Spot Sizes In Air Research Articles

Interferometric measurements of laser-induced shockwaves in air: Variable laser energy

When focused to a small spot size in air, a sufficiently energetic laser pulse initiates a rapidly expanding plasma. After a delay, a shockwave detaches from the plasma boundary and propagates. General features of the shockwaves can be deduced from condenser microphone measurements. However, the minimum range is limited by damage thresholds, and the presence of the microphone introduces a number of measurement artifacts. Distortion of the signal is caused by diffraction around the sensor, and the limited bandwidth does not allow rise times to be correctly quantified. In contrast, optical interferometry is a nonintrusive diagnostic for quantifying shockwave characteristics. In this study, a Nd:YAG laser is focused through a converging lens in order to generate laser-induced shockwaves. By using a variable attenuator, four laser energy outputs are examined: 25, 50, 75, and 100% of the maximum energy transmission. Heterodyne Mach–Zehnder interferometer measurements are made from 10 mm to 200 mm from the focal point of the lens. Virtual velocity signals, proportional to the time derivative of optical phase differences, are used to estimate density and pressure time histories, along with peak pressure as a function of distance.

Computer-assisted beam modeling for particle therapy.

PurposeTo develop a computer‐driven and thus less user‐dependent method, allowing for a simple and straightforward generation of a Monte Carlo (MC) beam model of a scanned proton and carbon ion beam delivery system.MethodsIn a first step, experimental measurements were performed for proton and carbon ion energies in the available energy ranges. Data included depth dose profiles measured in water and spot sizes in air at various isocenter distances. Using an automated regularization‐based optimization process (AUTO‐BEAM), GATE/Geant4 beam models of the respective beam lines were generated. These were obtained sequentially by using least square weighting functions with and without regularization, to iteratively tune the beam parameters energy, energy spread, beam sigma, divergence, and emittance until a user‐defined agreement was reached. Based on the parameter tuning for a set of energies, a beam model was semi‐automatically generated. The resulting beam models were validated for all centers comparing to independent measurements of laterally integrated depth dose curves and spot sizes in air. For one representative center, three‐dimensional dose cubes were measured and compared to simulations. The method was applied on one research as well as four different clinical beam lines for proton and carbon ions of three different particle therapy centers using synchrotron or cyclotron accelerator systems: (a) MedAustron ion therapy center, (b) University Proton Therapy Dresden, and (c) Center Antoine Lacassagne Nice.ResultsParticle beam ranges in the MC beam models agreed on average within 0.2 mm compared to measurements for all energies and beam lines. Spot sizes in air (full‐width at half maximum) at all positions differed by less than 0.4% from the measurements. Dose calculation with the beam model for the clinical beam line at MedAustron agreed better than 1.7% in absolute dose for a representative clinical case treated with protons. For protons, beam model generation, including geometry creation, data conversion, and validation, was possible within three working days. The number of iterations required for the optimization process to converge, was found to be similar for all beam line geometries and particle types.ConclusionThe presented method was demonstrated to work independently of the beam optics behavior of the different beam lines, particle types, and geometries. Furthermore, it is suitable for non‐expert users and requires only limited user interaction. Beam model validation for different beam lines based on different beam delivery systems, showed good agreement.

Commissioning and beam characterization of the first gantry-mounted accelerator pencil beam scanning proton system.

To present and discuss beam characteristics and commissioning process of the first gantry-mounted accelerator single room pencil beam scanning (PBS) proton system. The Mevion HYPERSCAN employs a design configuration with a synchrocyclotron mounted on the gantry to eliminate the traditional beamline and a nozzle that contains the dosimetry monitoring chambers, the energy modulator (Energy Selector (ES)), and an Adaptive Aperture (AA). To characterize the beam, we measured the integrated depth dose (IDDs) for 12 energies, from highest energy of 227MeV down to 28MeV with a range difference~2cm between the adjacent energies, using a large radius Bragg peak chamber; single-spot profiles in air at five locations along the beam central axis using radiochromic EBT3 film and cross compared with a scintillation detector; and determined the output using a densely packed spot map. To access the performance of AA, we measured interleaf leakage and the penumbra reduction effect. Monte Carlo simulation using TOPAS was performed to study spot size variation along the beam path, beam divergence, and energy spectrum. This proton system is calibrated to deliver 1Gy dose at 5cm depth in water using the highest beam energy by delivering 1 MU/spot to a 10×10 cm2 map with a 2.5mm spot spacing. The spot size in air varies from 4mm to 26mm from 227MeV to 28MeV at the isocenter planewith the nozzle retracted 23.6 cm from isocenter. The beam divergence of 28MeV beam is~52.7 mrad, which is nearly 22 times that of 227MeV proton beam. The binary design of the ES has resulted in shifts of the effective SSD toward the isocenter as the energy is modulated lower. The peaks of IDD curves have a constant 80%-80% width of 8.4mm at all energies. The interleaf leakage of the AA is less than 1.5% at the highest energy; and the AA can reduce the penumbra by 2mm to 13mm for the 227 and 28MeV energies at isocenter plane in air. The unique design of the HYPERSCAN proton system has yielded beam characteristics significantly different from that of other proton systems in terms of the Bragg peak shapes, spot sizes, and the penumbra sharpening effect of the AA. The combination of the ES and AA has made PBS implementation possible without using beam transport line and range shifter devices. Different considerations may be required in treatment planning optimization to account for different design and beam characteristics.

Benchmarking a GATE/Geant4 Monte Carlo model for proton beams in magnetic fields.

PurposeMagnetic resonance guidance in proton therapy (MRPT) is expected to improve its current performance. The combination of magnetic fields with clinical proton beam lines poses several challenges for dosimetry, treatment planning and dose delivery. Proton beams are deflected by magnetic fields causing considerable changes in beam trajectories and also a retraction of the Bragg peak positions. A proper prediction and compensation of these effects is essential to ensure accurate dose calculations. This work aims to develop and benchmark a Monte Carlo (MC) beam model for dose calculation of MRPT for static magnetic fields up to 1 T.MethodsProton beam interactions with magnetic fields were simulated using the GATE/Geant4 toolkit. The transport of charged particle in custom 3D magnetic field maps was implemented for the first time in GATE. Validation experiments were done using a horizontal proton pencil beam scanning system with energies between 62.4 and 252.7 MeV and a large gap dipole magnet (B = 0–1 T), positioned at the isocenter and creating magnetic fields transverse to the beam direction. Dose was measured with Gafchromic EBT3 films within a homogeneous PMMA phantom without and with bone and tissue equivalent material slab inserts. Linear energy transfer (LET) quenching of EBT3 films was corrected using a linear model on dose‐averaged LET method to ensure a realistic dosimetric comparison between simulations and experiments. Planar dose distributions were measured with the films in two different configurations: parallel and transverse to the beam direction using single energy fields and spread‐out Bragg peaks. The MC model was benchmarked against lateral deflections and spot sizes in air of single beams measured with a Lynx PT detector, as well as dose distributions using EBT3 films. Experimental and calculated dose distributions were compared to test the accuracy of the model.ResultsMeasured proton beam deflections in air at distances of 465, 665, and 1155 mm behind the isocenter after passing the magnetic field region agreed with MC‐predicted values within 4 mm. Differences between calculated and measured beam full width at half maximum (FWHM) were lower than 2 mm. For the homogeneous phantom, measured and simulated in‐depth dose profiles showed range and average dose differences below 0.2 mm and 1.2%, respectively. Simulated central beam positions and widths differed <1 mm to the measurements with films. For both heterogenous phantoms, differences within 1 mm between measured and simulated central beam positions and widths were obtained, confirming a good agreement of the MC model.ConclusionsA GATE/Geant4 beam model for protons interacting with magnetic fields up to 1 T was developed and benchmarked to experimental data. For the first time, the GATE/Geant4 model was successfully validated not only for single energy beams, but for SOBP, in homogeneous and heterogeneous phantoms. EBT3 film dosimetry demonstrated to be a powerful dosimetric tool, once the film response function is LET corrected, for measurements in‐line and transverse to the beam direction in magnetic fields. The proposed MC beam model is foreseen to support treatment planning and quality assurance (QA) activities toward MRPT.

Technical commissioning of the spot scanning system in Shanghai Proton Therapy Facility

In the field of particle therapy, the method of pencil beam scanning is of great potential for clinical application, now and in the future. The authors made strong effort to develop a spot scanning system for Shanghai Proton Therapy Facility. Design parameters and basic layout of the system are introduced. Functionalities and specifications of crucial devices are described in detail. Most of the devices in the system were designed in house by the authors themselves, including scanning nozzle, scanning magnets and their power supplies, beam monitors, irradiation control modules and safety interlock modules. During the technical commissioning stage in the fix beam room, the spot scanning system was tested and verified. Under conditions of the maximum dose rate and minimum dose rate, a) repeatability of the single spot dose is less than ± 0.1%; b) nonlinearity of the single spot dose is less than ± 0.1%; c) FWHM for spot size in air at isocenter varies from 8mm to 12mm for full energy, consistent with the design values; d) lateral dose distribution achieves a flatness of less than 2% for multiple proton energies. According to the results of technical commissioning, the spot scanning system is capable of producing a prescribed 3D dose distribution for target tumor successfully.

Scanned Proton Beam Performance and Calibration of the Shanghai Advanced Proton Therapy Facility

The Shanghai Advanced Proton Therapy facility (SAPT) is a hospital-based facility that began construction in December of 2014 with commissioning of the first scanned proton beam line starting in October of 2017. Proton beams are extracted from a synchrotron accelerator with energies between 70 and 235 MeV. Beam delivery uses the modulated scanning and energy stacking techniques to produce a maximal scanning area of 40 × 30 cm2 at the iso-center. Prior to clinical use, the beam delivery system was characterized and calibrated following the guidelines of the IEC 62667 medical electronic equipment standard including the spot size in air, spot position, depth dose distributions, and lateral dose profiles, as well as the beam monitor calibrations following the IAEA TRS-398 recommendations with small differences.•The measured dosimetric results showed that the full width at half maximum (FWHM) for the beam spot size in air varied approximately from 6 mm to 13 mm. The dose fall-off (DDF) derived from the measured depth dose in water varied from 4.7 mm at 235 MeV to 0.7 mm at 70 MeV. The homogeneity of the scanned field was better than 2% for various energies as expected.•Furthermore, the beam reproducibility and proportionality delivery accuracy was also stable with the results better than 0.1% and 1% respectively. Finally, the dose monitor calibration factor, its reproducibility and stability were tested. Reproducibility tests exhibited a standard deviation (SD) result of less than 1% during the test period.•All the measured dosimetric parameters showed that the design specifications were well achieved and the results are suitable for being used as a part of the clinical commissioning and quality assurance program for treating patients.

Modelling of a proton spot scanning system using MCNP6

The aim of this work was to model the characteristics of a clinical proton spot scanning beam using Monte Carlo simulations with the code MCNP6. The proton beam was defined using parameters obtained from beam commissioning at the Skandion Clinic, Uppsala, Sweden. Simulations were evaluated against measurements for proton energies between 60 and 226 MeV with regard to range in water, lateral spot sizes in air and absorbed dose depth profiles in water. The model was also used to evaluate the experimental impact of lateral signal losses in an ionization chamber through simulations using different detector radii. Simulated and measured distal ranges agreed within 0.1 mm for R90 and R80, and within 0.2 mm for R50. The average absolute difference of all spot sizes was 0.1 mm. The average agreement of absorbed dose integrals and Bragg-peak heights was 0.9%. Lateral signal losses increased with incident proton energy with a maximum signal loss of 7% for 226 MeV protons. The good agreement between simulations and measurements supports the assumptions and parameters employed in the presented Monte Carlo model. The characteristics of the proton spot scanning beam were accurately reproduced and the model will prove useful in future studies on secondary neutrons.

Differential evolution algorithm based photonic structure design: numerical and experimental verification of subwavelength λ/5 focusing of light.

Photonic structure designs based on optimization algorithms provide superior properties compared to those using intuition-based approaches. In the present study, we numerically and experimentally demonstrate subwavelength focusing of light using wavelength scale absorption-free dielectric scattering objects embedded in an air background. An optimization algorithm based on differential evolution integrated into the finite-difference time-domain method was applied to determine the locations of each circular dielectric object with a constant radius and refractive index. The multiobjective cost function defined inside the algorithm ensures strong focusing of light with low intensity side lobes. The temporal and spectral responses of the designed compact photonic structure provided a beam spot size in air with a full width at half maximum value of 0.19λ, where λ is the wavelength of light. The experiments were carried out in the microwave region to verify numerical findings, and very good agreement between the two approaches was found. The subwavelength light focusing is associated with a strong interference effect due to nonuniformly arranged scatterers and an irregular index gradient. Improving the focusing capability of optical elements by surpassing the diffraction limit of light is of paramount importance in optical imaging, lithography, data storage, and strong light-matter interaction.

SU‐F‐T‐153: Experimental Validation and Calculation Benchmark for a Commercial Monte Carlo Pencil Beam Scanning Proton Therapy Treatment Planning System in Water

Purpose:Eclipse proton Monte Carlo AcurosPT 13.7 was commissioned and experimentally validated for an IBA dedicated PBS nozzle in water. Topas 1.3 was used to isolate the cause of differences in output and penumbra between simulation and experiment.Methods:The spot profiles were measured in air at five locations using Lynx. PTW‐34070 Bragg peak chamber (Freiburg, Germany) was used to collect the relative integral Bragg peak for 15 proton energies from 100 MeV to 225 MeV. The phase space parameters (σx, σθ, ρxθ) number of protons per MU, energy spread and calculated mean energy provided by AcurosPT were identically implemented into Topas. The absolute dose, profiles and field size factors measured using ionization chamber arrays were compared with both AcurosPT and Topas.Results:The beam spot size, σx, and the angular spread, σθ, in air were both energy‐dependent: in particular, the spot size in air at isocentre ranged from 2.8 to 5.3 mm, and the angular spread ranged from 2.7 mrad to 6 mrad. The number of protons per MU increased from ∼9E7 at 100 MeV to ∼1.5E8 at 225 MeV. Both AcurosPT and TOPAS agree with experiment within 2 mm penumbra difference or 3% dose difference for scenarios including central axis depth dose and profiles at two depths in multi‐spot square fields, from 40 to 200 mm, for all the investigated single‐energy and multi‐energy beams, indicating clinically acceptable source model and radiation transport algorithm in water.Conclusion:By comparing measured data and TOPAS simulation using the same source model, the AcurosPT 13.7 was validated in water within 2 mm penumbra difference or 3% dose difference. Benchmarks versus an independent Monte Carlo code are recommended to study the agreement in output, filed size factors and penumbra differences.This project is partially supported by the Varian grant under the master agreement between University of Pennsylvania and Varian

The Influence of spot size on the expansion dynamics of nanosecond-laser-produced copper plasmas in atmosphere

Laser produced copper plasmas of different spot sizes in air were investigated using fast photography and optical emission spectroscopy (OES). The laser energy was 33 mJ. There were dramatic changes in the plasma plume expansion into the ambient air when spot sizes changed from ∼0.1 mm to ∼0.6 mm. A stream-like structure and a hemispherical structure were, respectively, observed. It appeared that the same spot size resulted in similar expansion dynamics no matter whether the target was located in the front of or behind the focal point, although laser-induced air breakdown sometimes occurred in the latter case. Plasma plume front positions agree well with the classic blast wave model for the large spot-size cases, while an unexpected stagnation of ∼80 ns occurred after the laser pulse ends for the small spot size cases. This stagnation can be understood in terms of the evolution of enhanced plasma shielding effects near the plasma front. Axial distributions of plasma components by OES revealed a good confinement effect. Electron number densities were estimated and interpreted using the recorded Intensified Charge Coupled Device (ICCD) images.

Experimental Investigation of 1064-nm IR Laser-Induced Air Plasma Using Optical Laser Shadowgraphy Diagnostics

Experimental measurements and analysis of pulsed 1064-nm Nd:YAG laser-induced air breakdown plasma at 760 torr has been carried out using high-speed and high-resolution laser shadowgraphy and optical diagnostics. Three different experimental laser energies and pulsewidths such as 170 mJ at 8 ns, 130 mJ at 7 ns, and 65 mJ at 12 ns are studied. The laser pulses were focused down to a ~ 7-μm spot size in air and the resulting laser flux densities range from 4 to 14 TW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A 532-nm laser shadowgraphy coupled with high-speed and high-resolution image capturing diagnostics has been established to investigate spatiotemporal evolution and hydrodynamic behavior of the 1064-nm laser-induced plasma and neutral-density shock during the formation, expansion, and collapsing stages. The observed plasma formations were aspherical due to absorption translation during the initial laser-energy coupling. The aspherical feature seeded the hydrodynamic instability leading to the ultimate destabilization of the hot gaseous core after approximately 10 μs. The active plasma lifetime through plasma self-luminescence measurements indicate variations from 200 to 500 ns for the three laser pulses. Shock propagation velocity and plasma volume for three laser pulse series indicate similarly shaped profiles at different expansion velocities. Early plasma expansion velocities of 20 km/s were measured, and using Hugoniot relations, the neutral shock pressures and temperatures were inferred, and the results at the early plasma expansion stage were found to be over 1000 atm and 4 eV.

SU-GG-T-89: Dosimetry Characteristics of the Discrete Spot Scanning Proton Beam at PTCH

Purpose: To present a summary of the dosimetry features of the discrete spot scanning proton beam at the Proton Therapy Center — Houston. Method and Materials: Dose is painted spot by spot, layer by layer using narrow discrete spot proton beams. The maximum scan area at isocenter is 30 × 30 cm2. Proton ranges (90% distal dose) span from 4.0 to 30.6 cm. Spot size depends on beam energy. The maximum and minimum monitor units per spot are 0.04 and 0.005. We have measured, using EBT film and ion chambers, the spot sizes in air, depth dose curve and profiles for various energies, and the linearity of the dose monitors. The absolute dose calibration standard for 1 cGy/MU has been established. Results: As expected, the spot is asymmetric, due to the beam acceleration characteristics. One sigma for the spot size in air varies from approximately 5 mm for 219 MeV to 15 mm for 72.5 MeV. The measured versus requested 90% depth dose agrees to within 1 millimeter over ranges of 10.5 cm to 30.6 cm. The dosimetry system is linear from 0.005 to 0.04 MUs. One MU is defined to deliver 1 cGy to 1 liter volume with 30.6 cm range for a 10 × 10 cm2 field. The non‐optimized spot plan penumbra at a depth of 25 cm for a beam with range 25.2 cm is approximately 1.5 cm and increases to approximately 1.7 cm at 10 cm depth for a beam with range of 10.5 cm. Spacing between spots to produce a ± 3% homogeneous dose volume has been studied. Conclusion: The dosimetric characteristics of the discrete spot scanning proton beam have been collected as part of our clinical commissioning program.

DEVELOPMENT OF AN EXTERNAL MICROBEAM-PIXE SYSTEM USING THIN FILM WINDOWS

A external microbeam system using polyimide windows has been developed for PIXE (Particle Induced X-ray Emission) analysis in air. A beamline consists of a magnetic quadrupole-doublet lens, cylindrical objective slits and double-deflection scanning coils. Polyimide films with a thickness of 3.5 μm or 7.5 μm were used for the exit windows. Spot sizes in air were measured by beam-induced fluorescence of CsI scintillation plates. A minimum spot size obtained was about 10 μm for a 1.5 MeV proton beam. PIXE-mapping images of a Cu grid were successfully obtained in air.