Beam Shaping Devices Research Articles

SU-E-T-124: Assessing Quality Assurance of Proton Scattering Nozzles Using Monte Carlo

Purpose: This study simulated regular QA procedures in order to examine relationships between offsets in beam-shaping devices and delivered dose. The goal was to provide insight into the cause of potential QA irregularities, and to investigate whether current QA procedures are adequate to detect equipment misalignments. Methods: The Monte Carlo package Topas, based on Geant4, was used to simulate scattering nozzle QA procedures. Scoring volumes in the nozzle geometry mimic online treatment verification devices including ion chambers and a multi-layer faraday cup range verifier, as well as weekly and monthly measurements of SOBP depth-dose curves and beam profiles. The study then examined the relationship between geometric perturbations to the Monte Carlo model and changes in scoring outputs. Small movements of equipment including scatterers, ion chambers, and range modulators were considered. Results: A number of different insights have come from our investigation. Of particular note, we found that the measurement of beam profile skewness in an upsteam ion chamber is inadequate to use for beam steering to correct for asymmetries in the delivered dose. Skewness alone cannot distinguish between an asymmetric beam due to scatterer misalignment and a simple offset in the ion chamber location. For misalignments in the range modulator, typically no significant change is seen in online measurements before a change occurs in the output factor or modulation width, reinforcing the need for regular measurement of standard beams to verify these quantities. Comparing simulation and measurement can give hints at actual misalignments of nozzle hardware. Conclusion: Monte Carlo techniques can give useful insight into potential QA pitfalls. Through simulation we found that while ion chamber skewness can detect beamline asymmetries, it is inadequate to use for beam steering to correct a tilted beam profile. Standard online measurements have difficulty detecting longitudinal dose abnormalities compared to frequent direct measurement.

Quantification of dose perturbations induced by external and internal accessories in ocular proton therapy and evaluation of their dosimetric impact

Proton scattering on beam shaping devices and protons slowing down on media with different densities within the treatment volume may produce dose perturbations and range variations that are not predicted by treatment planning systems. The aim of this work was to assess the dosimetric impact of elements present in ocular proton therapy treatments that may disturb the prescribed treatment plan. Both distal beam shaping devices and intraocular elements were considered. A wedge filter, tantalum fiducial marker, hemispherical compensator, two intraocular endotamponades (densities 0.97 and 1.92 g cm(-3)) and an intraocular eye lens (IOL) were considered in the study. For these elements, longitudinal dose distributions were measured and∕or calculated in water in beam alignment for a clinical spread-out Bragg peak. Under the same conditions, the unperturbed dose distributions were similarly measured and∕or calculated in the absence of the element. The dosimetric impact was assessed by comparison of unperturbed and perturbed dose distributions. Measurements and calculations were carried out with two methods. Measurements are based on EBT3 films with dedicated software, which makes use of a calibration curve and correction for the quenching effect. Calculations are based on the Monte Carlo (MC) code MCNPX and reproduce the experimental conditions. Both dose maps are obtained with a resolution of 300 dpi. The degree of disturbance of distal beam shaping devices is low for the wedge filter (2% overdose ripple all along the central axis) and moderate for the hemispherical compensator (two bands of variable overdose of up to 10% downstream the compensator lateral edges and -5% underdose on the plateau at off-axis distance of 5 cm). Tantalum clips produce important dose shadows (-20% behind the clip parallel to the beam and range reduction of 1.1 mm) and bands of overdose (15%). The presence of endotamponades modifies the dose distribution very significantly (-5% underdose on the plateau and 3 mm range prolongation for the tamponade with density 0.97 g cm(-3) and -15% underdose on plateau and 8 mm range reduction for that with density 1.92 g cm(-3)). No dose perturbations were found for the IOL. The high performance of EBT3 film and MC tools used was confirmed and good agreement was found between them (percentage of pixels passing the gamma test >87%). The degree of disturbance by external beam shaping devices remains low in ocular proton therapy and can be reduced by bringing accessories closer to the eye. Tantalum fiducial markers must be located in such a way that dose perturbation is not projected on the tumor. The treatment of patients with intraocular endotamponades must be carefully managed. It is fundamental that radiation oncologists and medical physicists are informed about the presence of such substances prior to the treatment.

Laser assisted direct write process with novel beam profiles

The Laser Assisted Direct Write (LADW) method can be used to generate electrical circuitry on a substrate by depositing metallic ink and curing the ink thermally with a laser. Compared to conventional oven curing, laser curing can be used in-situ, over complicated 3D contours, and selectively cure over heat sensitive substrates.The intensity distribution of the laser beam used has significant influence on many surface heating processes. A conventional Gaussian beam would lead to uneven heating across the beam path. To overcome this issue, Wellburn has designed a novel beam shaping device, which allows one to convert a Gaussian beam profile into an annular ring with a variable level of central plateau fill [1].Shang et al. have developed and verified a predictive Finite Element Method (FEM) model to simulate the laser curing process for a generic combination of processing parameters. Laser modes can be flexibly varied within this model. Hence it can be used to predict the performance of curing with differently intensity distributions.This study investigates the curing process under different intensity profiles obtained with the beam shaping device. Electrical resistivity and scratch resistance of cured samples are tested as measures of the curing quality achieved. Results obtained are then compared with the FEM model predictions, and indicate the best intensity distribution for this particular application.

Tailoring Gaussian Laser Beam Shape Through Controlled Etching of Single-Mode and Multimode Fibers: Simulation and Experimental Studies

A new class of all-fiber beam shaping devices has been realized by inverse etching the end face of single-mode and multimode fibers to form a concave cone tip. Concave tip fiber can convert a Gaussian beam profile to a flat top beam profile with a uniform intensity distribution. A flat top beam with intensity variation of approximately 5% and flat top diameter to spot diameter ratio of 67% has been achieved. This device can also change the beam shape from a Gaussian to a donut by moving the observation plane. A flat top multimode fiber beam delivery is attractive for applications which require high power and uniform intensity distribution. In single-mode fiber, concave tips could be used to reduce the beam spot size diameter, enabling efficient light coupling from a single-mode fiber to an integrated optical waveguide.

Pitfalls of tungsten multileaf collimator in proton beam therapy

Particle beam therapy is associated with significant startup and operational cost. Multileaf collimator (MLC) provides an attractive option to improve the efficiency and reduce the treatment cost. A direct transfer of the MLC technology from external beam radiation therapy is intuitively straightforward to proton therapy. However, activation, neutron production, and the associated secondary cancer risk in proton beam should be an important consideration which is evaluated. Monte Carlo simulation with FLUKA particle transport code was applied in this study for a number of treatment models. The authors have performed a detailed study of the neutron generation, ambient dose equivalent [H∗(10)], and activation of a typical tungsten MLC and compared with those obtained from a brass aperture used in a typical proton therapy system. Brass aperture and tungsten MLC were modeled by absorber blocks in this study, representing worst-case scenario of a fully closed collimator. With a tungsten MLC, the secondary neutron dose to the patient is at least 1.5 times higher than that from a brass aperture. The H∗(10) from a tungsten MLC at 10 cm downstream is about 22.3 mSv/Gy delivered to water phantom by noncollimated 200 MeV beam of 20 cm diameter compared to 14 mSv/Gy for the brass aperture. For a 30-fraction treatment course, the activity per unit volume in brass aperture reaches 5.3 × 10⁴ Bq cm(-3) at the end of the last treatment. The activity in brass decreases by a factor of 380 after 24 h, additional 6.2 times after 40 days of cooling, and is reduced to background level after 1 yr. Initial activity in tungsten after 30 days of treating 30 patients per day is about 3.4 times higher than in brass that decreases only by a factor of 2 after 40 days and accumulates to 1.2 × 10⁶ Bq cm(-3) after a full year of operation. The daily utilization of the MLC leads to buildup of activity with time. The overall activity continues to increase due to (179)Ta with a half-life of 1.82 yr and thus require prolonged storage for activity cooling. The H∗(10) near the patient side of the tungsten block is about 100 μSv/h and is 27 times higher at the upstream side of the block. This would lead to an accumulated dose for therapists in a year that may exceed occupational maximum permissible dose (50 mSv/yr). The value of H∗(10) at the upstream surface of the tungsten block is about 220 times higher than that of the brass. MLC is an efficient way for beam shaping and overall cost reduction device in proton therapy. However, based on this study, tungsten seems to be not an optimal material for MLC in proton beam therapy. Usage of tungsten MLC in clinic may create unnecessary risks associated with the secondary neutrons and induced radioactivity for patients and staff depending on the patient load. A careful selection of material for manufacturing of an optimal MLC for proton therapy is thus desired.

Dose verification of proton beam therapy using the Gafchromic EBT film

Abstract In proton beam therapy, dosimetric verification is important because proton beams are stopped within targets, and almost no dose proceeds beyond those targets. Patient-specific quality assurance (QA) checks the beam-delivery system using passive-scattering techniques, including portal calibrations and verification of beam-shaping devices, because there are many components to be checked, such as the range shifter, the aperture, and the compensator, in the beam-delivery system. Although the conventional measurement of dose distribution by scanning the ion chamber gives an accurate estimate of the dose, it requires considerable time and effort. While a mechanical check is useful to physically check the milling condition of the compensator, this capability is limited with respect to the investigation of the dose distribution. We investigated the usefulness of EBT films for their applications in the area of high-precision clinical QA of proton beams. Proton beams were delivered to films in a transverse (in solid water) or a longitudinal (in water) direction, with respect to the beam axis, and information about the aperture and the compensator shapes was obtained from the dose distributions measured with either film orientation. Quantitative comparisons between the treatment planning system (TPS) and the EBT film data were carried out using both distance-to-agreement and gamma index procedures. The percentage of points exceeding the acceptance criterion of the gamma index (i.e. 3%, 3 mm) was, on average, 2.4% and 8.3% in films oriented transversely and longitudinally to the beam axis, respectively. The results of transverse irradiations of EBT films were in good agreement with those of the TPS, although consistent disagreements in longitudinal irradiation were observed near the distal edge of the spread-out Bragg peak (SOBP), presumably due to differences in sensitivity in the linear energy transfer (LET). In addition to range, uncertainty (∼3 mm) caused by the non-water-equivalence of the EBT film was observed in longitudinal irradiation. The under-response of the EBT film in the high-LET region improved remarkably after we corrected the SOBP of the EBT film, using a weight function based on the Bragg peaks. Although our correction data obtained using the EBT film did not match the result of the chamber after the distal fall-off region, the EBT film could be a useful QA tool for both dose and range verification of aperture and compensator, after careful correction.

Laser beam shaping and mode conversion in optical fibers

A new class of all-fiber beam shaping devices has been realized by inverse etching the end face of single mode and multimode fibers to form a concave cone tip. Concave tip fiber can convert a Gaussian beam profile to a flat top beam profile with a uniform intensity distribution. A flat top beam with intensity variation of approx. 5% and flat top diameter to spot diameter ratio of 67% has been achieved. This device can also change the beam shape from a Gaussian to a donut by moving the observation plane. A flat top multimode fiber beam delivery is attractive for applications which require high power and uniform intensity distribution. In single mode fiber, concave tips could be used to reduce the beam spot size diameter, enabling efficient light coupling from a single mode fiber to an integrated optical waveguide.

Therapeutic techniques applied in the heavy-ion therapy at IMP

Superficially-placed tumors have been treated with carbon ions at the Institute of Modern Physics (IMP), Chinese Academy of Sciences (CAS), since November 2006. Up to now, 103 patients have been irradiated in the therapy terminal of the heavy ion research facility in Lanzhou (HIRFL) at IMP, where carbon-ion beams with energies up to 100 MeV/u can be supplied and a passive beam delivery system has been developed and commissioned. A number of therapeutic and clinical experiences concerning heavy-ion therapy have been acquired at IMP. To extend the heavy-ion therapy project to deep-seated tumor treatment, a horizontal beam line dedicated to this has been constructed in the cooling storage ring (CSR), which is a synchrotron connected to the HIRFL as an injector, and is now in operation. Therapeutic high-energy carbon-ion beams, extracted from the HIRFL-CSR through slow extraction techniques, have been supplied in the deep-seated tumor therapy terminal. After the beam delivery, shaping and monitoring devices installed in the therapy terminal at HIRFL-CSR were validated through therapeutic beam tests, deep-seated tumor treatment with high-energy carbon ions started in March 2009. The therapeutic techniques in terms of beam delivery system, conformal irradiation method and treatment planning used at IMP are introduced in this paper.

High-brilliance home-lab X-ray sources: status and future

Introduction Modern microfocus X-ray sources define the state-of-the-art for a number of applications such as protein crystallography and smallangle scattering in the home lab. These sources have anode spot sizes of 100 μm or smaller. They are usually combined with Montel multilayer optics as beam-shaping devices that image the source spot onto the position of the sample, magnifiying the beam to a suitable size. Multilayer optics deliver a parallel or focused monochromatic beam. Below results of three different microfocus sources are shown: a sealed tube source, a rotating anode source, and a liquid metal jet X-ray source. The power densities of these three X-ray sources range from several kW/mm2 for the sealed tube source, to about 20 kW/mm2 for the rotating anode source, and to more than 500 kW/mm2 for the liquid metal jet X-ray source.

SU-GG-T-474: Feasibility Study of MVCT Imaging Guided Adaptive Proton Therapy for Head and Neck Cancers

Purpose: Extending MVCT's potential for daily patient position and anatomy verification, we investigated the feasibility of using MVCT to assess the daily delivered dose for head and neck patients treated with proton beams. Method and Materials: A Rando head phantom was scanned on a Philips Brilliance Big Bore CT scanner to obtain the planning kVCT images. The corresponding MVCT images were acquired on a helical Tomotherapy unit. To establish CT-electron density curves, a Gammex RMI 465 electron density phantom was scanned on both kVCT and MVCT scanners. A typical 4-field plan was created for the phantom and dose distribution was calculated using pencil beam algorithm from the XIO treatment planning system. After MVCT and kVCT images manually registered, kVCT-based treatment plan with structure contours and beam configuration including beam shaping devices was mapped to MVCT images for dose re-computation. Dose map, isodose curves, dose profiles, gamma maps, and dose-volume histograms were used for dose comparison. Results: Although the noise level in MVCT images is higher than that with the comparabl kVCT the dose distribution computed on MVCT image is comparable to the dose distribution computed on the kVCT images. The dose comparison showed an agreement of higher than 97% between the two plans given the criteria of 3% dose difference and 3 mm distance to agreement. The biggest dose difference between two plans occurred around the periphery of the target. There was negligible difference in the DVH comparison. Conclusions: MVCT images acquired for daily setup verification in image-guided proton therapy are feasible for assessment of daily delivered proton dose distributions with accuracy comparable to that of kVCT-based dose calculation. The original treatment plan based on kVCT can be transferred to the daily MVCT image set to evaluate the actual dose distribution for adaptive proton therapy.

A high resolution scanning electron microscope for in situ investigation of swift heavy ion induced modification of solid surfaces

A new in situ high resolution electron microscope (HRSEM) setup has been designed and integrated into the materials science beamline (M-branch) of the universal linear accelerator at Gesellschaft für Schwerionenforschung, Darmstadt for in situ investigations of swift heavy ion induced surface modifications. Special ports for in situ experiments are installed at the HRSEM chamber to connect it to the high vacuum beamline, which is equipped with suitable beam control and shaping devices. In order to demonstrate the feasibility and power of this new instrument, first experiments were performed on a 50-nm-thick Fe(2)O(3) film on Si substrate, which exhibited submicrometer size holes due to irradiation induced dewetting in a previous experiment. We have demonstrated that with our new instrument, the development of individual dewetting holes as a function of the ion fluence can be investigated and even the interaction between them can be studied. To illustrate pattern formation during grazing incidence, 3.6 MeV/u (131)Xe ion irradiation was carried out on a 25-nm-thick NiO film on SiO(2)/Si at a tilt angle of 75 degrees. The SEM image sequence recorded during the experiment reveals the development of a lamellaelike structure also seen before in ex situ experiments. With our new in situ setup, however, we are able to not only investigate the overall average pattern formation, but also to track the formation and decay of individual linking structures, which would be hardly possible in a standard ex situ experiment.

Prototyping of Beam Shaping Diffraction Gratings by AFM Nanoscale Patterning

Diffractive gratings are often associated with the use of beam shaping device utilizing a monochromatic source. They could provide high flexibility in design and offer a precise control according to applications. The design of diffraction grating often makes several iterations through design and prototyping before completion. In this paper, we demonstrate a computer-aided design method and a mechanical method of prototyping diffractive grating optics for beam shaping, which aim at improve productivity through greater design flexibility, rapid fabrication and cost reduction. A description of the optical design is presented along with a discussion on the integrated patterning system.

Long-period grating and its application in laser beam shaping in the 10 μm wavelength region

Laser beam shaping is an important subject for industrial and medical applications since different applications may require different laser intensity distributions. We present an all-fiber beam-shaping device that could transform a Gaussian-shaped laser beam into an intensity uniform beam in the 1.0 microm wavelength region using a long-period grating (LPG). The device can be used to shape the beam of a Yb-doped high-power fiber laser. The mode structure of the LPGs made of HI1060 fiber is analyzed. The design parameters that affect the beam shaping are discussed, and the results of the beam shaping using two cladding modes, LP(03) and LP(04), are presented.

Laser beam shaping using a single-mode fiber abrupt taper

A new beam-shaping device was realized by an abrupt taper with a length of approximately 700 microm and a waist of approximately 40 microm. The insertion loss of the device is less than 3%. The diameter of the flat beam top can be up to approximately 900 microm with a small intensity variation (4%) and a small half-divergence angle (2.5 degrees). The conversion efficiency of the new device from a Gaussian-shaped to a flat-top profile is comparable with that of a long-period-gratings-based device, while keeping the fabrication cost low. The new device requires only a fusion splicer and standard SMF-28 fiber, eliminating the need for photolithographic procedures. The new device also has no obvious incident light polarization dependence.

Neutron production from beam-modifying devices in a modern double scattering proton therapy beam delivery system

In this work the neutron production in a passive beam delivery system was investigated. Secondary particles including neutrons are created as the proton beam interacts with beam shaping devices in the treatment head. Stray neutron exposure to the whole body may increase the risk that the patient develops a radiogenic cancer years or decades after radiotherapy. We simulated a passive proton beam delivery system with double scattering technology to determine the neutron production and energy distribution at 200 MeV proton energy. Specifically, we studied the neutron absorbed dose per therapeutic absorbed dose, the neutron absorbed dose per source particle and the neutron energy spectrum at various locations around the nozzle. We also investigated the neutron production along the nozzle's central axis. The absorbed doses and neutron spectra were simulated with the MCNPX Monte Carlo code. The simulations revealed that the range modulation wheel (RMW) is the most intense neutron source of any of the beam spreading devices within the nozzle. This finding suggests that it may be helpful to refine the design of the RMW assembly, e.g., by adding local shielding, to suppress neutron-induced damage to components in the nozzle and to reduce the shielding thickness of the treatment vault. The simulations also revealed that the neutron dose to the patient is predominated by neutrons produced in the field defining collimator assembly, located just upstream of the patient.

Near-Field Plates: Subdiffraction Focusing with Patterned Surfaces

Using a patterned, grating-like plate to control the electromagnetic near field, we demonstrate focusing well beyond the diffraction limit at approximately 1 gigahertz. The near-field plate consists of only capacitive elements and focuses microwaves emanating from a cylindrical source to a spot of size approximately lambda/20 (half-power beamwidth), where lambda is the free-space wavelength. These plates will find application in antennas, beam-shaping devices, nonradiative wireless power-transfer systems, microscopy, and lithography.

Fast reconfigurable and transient-less holographic beam-shaping realized by a AOM-SLM device

The reconfiguration time of a holographic beam-shaping device using a liquid crystal spatial light modulator (SLM) has been drastically decreased by five orders of magnitude to reach the microsecond range (~5 µs). The method involves an acousto-optic modulator (AOM) placed in front of the SLM which points the laser beam towards different pre-addressed regions of the liquid crystal plate. Because the addressing signal is not changed, the reconfiguration time is mainly governed by the AOM response time. Moreover, this method allows us to clean up the transient regime by suppressing the bleed effect usually observed in the standard SLM reconfiguration.

TU‐D‐M100F‐01: A Novel Quality Assurance Monitor for Real‐Time Verification of IMRT and IGART

Purpose: To develop an independent treatment verification method which can validate radiation field delivery in real‐time throughout the treatment course. The system is designed to capture common treatment delivery errors, and is intended to eliminate the need for pre‐treatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT) and enable the implementation of image guided adaptive radiation therapy. Method and Materials: A monitoring system, termed Integral Quality Monitor (IQM), has been developed that utilizes an area integrated energy fluence monitoring sensor (AIMS) positioned after the final beam shaping device (i.e. multileaf collimator (MLC)) and a signal prediction algorithm, IQM_Calc. The AIMS consists of a novel large area ionization chamber with a gradient oriented along the direction of the MLC motion. The measured signal from the AIMS can be compared in real‐time with the IQM_Calc predicted values. A prototype AIMS has been built with 2 mm thick Aluminum plates, an area of 22 cm × 22 cm and continuously varying electrode separation of 2 to 22 mm. The IQM_Calc uses a modified sector integration of MLC defined apertures and accounts for MLC characteristics such as: rounded leaf ends, transmission, and relative output factor. Testing of the IQM system was performed on Varian and Elekta linear accelerators. Results: Initial results for prostate IMRT fields show an average agreement of 2% between the measured IQM signals and the IQM_Calc results. For a 3 mm simulated MLC leaf positioning error, the signal of a prostate IMRT field changed by 2%. Conclusion: It is demonstrated that the prototype IQM system has the capability of verifying the accuracy of treatment delivery in real‐time. The system is also capable of capturing common treatment errors. The IQM system has the potential of playing an important role in the challenging QA environment of modern radiation therapy.

SU‐FF‐T‐135: Commissioning of Multi‐Leaf Collimator for Fast Neutron Therapy

Purpose: To collect and analyze data required for modeling of d(48.5)+Be fast neutron beam and shaped by newly installed multi‐leaf collimator (MLC) replacing previously used multi‐rod collimator (MRC). Method and Materials: The MLC is of semi‐focused design and consists of 60 leaf pairs operated under vision control. The leaf ends are straight, while the sides of the leaves are tapered to match the beam divergence and project 5 mm at isocenter plane. The leaf thickness is 300 mm of steel, and the MLC is the primary beam shaping device. The measurements along the central axis as well as lateral profiles were done in a water phantom. In the build‐up region the central axis depth dose curves were combined with data obtained from the measurements using thin window extrapolation ionization chamber. The output factors were defined at 0.9 cm depth in phantom and in free air by means of small volume Tissue Equivalent (TE) ionization chamber and miniature pin diode. In addition TE/Mg(Ag) paired ionization chambers were used to separate the neutron and gamma components in phantom as well as in free air. Results: No significant variations of the central axis depth doses were observed compared to MRC. Penumbra measured between tapered leaf sides was smaller than penumbra measured between straight leaf ends. The differences were largest at shallow depths and for large field sizes: 7 mm for 25 × 25 cm2 field decreasing to less than1 mm for fields smaller than 15 × 15 cm2. Linear relationship between the actual leaf end position and field size was observed. The gamma component at 0.9 cm depth in phantom increased with the field size from 1.5% to 3% and from 0.4% to 0.74% in air. Conclusion: A set of beam data measured for neutron MLC was used in beam modeling.

Design of a Reflective Aspherical Surface of a Compact Beam-Shaping Device

A compact beam-shaping device with a reflective aspherical surface is proposed. The device converts a circular symmetric Gaussian beam from a laser into a uniform distribution on a target plane. The device consists of a laser, a reflective aspherical surface formed on the base plane inclined by 45° against the optical axis, and a spacer. The surface is designed for an optical device used as a transmitter of indoor wireless optical communication, which is one of the promising applications. The designed surface is obtained by approximation using polynomial. Beam shaping of a simulated surface and a uniform intensity distribution on the target plane is obtained. The intensity distribution generated by the surface is numerically simulated and evaluated if the surface is misaligned. It is clarified that the generated distribution is tolerable for the misalignment.