Collimation System Research Articles

Design and dosimetric characterization of a transportable proton minibeam collimation system

BackgroundProton Minibeam Radiation Therapy has shown to widen the therapeutic window compared to conventional radiation treatment in pre-clinical studies. The underlying biological mechanisms, however, require more research.PurposeThe purpose of this study was to develop and characterize a mechanical collimation setup capable of producing 250µm wide proton minibeams with a center-to-center distance of 1000µm.MethodsTo find the optimal arrangement Monte Carlo simulations were employed using the Geant4 toolkit TOPAS to maximize key parameters such as the peak-to-valley dose ratio (PVDR) and the valley dose rate. The experimental characterization of the optimized setup was carried out with film dosimetry at the University Proton Therapy beamline in Dresden and the proton beamline of the University of Washington Medical Center in Seattle with 150MeV and 50.5MeV, respectively. A microDiamond detector (PTW, Freiburg, Germany) was utilized at both beamlines for online proton minibeam dosimetry.ResultsA PVDR of 10 was achieved in Dresden and a PVDR of 14 in Seattle. Dosimetry measurements were carried out with EBT3 films at a depth of 5mm in a polymethylmethacrylate (PMMA) phantom. When comparing film dosimetry with the microDiamond, excellent agreement was observed in the valleys. However, the peak dose showed a discrepancy of approximately 10% in the 150MeV beam and 20% in the 50.5MeV beam between film and microDiamond.DiscussionThe characteristics of the minibeams generated with our system compares well with those of other collimated minibeams despite being smaller. The deviations of microDiamond measurements from film readings might be subject to the diamond detector responding differently in the peak and valley regions. Applying previously reported correction factors aligns the dose profile measured by the microDiamond with the profile acquired with EBT3 films in Dresden.ConclusionThe novel proton minibeam system can be operated independently of specific beamlines. It can be transported easily and hence used for inter-institutional comparative studies. The quality of the minibeams allows us to perform in vitro and in vivo experiments in the future. The microDiamond was demonstrated to have great potential for online dosimetry for proton minibeams, yet requires more research to explain the observed discrepancies.

Monte Carlo source verification of Elekta Synergy for pMLC collimated electron beams

IntroductionIn previous works, an electron beam source was developed that models its energy spectrum with a Lévy distribution. This source is used in BEAMnrc Monte Carlo simulation. It was benchmarked against water tank measurements for electrons delivered through an applicator collimation system, and it was found to agree with 2%/2 mm. This study uses this electron source further by investigating its ability to simulate electrons collimated by the photon multi-leaf collimators for MERT applications. Simulated and Gafchromic EBT3 depth dose and profile data were compared. MethodMeasured data were obtained from film exposures in a mini phantom with water-equivalent slabs at SSDs of 60 and 70 cm, respectively. Output factors and R90 values were calculated from film strip data taken in the depth direction. Corresponding dose data were simulated in BEAMnrc to obtain phase-space files for each beam and used in DOSXYZnrc to obtain dose data in a 3D water tank with 2 mm side length subvoxels. ResultsSimulation data corresponded with film data within 3 %/2 mm in most cases. Uncertainties in film measurements prevent a more accurate agreement. The output factors showed good agreement and could be fitted satisfactorily with an analytic function. A clinical example of a scalp treatment showed good agreement between simulated and film data. ConclusionIt is concluded that the electron source with the Lévy-based energy distribution can model electron dose for photon multi-leaf collimated electron beams within 3 %/2 mm.

The smearing function for a multi-slit very small angle neutron scattering instrument

Besides traditional pinhole geometry, the multi-slit very small angle neutron scattering instrument (MS-VSANS) at the China Spallation Neutron Source also utilizes a multi-slit collimation system to focus neutrons. Using the special focusing structures, the minimum scattering vector magnitude (q) can reach 0.00028 Å−1. The special structures also lead to a significantly different smearing function. By comparing the results of theoretical calculations with experimental data, we have validated the feasibility of a smearing method based on a mature theory for slit smearing. We use the weight-averaged intensity of neutron wavelength as a representative to evaluate the effect from a broad wavelength distribution, concentrating on the effect from the geometry of the multi-slit structures and the detector. The consistency of the theoretical calculation of the smearing function with experimental VSANS scattering profiles for a series of polystyrene standards of different diameters proves the feasibility of the smearing method. This marks the inaugural use of real experimental data from an instrument employing a multi-slit collimation system.

A Long-Range Lidar Optical Collimation System Based on a Shaped Laser Source

Semiconductor near-infrared lasers have been widely used in lidar systems. However, various source types have different shapes and divergence angles, causing more difficulties for long-distance detection. In this paper, an optical collimation system is designed for a long-range lidar system with a shaped laser source (the wavelength is 905 nm, the emitted spot size is 50 µm long by 10 µm wide, and the divergence angles are 33°and 15°, respectively, which are unconventional). On the basis of the traditional method of aspheric lens setting, a pair of asymmetric aspherical lenses were designed using an extended polynomial. The simulation results show that the spot shapes are all close to circular from 100 mm to 30 m and the spot size always remains the same value. The corrected optical system is put into the designed lidar system for verification. It results show that the average divergence angles in the long and short axis directions are 0.06°and 0.07°, which satisfy the project requirements. This optical system designed provides a collimation scheme and expands the application of vehicle-mounted lidar in the field of long-range detection.

Exploring the potential of resonance islands and bent crystals for a slow extraction from circular hadron accelerators

New developments in accelerator physics have broadened the set of available techniques for manipulating charged-particle beams. Adiabatic trapping and transport of a beam in resonance islands has been studied and successfully implemented at the CERN Proton Synchrotron to perform multiturn extraction. Bent crystals have been successfully installed in the CERN Large Hadron Collider, improving the cleaning performance of the collimation system, and at the CERN Super Proton Synchrotron for reducing losses at the extraction septum in the case of slow extraction. In this Letter, we discuss the potential of the combined use of resonance islands and bent crystals to devise a technique to perform slow extraction in circular hadron accelerators. The proposed approach is promising, particularly for applications with high-intensity beams, as it could dramatically reduce the losses on the extraction devices. Published by the American Physical Society 2024

Semiconductor laser collimation and shaping design based on the combination of a rotationally symmetrical lens and a cylindrical lens

A dual-lens collimation and shaping system which is composed of a rotationally symmetric lens and an aspherical cylindrical lens is proposed in this paper. The system design method is discussed in detail, and the divergent elliptical laser beams are finally converted into collimated circular beams. The simulation results show that the maximum divergence angle of the beams can be collimated to 2.58 µrad, the standard deviation of the edge beam radius samples (SDRS) can be decreased from the original 4.227 to 0.135 mm, and the ratio of beam waist (RBW) drops from 1.554 to 1.0093 in the case of the output beam radius of 40 mm. The effects of transmission distance, astigmatism, wavelength deviation, light source offset, and lens offset on the collimation shaping effect are discussed. The collimation system can be widely used in long-distance optical communication systems and the design method in this paper can provide some new ideas for optical design researchers.

Dosimetric calibration of anatomy-specific ultra-high dose rate electron irradiation platform for preclinical FLASH radiobiology experiments.

FLASH radiation therapy (RT) offers a promising avenue for the broadening of the therapeutic index. However, to leverage the full potential of FLASH in the clinical setting, an improved understanding of the biological principles involved is critical. This requires the availability of specialized equipment optimized for the delivery of conventional (CONV) and ultra-high dose rate (UHDR) irradiation for preclinical studies. One method to conduct such preclinical radiobiological research involves adapting a clinical linear accelerator configured to deliver both CONV and UHDR irradiation. We characterized the dosimetric properties of a clinical linear accelerator configured to deliver ultra-high dose rate irradiation to two anatomic sites in mice and for cell-culture FLASH radiobiology experiments. Delivered doses of UHDR electron beams were controlled by a microcontroller and relay interfaced with the respiratory gating system. We also produced beam collimators with indexed stereotactic mouse positioning devices to provide anatomically specific preclinical treatments. Treatment delivery was monitored directly with an ionization chamber, and charge measurements were correlated with radiochromic film measurements at the entry surface of the mice. The setup for conventional dose rate irradiation utilized the same collimation system but at increased source-to-surface distance. Monte Carlo simulations and film dosimetry were used to characterize beam properties and dose distributions. The mean electron beam energies before the flattening filter were 18.8 MeV (UHDR) and 17.7 MeV (CONV), with corresponding values at the mouse surface of 17.2 and 16.2 MeV. The charges measured with an external ion chamber were linearly correlated with the mouse entrance dose. The use of relay gating for pulse control initially led to a delivery failure rate of 20% (±1 pulse); adjustments to account for the linac latency improved this rate to<1/20. Beam field sizes for two anatomically specific mouse collimators (4 × 4 cm2 for whole-abdomen and 1.5 × 1.5 cm2 for unilateral lung irradiation) were accurate within<5% and had low radiation leakage (<4%). Normalizing the dose at the center of the mouse (∼0.75cm depth) produced UHDR and CONV doses to the irradiated volumes with>95% agreement. We successfully configured a clinical linear accelerator for increased output and developed a robust preclinical platform for anatomically specific irradiation, with highly accurate and precise temporal and spatial dose delivery, for both CONV and UHDR irradiation applications.

A mini review of plan quality and secondary cancer risk in CyberKnife M6 radiosurgery for benign intracranial tumors.

With advancements in medical technology, stereotactic radiosurgery (SRS) has become an essential option for treating benign intracranial tumors. Due to its minimal side effects and high local control rate, SRS is widely applied. This paper evaluates the plan quality and secondary cancer risk (SCR) in patients with benign intracranial tumors treated with the CyberKnife M6 system. The CyberKnife M6 robotic radiosurgery system features both multileaf collimator (MLC) and IRIS variable aperture collimator systems, providing different treatment options. The study included 15 patients treated with the CyberKnife M6 system, examining the differences in plan quality and SCR between MLC and IRIS systems. Results showed that MLC and IRIS plans had equal PTV (planning target volume) coverage (98.57% vs. 98.75%). However, MLC plans demonstrated better dose falloff and conformity index (CI: 1.81 ± 0.26 vs. 1.92 ± 0.27, P = 0.025). SCR assessment indicated that MLC plans had lower cancer risk estimates, with IRIS plans having average LAR (lifetime attributable risk) and EAR (excess absolute risk) values approximately 25% higher for cancer induction and 15% higher for sarcoma induction compared to MLC plans. The study showed that increasing tumor volume increases SCR probability, but there was no significant difference between different plans in PTV and brainstem analyses.

Determination of position and size of non-flaws at albedo flaw detection

In this article the methods of determining the position and size of non-flaws in albedo flaw detection are considered. Analytical and numerical solutions of the problem of determining the location of non-flaws on the basis of known parameters of the collimation system are shown. The dependence of the location of the flaw on the parameters of the collimation system is shown. It is proposed to determine not the true size of the flaw, but its equivalent area, similar to ultrasonic flaw detection.

Photoneutron production mechanisms, their characteristics, and shielding strategies in high-energy linac environment: A review

Radiotherapy, a mainstay cancer treatment for patients worldwide, utilizes medical linear accelerators (linacs) with photon energies ranging from 4 MV to 25 MV. However, concerns arise at higher energies (>6 MV, particularly >10 MV). These high-energy photons interact with high-atomic number materials in the linac head and collimation system, generating unwanted neutrons through (γ,n) reactions. This neutron contamination, present in both photon and electron beams, is a significant issue. Neutrons, with their high Linear Energy Transfer (LET), are more effective at causing clustered DNA damage (single and double-strand breaks). These neutrons not only impact shielding requirements in treatment rooms but also increase out-of-field radiation doses for patients receiving high-energy photon therapy. Therefore, for radiotherapy treatments exceeding 6 MV, additional precautions become crucial, including enhanced door shielding and optimized treatment planning. This review discusses in detail the multifaceted aspects of neutron production and shielding requirements during radiotherapy.

Temperature distribution in the PGAA system: Collimator, shutter, and filter in TRIGA Mark II reactor

This article examines the impact of temperature on the steel collimator cap and the primary beam shutter. These components will be used to implement the Prompt Gamma Activation Analysis (PGAA) technique in the Moroccan TRIGA Mark-II research reactor. The steel collimator plug is essential for forming the neutron beam, while the main role of the shutter is to stop the beam when the channel is inactive. This study analyzes the effect of temperature on the collimator and shutter system, particularly focusing on the variation in maximum temperature over a month of operation with 8-h cycles per day, the behavior of temperature over 24 h, the total heat flux as a function of the length of the experimental device, the temperature distribution in mild steel (E235) and 304L stainless steel materials, and the total displacement and strain gradient as a function of temperature. All calculations were performed using COMSOL Multiphysics simulation software, based on the finite element method.

Patient-specific quality assurance of dynamically-collimated proton therapy treatment plans.

The dynamic collimation system (DCS) provides energy layer-specific collimation for pencil beam scanning (PBS) proton therapy using two pairs of orthogonal nickel trimmer blades. While excellent measurement-to-calculation agreement has been demonstrated for simple cube-shaped DCS-trimmed dose distributions, no comparison of measurement and dose calculation has been made for patient-specific treatment plans. To validate a patient-specific quality assurance (PSQA) process for DCS-trimmed PBS treatment plans and evaluate the agreement between measured and calculated dose distributions. Three intracranial patient cases were considered. Standard uncollimated PBS and DCS-collimated treatment plans were generated for each patient using the Astroid treatment planning system (TPS). Plans were recalculated in a water phantom and delivered at the Miami Cancer Institute (MCI) using an Ion Beam Applications (IBA) dedicated nozzle system and prototype DCS. Planar dose measurements were acquired at two depths within low-gradient regions of the target volume using an IBA MatriXX ion chamber array. Measured and calculated dose distributions were compared using 2D gamma analysis with 3%/3mm criteria and low dose threshold of 10% of the maximum dose. Median gamma pass rates across all plans and measurement depths were 99.0% (PBS) and 98.3% (DCS), with a minimum gamma pass rate of 88.5% (PBS) and 91.2% (DCS). The PSQA process has been validated and experimentally verified for DCS-collimated PBS. Dosimetric agreement between the measured and calculated doses was demonstrated to be similar for DCS-collimated PBS to that achievable with noncollimated PBS.

Characterization of accurate 3D collimator-detector response function for single- and multi-lofthole collimated SPECT cameras.

Collimator-detector response function (CDRF) of a SPECT scanner refers to the image generated from a point source of activity. This research aims to characterize the CDRF of a breast-dedicated SPECT imager equipped with a lofthole collimator using GATE Monte Carlo simulation. To do so, a cylindrical multi-lofthole collimation system with lofthole apertures dedicated to breast imaging was modeled using the GATE Monte Carlo simulator. The dependency of the CDRF on the source-to-collimator distance of a single-lofthole as well as 8-lofthole collimations was assessed and then compared. In addition, the 3D-sensitivity map of the 8-lofthole collimation was derived. Finally, fair comparisons were conducted between the response of the 8-lofthole collimator and that of an 8-pinhole and also existing analytical derivations. In all cases, a data acquisition period of 5.0 min with an in-air 99mTc point source was considered. For the single-lofthole collimator, 4.5 times increasing the magnification factor leads to a 16- and twofold improvement in the sensitivity and spatial resolution, respectively. In the single-lofthole collimator, the resolution and sensitivity are degraded as the source-to-aperture distance increases. For the cylindrical 8-lofthole collimator, the findings confirm that CDRF strongly depends on source-to-aperture distance and angle of photon incidence. For a 30 mm in-plane offset point, a 25% increase in sensitivity is observed compared to that of the center of the FOV. Increasing the angle from 0 to 34 results in a 50% reduction in sensitivity. Furthermore, the findings illustrate that spatial resolution follows a quadratic function as where d is an offset along the x-, y-, and z-axis, and R0 is the spatial resolution at the center of the FOV. In conclusion, both spatial resolution and sensitivity of the lofthole collimation are considerably angle- and offset-dependent within the FOV of single- and multi-lofthole collimated SPECT imagers.

A Projection Integration Method for X-ray Collimator Design.

One of the recent trends in radiation therapy is to increaseconformal and accurate dose deliverysuch as in stereotactic radiosurgery (SRS). Treating small lesions and brain disorders requires the accurate placement of small radiation fields deep inside the human cranium.To design a collimator meeting these requirements, a new numerical concept was developed, which is presented here. The algorithm proposed here can generate beam profiles of plural collimation apertures and arbitrary initial beam spot distributions in a time-efficient method. It is an ideal tool to optimize collimator design for penumbra, dose rate, and field size. The intensity of the source beam spot is divided into slices, and each slice is projected onto the treatment plane at the isocenter through the collimator apertures.The illuminated field range and intensity are determined by geometry and the intensity of that slice of beam source, respectively.By integrating the projected intensity across all the slicesof the source profile, the profile on the treatment plane is obtained. The algorithm is used to generate beam profiles of a conical pencil beam collimator system and compare them to the Monte Carlo simulation as well as measurements. It canalso be used to demonstrate the impact of collimator shape on the beam penumbra, dose rate, and field size. The projection integration method provides a quick and informative tool for collimator design. The results were validated with the Monte Carlo simulation and measurements. This method was demonstrated to be effective for optimizing beam characteristics.

Analysis of Maintenance Methods and Process Improvement of Vacuum Laser Collimation System for Dams

The method and process of vacuum laser collimation system maintenance is one of the key technologies directly related to the accuracy of dam vacuum laser collimation system measurement value. Taking the maintenance methods and processes of vacuum laser collimation system in Lianlu and Xiacheng as the background, a new set of dam vacuum laser collimation system maintenance methods and processes are proposed, aiming to make up for the gaps in the maintenance of vacuum laser collimation system. It was found that the vacuum pipeline operates at vacuum values between 400 Pa and 900 Pa. The influence of light on the vacuum laser is avoided by correcting the system and shading the transmitting end as well as the connecting part of the receiving end and the vacuum pipeline with a rubber sheet. This new maintenance method and process also provides a useful reference for the safety monitoring of similar projects.

High-accuracy attitude angle measurement system using laser collimation

We propose a high-precision attitude angle measurement system utilizing laser collimation to rectify environmental errors that significantly impact the precision of high-accuracy star sensors. Environmental factors, primarily temperature variations, can induce shifts in the optical axis of star sensors deployed in orbit, leading to errors in attitude angle measurements. Leveraging the conventional star sensor architecture, we have augmented the system with an additional laser path and a four-quadrant detector (4QD) positioned in the image plane. This setup converts angular deviations of the optical axis into positional shifts of the laser spot, facilitating the measurement of environmental errors. Both simulations and experimental findings demonstrate that the laser collimation system effectively mitigates environmental errors, thereby enhancing the attitude accuracy of the star sensors.

The IAEA remote audit of small field dosimetry for testing the implementation of the TRS-483 code of practice.

The TRS‑483, an IAEA/AAPM International Code of Practice on dosimetry of small static photon fields, underwent testing via an IAEA coordinated research project (CRP). Alongside small field output factors (OFs) measurements using active dosimeters by CRP participants, the IAEA Dosimetry Laboratory received a mandate to formulate a remote small field dosimetry audit method using its passive dosimetry systems. This work aimed to develop a small field dosimetry audit methodology employing radiophotoluminescent dosimeters (RPLDs) and radiochromic films. The methodology was subsequently evaluated through a multicenter pilot study with CRP participants. The developments included designing and manufacturing a dosimeter holder set and the characterization of an RPLD system for measurements in small photon fields using the new holder. The audit included verification of small field OFs and lateral beam profiles for small fields. At first, treatment planning system (TPS) calculated OFs were checked against a reference data set that was available for conventional linacs. Second, calculated OFs were verified through the RPLD measurement of point doses in a machine-specific reference field, 4cm × 4cm, 2cm × 2cm, and 1cm × 1cm, corresponding size circularfields or nearest achievable field sizes. Lastly, profile checks in in-plane and cross-plane directions were done for the two smallest fields by comparing film measurements with TPS calculations at 20%, 50%, and 80% isodose levels. RPLD correction factors for small field measurements were approximately unity. However, they influenced the dose determination's overall uncertainty in small fields, estimated at 2.30% (k=1 level). Considering the previous experience in auditing reference beam output following the TRS-398 Code of Practice, the acceptance limit of 5% for the ratio of the dose determined by RPLD to the dose calculated by TPS, DRPLD/DTPS, was considered adequate. The multicenter pilot study included 15 participants from 14 countries (39 beams). Consistent with the previous findings, the results of the OF check against the reference data confirmed that TPSs tend to overestimate OFs for the smallest fields included in this exercise. All except three RPLD measurement results were within the acceptance limit, and the spread of results increased for smaller field sizes. The differences between the film measured and TPS calculated dose profiles were within 3mm for most of the beams checked; deviated results revealed problems with TPS commissioning and calibration of the treatment unit collimation systems. The newly developed small field dosimetry audit methodology proved effective and successfully complemented the CRP OF measurements by participants with RPLD audit results.

Design of a long depth of focus Gaussian beam shaping system via aspheric lenses

This paper presents a beam shaping system that uses a Galilean beam shaper. The light mapping function and the aspherical coefficient, which determines the characteristics of the aspheric lens, are derived based on the law of conservation of energy. Furthermore, optical software (i.e., ZEMAX) was utilized to simulate the design of the shaping system. In addition, a spherical collimation system is introduced to achieve the shaping system with long depth of focus. The results show that the output flat-top beam has the uniformity over 90% within the 300mm depth of focus range and the divergence angle no more than 0.6°, which is in line with the theoretical expectation. Compared to the previous works, our beam shaping system is characterized by long depth of focus, simple structure and good beam shaping effect, which makes the present shaping beam more flexible for the application in the field of laser processing.

High intensity beam dynamics assessment and challenges for HL-LHC

The High Luminosity (HL-LHC) project aims to increase the integrated luminosity of CERN's Large Hadron Collider (LHC) by an order of magnitude compared to its initial design. This requires a large increase in bunch intensity and beam brightness compared to the first three LHC runs, and hence poses serious collective-effects challenges, related in particular to electron cloud, instabilities from beam-coupling impedance, and beam-beam effects. Here, we present the associated constraints and the mitigation measures proposed to achieve the baseline performance of the upgraded LHC machine. We also discuss the interplay of these mitigation measures with other aspects of the accelerator, such as optics, physical and dynamic apertures, the collimation system, and crab cavities. Additional potential sources of intensity limitations are also briefly discussed.

Performance study of the dynamic multi-leaf collimator system (DMLC-120)

BackgroundOur research focused on the design of the Dynamic Multi-Leaf Collimator (DMLC-120), which can be installed on the medical linear accelerator (KylinRay-MELAC). This innovative device, consisting of 60 leaf pairs, aims to significantly enhance the precision of radiation therapy. PurposeTo address the challenge of high leakage rates observed in traditional arc-end multi-leaf collimators upon leaf closure, we introduced a novel design that combines arc and flat end-face structures. This study evaluates the performance of this composite design, intending to establish a new standard for clinical dose modeling and planning systems, thereby improving the accuracy of radiotherapy delivery. MethodsBefore assessing performance, we conducted beam commissioning. Following established guidelines, we utilized an Electronic Portal Imaging Device (EPID) for assessing leaf positioning accuracy and reproducibility. The investigation of beam transmission, leakage, and the tongue and groove effect was facilitated through the use of EBT3 radiotherapy verification films and analysis with an EPSON EXPRESSION 12000XL scanner. Output factors, percent depth dose (PDD), and beam profiles were accurately determined using an IBA phantom equipped with an ionization chamber. ResultsThe composite end-face structure demonstrated exceptional performance, with leaf position accuracy and reproducibility deviations at 0.3 mm and 0.11 mm, respectively. Leakage through the leaves averaged at 0.98%, with a penumbra of 6.6 mm for a 10 × 10 cm2 field, indicating a significant improvement over traditional designs. ConclusionsThe innovative composite end-face structure for the arc-end MLC significantly mitigates the issue of high leakage rates during leaves closure. Our research demonstrated that all MLC performance levels were within the acceptable range based on the protocols, which were well below the specified tolerance limits, supporting highly accurate radiotherapy delivery.