improve the work in quality controls of x-ray systems.

Dosimetric and mechanical equivalency of Varian TrueBeam linear accelerators.

PurposeTo investigate and improve the level of equivalency of Varian TrueBeam linear accelerators (linacs) in energy‐, dosimetric leaf gap‐ (DLG) and jaw calibration.MethodsEight linacs with four photon energies: 6 MV, 6 MV FFF, 10 MV FFF, and 15 MV, and three electron energies (on two linacs): 6, 9, and 12 MeV were commisioned and beam‐matched. Initially, symmetry of lateral profiles was calibrated for maximum field size. Energy‐matching was then performed for photons by adjusting diagonal profiles at maximum field size and depth of maximum dose to coincide with the reference linac, and for electrons by matching the range at percentage depth of ionization of 90%, 80%, and 50%. Calibration of DLG was performed for 6 MV and evaluated among the linacs. The relationship between DLG and the Gap value was investigated. A method using electronic portal imaging device (EPID) was developed and implemented for jaw calibration.ResultsSymmetry calibration for photons (electrons) was within 1% (0.7%), further improving the vendor's acceptance criteria. Photon and electron energy‐matching was within 0.5% and 0.1 mm, respectively. Calibration of DLG was within 0.032 mm among the linacs and utilizing the relationship between DLG and the Gap value resulted in an empirical calibration method which was implemented to simplify DLG adjustment. Using EPID‐based method of calibration, evaluation of the jaw‐positioning among the linacs for 30 cm × 30 cm field size was within 0.4 mm and in the junction area within 0.2 mm. Dose delivery error of VMAT‐plans were at least 99.2% gamma pass rate (1%, 1 mm).ConclusionsHigh level of equivalency, beyond clinically accepted criteria, of TrueBeam linacs could be achieved which reduced dose delivery systematic errors and increased confidence in interchanging patients among linacs.

Technical and dosimetric realization of in vivo x-ray microbeam irradiations at the Munich Compact Light Source.

X-ray microbeam radiation therapy is a preclinical concept for tumor treatment promising tissue sparing and enhanced tumor control. With its spatially separated, periodic micrometer-sized pattern, this method requires a high dose rate and a collimated beam typically available at large synchrotron radiation facilities. To treat small animals with microbeams in a laboratory-sized environment, we developed a dedicated irradiation system at the Munich Compact Light Source (MuCLS). A specially made beam collimation optic allows to increase x-ray fluence rate at the position of the target. Monte Carlo simulations and measurements were conducted for accurate microbeam dosimetry. The dose during irradiation is determined by a calibrated flux monitoring system. Moreover, a positioning system including mouse monitoring was built. We successfully commissioned the in vivo microbeam irradiation system for an exemplary xenograft tumor model in the mouse ear. By beam collimation, a dose rate of up to 5.3Gy/min at 25keV was achieved. Microbeam irradiations using a tungsten collimator with 50μm slit size and 350μm center-to-center spacing were performed at a mean dose rate of 0.6Gy/min showing a high peak-to-valley dose ratio of about 200 in the mouse ear. The maximum circular field size of 3.5mm in diameter can be enlarged using field patching. This study shows that we can perform in vivo microbeam experiments at the MuCLS with a dedicated dosimetry and positioning system to advance this promising radiation therapy method at commercially available compact microbeam sources. Peak doses of up to 100Gy per treatment seem feasible considering a recent upgrade for higher photon flux. The system can be adapted for tumor treatment in different animal models, for example, in the hind leg.

Feasibility of proton FLASH irradiation using a synchrocyclotron for preclinical studies.

It has been recently shown that radiotherapy at ultrahigh dose rates (>40Gy/s, FLASH) has a potential advantage in sparing healthy organs compared to that at conventional dose rates. The purpose of this work is to show the feasibility of proton FLASH irradiation using a gantry-mounted synchrocyclotron as a first step toward implementing an experimental setup for preclinical studies. A clinical Mevion HYPERSCAN® synchrocyclotron was modified to deliver ultrahigh dose rates. Pulse widths of protons with 230MeV energy were manipulated from 1 to 20μs to deliver in conventional and ultrahigh dose rate. A boron carbide absorber was placed in the beam for range modulation. A Faraday cup was used to determine the number of protons per pulse at various dose rates. Dose rate was determined by the dose measured with a plane-parallel ionization chamber with respect to the actual delivery time. The integral depth dose (IDD) was measured with a Bragg ionization chamber. Monte Carlo simulation was performed in TOPAS as the secondary check for the measurements. Maximum protons charge per pulse, measured with the Faraday cup, was 54.6pC at 20μs pulse width. The measured IDD agreed well with the Monte Carlo simulation. The average dose rate measured using the ionization chamber showed 101Gy/s at the entrance and 216Gy/s at the Bragg peak with a full width at half maximum field size of 1.2cm. It is feasible to deliver protons at 100 and 200Gy/s average dose rate at the plateau and the Bragg peak, respectively, in a small~1cm2 field using a gantry-mounted synchrocyclotron.