Collimator Aperture Research Articles

SU‐E‐T‐453: A Novel Daily QA System for Robotic Image Guided Radiosurgery with Variable Aperture Collimator

Purpose:A novel end‐to‐end system using a CCD camera and a scintillator based phantom that is capable of measuring the beam‐by‐beam delivery accuracy of Robotic Radiosurgery has been developed and reported in our previous work. This work investigates its application to end‐to‐end type daily QA for Robotic Radiosurgery (Cyberknife) with Variable Aperture Collimator (Iris).Methods:The phantom was first scanned with a CT scanner at 0.625 slice thickness and exported to the Cyberknife Muliplan (v4.6) treatment planning system. An isocentric treatment plan was created consisting of ten beams of 25 Monitor Units each using Iris apertures of 7.5, 10, 15, 20, and 25 mm. The plan was delivered six times in two days on the Cyberknife G4 system with fiducial tracking on the four metal fiducials embedded in phantom with re‐positioning between the measurements. The beam vectors (X, Y, Z) are measured and compared with the plan from the machine delivery file (XML file). The Iris apertures (FWHM) were measured from the beam flux map and compared with the commissioning data.Results:The average beam positioning accuracies of the six deliveries are 0.71 ± 0.40 mm, 0.72 ± 0.44 mm, 0.74 ± 0.42 mm, 0.70 ± 0.40 mm, 0.79 ± 0.44 mm and 0.69 ± 0.41 mm respectively. Radiation beam width (FWHM) variations are within ±0.05 mm, and they agree with the commissioning data within 0.22 mm. The delivery time for the plan is about 7 minutes and the results are given instantly.Conclusion:The experimental results agree with stated sub‐millimeter delivery accuracy of Cyberknife system. Beam FWHM variations comply with the 0.2 mm accuracy of the Iris collimator at SAD. The XRV‐100 system has proven to be a powerful tool in performing end‐to‐end type tests for Robotic Image Guided Radiosurgery Daily QA.

The neutron dose equivalent around high energy medical electron linear accelerators

The measurement of neutron dose equivalent was made in four dual energy linear accelerator rooms. Two of the rooms were reconstructed after decommissioning of 60Co units, so the main limitation was the space. The measurements were performed by a nuclear track etched detectors LR-115 associated with the converter (radiator) that consist of 10B and with the active neutron detector Thermo BIOREM FHT 742. The detectors were set at several locations to evaluate the neutron ambient dose equivalent and/or neutron dose rate to which medical personnel could be exposed. Also, the neutron dose dependence on collimator aperture was analyzed. The obtained neutron dose rates outside the accelerator rooms were several times smaller than the neutron dose rates inside the accelerator rooms. Nevertheless, the measured neutron dose equivalent was not negligible from the aspect of the personal dosimetry with almost 2 mSv a year per person in the areas occupied by staff (conservative estimation). In rooms with 15 MV accelerators, the neutron exposure to the personnel was significantly lower than in the rooms having 18 MV accelerators installed. It was even more pronounced in the room reconstructed after the 60Co decommissioning. This study confirms that shielding from the neutron radiation should be considered when building vaults for high energy linear accelerators, especially when the space constraints exist.

Secondary doses to healthy tissues during proton therapy treatments: influence of irradiation parameters

Secondary doses to healthy tissues during proton therapy treatments: influence of irradiation parameters

PD-0042: Development of a new virtual source model for portal image prediction

Purpose/Objective: Electronic Portal Imaging Devices (EPIDs) are widely used for quality assurance in radiotherapy, and also for dosimetric verifications. For this latter application, the image obtained during the treatment session can be compared to a pre-calculated reference image in order to highlight dose delivery errors. A Monte Carlo(MC)-based method to predict high-resolution reference images was recently developed. However, the use of this method in clinical routine is still hampered by the need to store huge phase space (PS) files. This study aims at developing a new virtual source model (VSM) which retains all the existing correlations between particle characteristics stored in the PS file, and hence to provide a more compact way to model the irradiation beam, while keeping the accuracy of using a PS file. Materials and Methods: A model of the studied linac (Elekta Synergy) was first developed using both the MC codes EGSnrc and PENELOPE, and commissioned. Using this model, a reference PSF (PSFr) was calculated after the flattening filter using EGSnrc, where the axial symmetries assumed, and particles were sorted out in three sub-sources according to the position of their last interaction (target, primary collimator or flattening filter). Each particle is described by its radial position (r) in the PS plane,its energy (E), and its polar and azimuthal angles (phi, theta), representing the particle deviation compared to its direction after bremsstrahlung, and the orientation of this deviation. For each subsource, a 4D histogram was built by storing the particle distributions accordingto their r, E, phi, theta values. Our VSM hence contains all correlations existing between these four variables contrary to other existing VSMs. This new VSM is now being implemented in the PENELOPE code, and its accuracy is being tested by comparing first the PSF build using our VSM with the PSFr, and then dose distributions in water and portal images calculated with the MSV and PSFr. Results: Figure 1 shows for each sub-source the correlated 2D histogram depicting particle energy against their radial position. Some characteristics are shared by all sources. Few particles are beyond the collimator aperture (4.5 cm from the linac axis). The annihilation photons at 511 keV are always visible. However, the content of a histogram depends widely of the sub-source it represents: the target produces the most energetic particles. The flattening filter and the primary collimator sub-sources generate particles beyond 4.5 cm of lower energy, because of their large deflection angle. Conclusions: A new VSM taking into account for all correlations between particle characteristics was developed and implemented in the PENELOPE code. It will be used to calculate accurate reference portal images for quality assurance in radiotherapy.

Advantages and limitations of navigation-based multicriteria optimization (MCO) for localized prostate cancer IMRT planning

Efficacy of inverse planning is becoming increasingly important for advanced radiotherapy techniques. This study’s aims were to validate multicriteria optimization (MCO) in RayStation (v2.4, RaySearch Laboratories, Sweden) against standard intensity-modulated radiation therapy (IMRT) optimization in Oncentra (v4.1, Nucletron BV, the Netherlands) and characterize dose differences due to conversion of navigated MCO plans into deliverable multileaf collimator apertures. Step-and-shoot IMRT plans were created for 10 patients with localized prostate cancer using both standard optimization and MCO. Acceptable standard IMRT plans with minimal average rectal dose were chosen for comparison with deliverable MCO plans. The trade-off was, for the MCO plans, managed through a user interface that permits continuous navigation between fluence-based plans. Navigated MCO plans were made deliverable at incremental steps along a trajectory between maximal target homogeneity and maximal rectal sparing. Dosimetric differences between navigated and deliverable MCO plans were also quantified. MCO plans, chosen as acceptable under navigated and deliverable conditions resulted in similar rectal sparing compared with standard optimization (33.7 ± 1.8Gy vs 35.5 ± 4.2Gy, p = 0.117). The dose differences between navigated and deliverable MCO plans increased as higher priority was placed on rectal avoidance. If the best possible deliverable MCO was chosen, a significant reduction in rectal dose was observed in comparison with standard optimization (30.6 ± 1.4Gy vs 35.5 ± 4.2Gy, p = 0.047). Improvements were, however, to some extent, at the expense of less conformal dose distributions, which resulted in significantly higher doses to the bladder for 2 of the 3 tolerance levels. In conclusion, similar IMRT plans can be created for patients with prostate cancer using MCO compared with standard optimization. Limitations exist within MCO regarding conversion of navigated plans to deliverable apertures, particularly for plans that emphasize avoidance of critical structures. Minimizing these differences would result in better quality treatments for patients with prostate cancer who were treated with radiotherapy using MCO plans.

The active area shadow-shielding effect on detection efficiency of collimated broad energy germanium detectors

The ISOCS calibration, when utilized for a BEGe detector with a small angled collimator, produces inaccuracies of about 19% for gamma rays with energies greater than 0.4MeV. Such a discrepancy is caused by the collimator algorithms currently utilized in the ISOCS software which, originally developed for HPGe detectors, are less suited for BEGe detectors. ISOCS's errors are due to the different crystal configurations of broad energy detectors compared to coaxial detectors, i.e. to a different importance of the active area portion obscured by the collimator. This work proposes some solutions for the problem, either using the ISOCS software or implementing a stochastic calibration procedure. In particular, the present work considers a virtual collimator that, maintaining its angular aperture, is capable of continuously enlarging its bottom collimator's aperture cone radius, to expose growing active area portions. In such a way two goals may be achieved: the mathematical characterization of ISOCS’ errors and the minimization of observed errors by means of the stochastic calibration procedure.Different reference set-ups are considered in order to test source geometry effects, source materials and different detectors. In particular, a 220L drum, a 2m3 box filled with uniformly contaminated cellulose or PVC, and small BE3825 and large BE5030 Canberra detectors are considered. Detection efficiencies calculated by ISOCS software are compared against a completely stochastic MCNPX simulation procedure, that is unaffected by any algorithmic correction. MCNPX simulations demonstrate, when widening collimators’ cone but maintaining the angular aperture unchanged, that ISOCS and MCNPX difference percentage between efficiency data points reduces, depending on energy, by more than 50%. This happens as far as the shadow-shielded portion of detector's active area reduces.

X-RAY COLLIMATOR DESIGN USING MONTE CARLO SIMULATIONS

The purpose of this study was to apply Monte Carlo (MC) based simulations to design a new collimation system to achieve a larger beam size (2 ± 0.1 cm) and tight penumbra (< 1 mm) with a kV X-ray small animal irradiator. The BEAMnrc and DOSXYZnrc MC-based programs were adapted to simulate the system. First the aperture of the primary collimator was expanded until the desired size of the beam was obtained. Subsequently, the beam was trimmed by reducing the aperture size of the secondary collimator in order to eliminate low energy scattered photons and sharpen the penumbra. Finally, the new collimator was constructed and the resultant dose distributions were evaluated with EBT2 film measurements. From the MC computed dose profiles, a 2.1 cm FWHM (1.9 cm width at 95% of the dose) and a sharp penumbra (< 1.0 mm) at 1.0 cm depth in water were obtained. Dose distribution comparisons between the EBT2 film measurements and the MC calculations showed acceptable agreement. MC-based calculation is an effective tool to expedite the creation of new collimator designs and avoid costly machining.

An approach to increasing the resolution of industrial CT images based on an aperture collimator

The spatial resolution of CT images is dominated by the focal spot size when it is large relative to the detector cells. We propose an approach to increase the spatial resolution by utilizing an aperture collimator. The aperture collimator is specially designed and placed in front of the X-ray source so that the rays penetrating the collimator form a set of narrow fan beams. Then an iterative algorithm is introduced to reconstruct CT images from the data obtained by scanning the narrow fan beams. Numerical experiments show that the proposed approach could significantly increase the resolution of the CT images. Furthermore, this approach is also robust against some challenging cases, such as the examination of low contrast object, reconstruction based on multi-energy data and perturbation of geometric errors in CT systems.

Parallel Hole and Rotating Slat Collimators: Comparative Study Using Digital Contrast Phantoms

The parallel hole (PH) collimator is the most commonly used collimator in single photon emission computed tomography (SPECT) imaging. A well-known limitation of the PH collimator is its low geometric efficiency. For better geometric efficiency without compromising the resolution, a rotating slat (RS) collimator is a potential alternative. In our previous work, a linear relation between the image resolution and the optimal collimator aperture was derived for both the PH and the RS collimator systems, and the two collimator systems were compared with the optimized geometries using different uniform phantoms. In this study, the PH and the RS collimator system were further compared using digital contrast phantoms. Three figures of merit (FOMs) were calculated for comparison, i.e., the contrast-to-noise ratio (CNR) in a pixel of interest, the CNR in a region of interest, and the signal-to-noise ratio (SNR) of a channelized Hotelling observer (CHO). All the FOMs were calculated analytically using an efficient approximation method. The comparison was done in both planar imaging and volume imaging. The size of the digitized phantom was varied, resulting in a detector area coverage from 2.5% to 40%. The main results are: (1) With the optimal collimator apertures, the gains (RS over PH) in all FOMs show the same trends in the system comparison. (2) The performance of the two collimators is similar for cold spot imaging with the largest phantom given in this study, while RS outperforms PH for all the other cases. (3) For system optimization, the optimal collimator apertures depend on the FOM to be optimized, however the ratio of the apertures (PH/RS) is always around √2, a value that we derived analytically in our previous work.

Experimental verification of dose calculation using the simplified Monte Carlo method with an improved initial beam model for a beam-wobbling system

A beam delivery system using a single-radius-beam-wobbling method has been used to form a conformal irradiation field for proton radiotherapy in Japan. A proton beam broadened by the beam-wobbling system provides a non-Gaussian distribution of projection angle different in two mutually orthogonal planes with a common beam central axis, at a certain position. However, the conventional initial beam model for dose calculations has been using an approximation of symmetric Gaussian angular distribution with the same variance in both planes (called here a Gaussian model with symmetric variance (GMSV)), instead of the accurate one. We have developed a more accurate initial beam model defined as a non-Gaussian model with asymmetric variance (NonGMAV), and applied it to dose calculations using the simplified Monte Carlo (SMC) method. The initial beam model takes into account the different distances of two beam-wobbling magnets from the iso-center and also the different amplitudes of kick angle given by each magnet. We have confirmed that the calculation using the SMC with NonGMAV reproduced the measured dose distribution formed in air by a mono-energetic proton beam passing through a square aperture collimator better than with the GMSV and with a Gaussian model with asymmetric variance (GMAV) in which different variances of angular distributions are used in the two mutually orthogonal planes. Measured dose distributions in a homogeneous phantom formed by a modulated proton beam passing through a range shifter and an L-shaped range compensator, were consistent with calculations using the SMC with GMAV and NonGMAV, but in disagreement with calculations using the SMC with GMSV. Measured lateral penumbrae in a lateral direction were reproduced better by calculations using the SMC with NonGMAV than by those with GMAV, when an aperture collimator with a smaller opening was used. We found that such a difference can be attributed to the non-Gaussian angular distribution of the initial beam at a lateral position for the beam-wobbling system. Calculations using the SMC with NonGMAV are effective to reproduce dose distributions formed by a beam-wobbling system more accurately than that with GMSV or that with GMAV.

SU‐E‐T‐603: Evaluation of MLC‐Based Robotic Radiotherapy

Purpose: A multi‐leaf collimator (MLC) system has been introduced for the CyberKnife M6 system, enabling conformal shaping of beam apertures in robotic radiotherapy. Through a multi‐disease site study, the potential for the use of MLC‐based robotic delivery is assessed for conventionally fractionated regimens. Methods: The InCise MLC, consisting of 41 leaf pairs, with leaf thickness of 2.5 mm at 800 mm SAD, and a maximum field‐of‐view field size of 120 mm × 100 mm was used in conjunction with the MultiPlan 5.0 platform for comparative planning study relative to the pseudo‐conical IRIS variable aperture collimator. Conformal avoidance technique with relaxed convergence was used for the optimization, and identical dose‐volume optimization parameters and normalization (95%) for the comparative studies. Disease sites, including the prostate, partial breast, and peri‐orbit carcinomas were evaluated, and the results were quantified with respect to total monitor units (MUs), treatment time, conformality, and dose‐volume coverage. Results: The MLC system results on average of 3– 4 modulated segments for a series of non‐coplanar nodes. Relative to plans generated by the IRIS variable aperture collimators, the MLC‐based plans resulted in a 38±10% reduction of both the MUs and treatment time. While similar dosimetry is achieved by both systems, for a dose rate of 1000 MU/min, it is shown that treatment times of less than 20 minutes are possible for most conventionally fractionated treatments with the MLC system. Conclusion: A limiting factor in the adaptation of the robotic delivery for conventional radiotherapy has been the prolonged treatment times. The introduction of compact MLC fixture enables reduction of MUs and treatment time, making conventionally fractionated treatments feasible to more efficiently leverage the higher delivering accuracy and intrafraction motion correction capabilities of robotic delivery.

MO‐D‐105‐06: Dose‐Area Product as a Method for Small Field Geometric QA

Purpose: To investigate the use of dose‐area product (DAP) as a highresolution tool for QA of small field sizes as an alternative to more complex alternatives such as film or water tank scanning. Method: An 81 mm diameter PTW Bragg Peak parallel plate chamber was used in combination with an Iris variable circular aperture collimator fitted to a CyberKnife System and a vendor‐supplied alignment jig. The Iris collimator is designed to deliver field diameters between 5 mm and 60 mm, with a tolerance of ±0.2 mm. First the sensitivity of the chamber reading to influence parameters (SSD, angle misalignment, lateral displacement, output variation) was investigated. Next the dependency of DAP on field diameter was investigated. Finally the sensitivity of the method to detect small field size changes consistent with the device specification was evaluated. Comparisons were made with film and diode water scans using four CyberKnife systems and three chambers. Results: Expected variations in chamber position/orientation within the jig resulted in measurement changes <0.2%. Measurement variations due to LINAC output changes can be removed by normalising measurements to a fixed collimator reading obtained at the same time. The variation in chamber reading with field size followed a quadratic dependency over the 5mm ‐ 60mm range. The change in chamber reading associated with a 0.2 mm field diameter change ranged from 10% at 5 mm nominal to 0.5% at 60 mm, and was reasonably consistent across tests performed on four units. Measurements were more consistent than routine film tests, and were stable over two years. Conclusion: DAP provides a practical, efficient, robust, and sensitive method for routine QA of field aperture changes as small as ±0.2mm across the field size range 5 mm – 60 mm. The method may also have applications for QA of small fields formed by MLC.

Automatic Prostate Tracking and Motion Assessment in Volumetric Modulated Arc Therapy With an Electronic Portal Imaging Device

To assess the prostate intrafraction motion in volumetric modulated arc therapy treatments using cine megavoltage (MV) images acquired with an electronic portal imaging device (EPID). Ten prostate cancer patients were treated with volumetric modulated arc therapy using a Varian TrueBeam linear accelerator equipped with an EPID for acquiring cine MV images during treatment. Cine MV images acquisition was scheduled for single or multiple treatment fractions (between 1 and 8). A novel automatic fiducial detection algorithm that can handle irregular multileaf collimator apertures, field edges, fast leaf and gantry movement, and MV image noise and artifacts in patient anatomy was used. All sets of images (approximately 25,000 images in total) were analyzed to measure the positioning accuracy of implanted fiducial markers and assess the prostate movement. Prostate motion can vary greatly in magnitude among different patients. Different motion patterns were identified, showing its unpredictability. The mean displacement and standard deviation of the intrafraction motion was generally less than 2.0 ± 2.0 mm in each of the spatial directions. In certain patients, however, the percentage of the treatment time in which the prostate is displaced more than 5 mm from its planned position in at least 1 spatial direction was 10% or more. The maximum prostate displacement observed was 13.3 mm. Prostate tracking and motion assessment was performed with MV imaging and an EPID. The amount of prostate motion observed suggests that patients will benefit from its real-time monitoring. Megavoltage imaging can provide the basis for real-time prostate tracking using conventional linear accelerators.

ALBIRA: A small animal PET/SPECT/CT imaging system

The authors have developed a trimodal PET∕SPECT∕CT scanner for small animal imaging. The gamma ray subsystems are based on monolithic crystals coupled to multianode photomultiplier tubes (MA-PMTs), while computed tomography (CT) comprises a commercially available microfocus x-ray tube and a CsI scintillator 2D pixelated flat panel x-ray detector. In this study the authors will report on the design and performance evaluation of the multimodal system. X-ray transmission measurements are performed based on cone-beam geometry. Individual projections were acquired by rotating the x-ray tube and the 2D flat panel detector, thus making possible a transaxial field of view (FOV) of roughly 80 mm in diameter and an axial FOV of 65 mm for the CT system. The single photon emission computed tomography (SPECT) component has a dual head detector geometry mounted on a rotating gantry. The distance between the SPECT module detectors can be varied in order to optimize specific user requirements, including variable FOV. The positron emission tomography (PET) system is made up of eight compact modules forming an octagon with an axial FOV of 40 mm and a transaxial FOV of 80 mm in diameter. The main CT image quality parameters (spatial resolution and uniformity) have been determined. In the case of the SPECT, the tomographic spatial resolution and system sensitivity have been evaluated with a (99m)Tc solution using single-pinhole and multi-pinhole collimators. PET and SPECT images were reconstructed using three-dimensional (3D) maximum likelihood and ordered subset expectation maximization (MLEM and OSEM) algorithms developed by the authors, whereas the CT images were obtained using a 3D based FBP algorithm. CT spatial resolution was 85 μm while a uniformity of 2.7% was obtained for a water filled phantom at 45 kV. The SPECT spatial resolution was better than 0.8 mm measured with a Derenzo-like phantom for a FOV of 20 mm using a 1-mm pinhole aperture collimator. The full width at half-maximum PET radial spatial resolution at the center of the field of view was 1.55 mm. The SPECT system sensitivity for a FOV of 20 mm and 15% energy window was 700 cps∕MBq (7.8 × 10(-2)%) using a multi-pinhole equipped with five apertures 1 mm in diameter, whereas the PET absolute sensitivity was 2% for a 350-650 keV energy window and a 5 ns timing window. Several animal images are also presented. The new small animal PET∕SPECT∕CT proposed here exhibits high performance, producing high-quality images suitable for studies with small animals. Monolithic design for PET and SPECT scintillator crystals reduces cost and complexity without significant performance degradation.

Scientific Design of the New Neutron Radiography Facility (SANRAD) at SAFARI-1 for South Africa

Abstract The final scientific design for an upgraded neutron radiography/tomography facility at beam port no.2 of the SAFARI-1 nuclear research reactor has been performed through expert advice from Physics Consulting, FRMII in Germany and IPEN, Brazil. A need to upgrade the facility became apparent due to the identification of various deficiencies of the current SANRAD facility during an IAEA-sponsored expert mission of international scientists to Necsa, South Africa. A lack of adequate shielding that results in high neutron background on the beam port floor, a mismatch in the collimator aperture to the core that results in a high gradient in neutron flux on the imaging plane and due to a relative low L/D the quality of the radiographs are poor, are a number of deficiencies to name a few.The new design, based on results of Monte Carlo (MCNP-X) simulations of neutron- and gamma transport from the reactor core and through the new facility, is being outlined. The scientific design philosophy, neutron optics and imaging capabilities that include the utilization of fission neutrons, thermal neutrons, and gamma-rays emerging from the core of SAFARI-1 are discussed.

The Relativistic Electron-Proton Telescope (REPT) Instrument on Board the Radiation Belt Storm Probes (RBSP) Spacecraft: Characterization of Earth’s Radiation Belt High-Energy Particle Populations

Particle acceleration and loss in the million electron Volt (MeV) energy range (and above) is the least understood aspect of radiation belt science. In order to measure cleanly and separately both the energetic electron and energetic proton components, there is a need for a carefully designed detector system. The Relativistic Electron-Proton Telescope (REPT) on board the Radiation Belt Storm Probe (RBSP) pair of spacecraft consists of a stack of high-performance silicon solid-state detectors in a telescope configuration, a collimation aperture, and a thick case surrounding the detector stack to shield the sensors from penetrating radiation and bremsstrahlung. The instrument points perpendicular to the spin axis of the spacecraft and measures high-energy electrons (up to ∼20 MeV) with excellent sensitivity and also measures magnetospheric and solar protons to energies well above E=100 MeV. The instrument has a large geometric factor (g=0.2 cm2 sr) to get reasonable count rates (above background) at the higher energies and yet will not saturate at the lower energy ranges. There must be fast enough electronics to avert undue dead-time limitations and chance coincidence effects. The key goal for the REPT design is to measure the directional electron intensities (in the range 10−2–106 particles/cm2 s sr MeV) and energy spectra (ΔE/E∼25 %) throughout the slot and outer radiation belt region. Present simulations and detailed laboratory calibrations show that an excellent design has been attained for the RBSP needs. We describe the engineering design, operational approaches, science objectives, and planned data products for REPT.

Proof of concept of MRI-guided tracked radiation delivery: tracking one-dimensional motion

In radiotherapy one aims to deliver a radiation dose to a tumour with high geometrical accuracy while sparing organs at risk (OARs). Although image guidance decreases geometrical uncertainties, treatment of cancer of abdominal organs is further complicated by respiratory motion, requiring intra-fraction motion compensation to fulfil the treatment intent. With an ideal delivery system, the optimal method of intra-fraction motion compensation is to adapt the beam collimation to the moving target using a dynamic multi-leaf collimator (MLC) aperture. The many guidance strategies for such tracked radiation delivery tested up to now mainly use markers and are therefore invasive and cannot deal with target deformations or adaptations for OAR positions. We propose to address these shortcomings using the online MRI guidance provided by an MRI accelerator and present a first step towards demonstration of the technical feasibility of this proposal. The position of a phantom subjected to one-dimensional (1D) periodic translation was tracked using a fast 1D MR sequence. Real-time communication with the MR scanner and control of the MLC aperture were established. Based on the time-resolved position of the phantom, tracked radiation delivery to the phantom was realized. Dose distributions for various delivery conditions were recorded on a gafchromic film. Without motion a sharply defined dose distribution is obtained, whereas considerable blur occurs for delivery to a moving phantom. With compensation for motion, the sharpness of the dose distribution is nearly restored. The total latency in our motion management architecture is approximately 200 ms. Combination of the recorded phantom and aperture positions with the planned dose distribution enabled the reconstruction of the delivered dose in all cases, which illustrates the promise of online dose accumulation and confirms that latency compensation could further enhance our results. For a simple 1D tracked delivery scenario, the technical feasibility of MRI-guided tracked radiation delivery is confirmed. More generic tracking scenarios require advanced MRI, leading to increased acquisition time and more challenging image processing problems. Latency compensation is therefore an important subject of future investigations.

A novel 2D binary collimator for IMRT dose delivery: dosimetric characterization using Monte Carlo simulations

We present a novel technical concept of a two-dimensional binary multileaf collimator (2D-bMLC) especially designed for fast dose delivery in rotational IMRT. The 2D-bMLC consists of individually controlled absorber channels, which are arranged side by side forming a 2D collimator aperture. In each channel three separate tungsten modules are arranged behind each other. To open and close an element, the central module is shifted between two positions. The purpose of this work is the presentation of the 2D-bMLC concept and its dosimetric evaluation. To determine the dosimetric properties, we designed a Monte Carlo model of an exemplary 2D-bMLC, consisting of 30 × 30 elements. A virtual source model of a flattening filter-free 7 MV linac was used to characterize the linac phase space. A primary radiation efficiency factor of 43% was calculated for the open 2D-bMLC by dividing the integral dose scored for a 2D-bMLC field by the integral dose scored for an open field with the same dimensions. The leakage calculated for the closed collimator was below 0.5%. Following the primary photon fluence distribution, the bixel intensity decreases with the distance of the element to the central axis of the treatment machine. From the collimator field’s center toward its borders, the geometric bixel widths increase in a symmetric and predictable manner by up to 4%. The increase is explained by the specific design of the 2D-bMLC. Abutting element beams exhibit a slight tongue-and-groove effect if opened sequentially. This effect as well as the primary radiation efficiency is basically affected by the source size and the dimensions of the collimator elements. We successfully established and evaluated a dosimetric model of the 2D-bMLC. The results are promising, and we will therefore investigate on real patient plans, if the concept could be advantageous for fast rotational IMRT treatments.

On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the correction factors for microchamber and diode detectors

Purpose:To measure the output factors (OFs) of the small fields formed by the variable aperture collimator system (iris) of a CyberKnife (CK) robotic radiosurgery system, and determine the correction factors for a microchamber and four diode detectors.Methods:OF measurements were performed using a PTW PinPoint 31014 microchamber, four diode detectors (PTW‐60017, −60012, −60008, and the SunNuclear EDGE detector), TLD‐100 microcubes, alanine dosimeters, EBT films, and polymer gels for the 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm iris collimators at 650 mm, 800 mm, and 1000 mm source to detector distance (SDD). The alanine OF measurements were corrected for volume averaging effects using the 3D dose distributions registered in polymer gel dosimeters. correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymer gel, EBT, alanine, and TLD results.Results:Experimental OF results are presented for the array of dosimetric systems used. The PinPoint microchamber was found to underestimate small field OFs, and a correction factor ranging from 1.127 ± 0.022 (for the 5 mm iris collimator) to 1.004 ± 0.010 (for the 15 mm iris collimator) was determined at the reference SDD of 800 mm. The PinPoint correction factor was also found to increase with decreasing SDD; values equal to 1.220 ± 0.028 and 1.077 ± 0.016 were obtained for the 5 mm iris collimator at 650 mm and 1000 mm SDD, respectively. On the contrary, diode detectors were found to overestimate small field OFs and a correction factor equal to 0.973 ± 0.006, 0.954 ± 0.006, 0.937 ± 0.007, and 0.964 ± 0.006 was measured for the PTW‐60017, −60012, −60008 and the EDGE diode detectors, respectively, for the 5 mm iris collimator at 800 mm SDD. The corresponding correction factors for the 15 mm iris collimator were found equal to 0.997 ± 0.010, 0.994 ± 0.009, 0.988 ± 0.010, and 0.986 ± 0.010, respectively. No correlation of the diode correction factors with SDD was observed.Conclusions:This work demonstrates an experimental procedure for the determination of the correction factors required to obtain small field OF results of increased accuracy.

Monte Carlo simulation of a compact microbeam radiotherapy system based on carbon nanotube field emission technology

Microbeam radiation therapy (MRT) is an experimental radiotherapy technique that has shown potent antitumor effects with minimal damage to normal tissue in animal studies. This unique form of radiation is currently only produced in a few large synchrotron accelerator research facilities in the world. To promote widespread translational research on this promising treatment technology we have proposed and are in the initial development stages of a compact MRT system that is based on carbon nanotube field emission x-ray technology. We report on a Monte Carlo based feasibility study of the compact MRT system design. Monte Carlo calculations were performed using EGSnrc-based codes. The proposed small animal research MRT device design includes carbon nanotube cathodes shaped to match the corresponding MRT collimator apertures, a common reflection anode with filter, and a MRT collimator. Each collimator aperture is sized to deliver a beam width ranging from 30 to 200 μm at 18.6 cm source-to-axis distance. Design parameters studied with Monte Carlo include electron energy, cathode design, anode angle, filtration, and collimator design. Calculations were performed for single and multibeam configurations. Increasing the energy from 100 kVp to 160 kVp increased the photon fluence through the collimator by a factor of 1.7. Both energies produced a largely uniform fluence along the long dimension of the microbeam, with 5% decreases in intensity near the edges. The isocentric dose rate for 160 kVp was calculated to be 700 Gy∕min∕A in the center of a 3 cm diameter target. Scatter contributions resulting from collimator size were found to produce only small (<7%) changes in the dose rate for field widths greater than 50 μm. Dose vs depth was weakly dependent on filtration material. The peak-to-valley ratio varied from 10 to 100 as the separation between adjacent microbeams varies from 150 to 1000 μm. Monte Carlo simulations demonstrate that the proposed compact MRT system design is capable of delivering a sufficient dose rate and peak-to-valley ratio for small animal MRT studies.