Multiple Field Sizes Research Articles

ResGEM: Multi-scale Graph Embedding Network for Residual Mesh Denoising.

Mesh denoising is a crucial technology that aims to recover a high-fidelity 3D mesh from a noise-corrupted one. Deep learning methods, particularly graph convolutional networks (GCNs) based mesh denoisers, have demonstrated their effectiveness in removing various complex real-world noises while preserving authentic geometry. However, it is still a quite challenging work to faithfully regress uncontaminated normals and vertices on meshes with irregular topology. In this paper, we propose a novel pipeline that incorporates two parallel normal-aware and vertex-aware branches to achieve a balance between smoothness and geometric details while maintaining the flexibility of surface topology. We introduce ResGEM, a new GCN, with multi-scale embedding modules and residual decoding structures to facilitate normal regression and vertex modification for mesh denoising. To effectively extract multi-scale surface features while avoiding the loss of topological information caused by graph pooling or coarsening operations, we encode the noisy normal and vertex graphs using four edge-conditioned embedding modules (EEMs) at different scales. This allows us to obtain favorable feature representations with multiple receptive field sizes. Formulating the denoising problem into a residual learning problem, the decoder incorporates residual blocks to accurately predict true normals and vertex offsets from the embedded feature space. Moreover, we propose novel regularization terms in the loss function that enhance the smoothing and generalization ability of our network by imposing constraints on normal consistency. Comprehensive experiments have been conducted to demonstrate the superiority of our method over the state-of-the-art on both synthetic and real-scanned datasets.

Advancing the Role of Palliative Proton Therapy Through Diagnostic Scan-Based Planning

Single-fraction palliative spine radiation is an attractive option for patients with poor prognosis because of its convenience and effectiveness in terms of pain relief. However, the associated large doses to normal tissues can lead to toxicities that negatively impact quality of life. Proton therapy offers a dosimetric solution to this problem but has not historically been used for palliative treatment because of inherent cost and time needed for planning. The purpose of this study was to assess a process for delivering cost-effective and efficient palliative proton therapy by using diagnostic scan-based planning (DSBP) and prefabricated treatment delivery devices.We designed and characterized a reusable proton aperture system capable of adjusting to multiple field sizes for spine treatment. Next, we retrospectively identified 10 patients treated with proton therapy for thoracic malignancies in 2019 who also had a diagnostic CT of the thorax within 4 months of simulation. For each case, one physician contoured T6-T9 as the target volume on both the diagnostic and simulation scans. Using the reusable proton aperture system, we generated passive-scatter proton plans on the diagnostic scans using a single PA beam with no custom range compensator to treat T6-T9 to 8 Gy x 1. The plans were then transferred to the simulation scans to compare target coverage and doses to normal tissues, followed by robustness analysis with 5mm X, Y, and Z shifts. Finally, we generated AP/PA photon plans on the simulation scans to compare doses to normal tissues between proton and photon plans. Results were compared using the Wilcoxon signed-rank test.Median D95% on the diagnostic scan plans was 101% (range, 100%-102%) of the prescription dose using the same 12L x 7W aperture for each case. Median Dmax was 107% (range, 105-108%). When transferred to simulation scans, coverage remained excellent and hot spots acceptable for all cases with a median D95% of 101% (range, 100%-102%) and a median Dmax of 106% (range, 105%-106%). Heart and esophagus mean and max doses did not vary significantly between the diagnostic scan and the simulation scan proton plans (P > 0.2 for all). Robustness analysis with 5mm X, Y, and Z shifts showed that coverage remained acceptable with D95% > 98% for all cases. Compared to the proton plans, mean heart dose was markedly higher for the AP/PA photon plans (median 4.7 Gy v. 0.4 Gy, P < 0.01), as was mean esophagus dose (median 7.1 Gy v. 4.9 Gy, P < 0.01). Heart and esophagus max doses were not significantly different between the proton and photon plans (P > 0.05).Palliative proton DSBP is technically feasible and robust with superior sparing of normal tissues compared to AP/PA photon plans. By eliminating the need for simulation and custom devices, the cost-benefit ratio for palliative proton therapy may become more acceptable, particularly in a bundled payment environment.

Scale- and Context-Aware Convolutional Non-Intrusive Load Monitoring

Non-intrusive load monitoring addresses the challenging task of decomposing the aggregate signal of a household's electricity consumption into appliance-level data without installing dedicated meters. By detecting load malfunction and recommending energy reduction programs, cost-effective non-intrusive load monitoring provides intelligent demand-side management for utilities and end users. In this paper, we boost the accuracy of energy disaggregation with a novel neural network structure named scale- and context-aware network, which exploits multi-scale features and contextual information. Specifically, we develop a multi-branch architecture with multiple receptive field sizes and branch-wise gates that connect the branches in the sub-networks. We build a self-attention module to facilitate the integration of global context, and we incorporate an adversarial loss and on-state augmentation to further improve the model's performance. Extensive simulation results tested on open datasets corroborate the merits of the proposed approach, which significantly outperforms state-of-the-art methods.

Axonal Anatomy Optimizes Spatial Encoding in the Rat Entorhinal-Dentate System: A Computational Study.

The network architecture connecting neural regions is defined by the organization and anatomical properties of the projecting axons, but its contributions to neural encoding and system function are difficult to study experimentally. Using a large-scale, spiking neuronal network model of rat dentate gyrus, the role of the anatomy of the entorhinal-dentate axonal projection was evaluated in the context of spatial encoding by incorporating grid cell activity to provide physiological, spatially-correlated input. The dorso-ventral extents of the entorhinal axon terminal fields were varied to generate different feedforward architectures, and the resulting spatial representations and spatial information scores of the network were evaluated. Position was decoded from the population activity using a point process filter to investigate the contributions of network architecture on spatial encoding. The model predicted the emergence of anatomical gradients within the dentate gyrus for place field size and spatial information along its dorso-ventral axis, which were dependent on the extents of the entorhinal axon terminal fields. The decoding results revealed an optimal performance at an axon terminal field extent of 2mm that lies within the biological range. The axonal anatomy mediates a tradeoff between encoding multiple place field sizes or achieving a high spatial information score, and the combination of both properties is necessary to maximize spatial encoding by a network. In total, this paper establishes a mechanistic neuronal network model that, in concert with information-theoretic and statistical methods, can be used to investigate how lower level properties contribute to higher level function.

In-vivo EPID dosimetry for IMRT and VMAT based on through-air predicted portal dose algorithm

We have adapted the methodology of Berry et al. (2012) for Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) treatments at a fixed source to imager distance (SID) based on the manufacturer’s through-air portal dose image prediction algorithm. In order to fix the SID a correction factor was introduced to account for the change in air gap between patient and imager. Commissioning data, collected with multiple field sizes, solid water thicknesses and air gaps, were acquired at 150 cm SID on the Varian aS1200 EPID. The method was verified using six IMRT and seven VMAT plans on up to three different phantoms. The method’s sensitivity and accuracy were investigated by introducing errors. A global 3%/3 mm gamma was used to assess the differences between the predicted and measured portal dose images. The effect of a varying air gap on EPID signal was found to be significant – varying by up to 30% with field size, phantom thickness, and air gap. All IMRT plans passed the 3%/3 mm gamma criteria by more than 95% on the three phantoms. 23 of 24 arcs from the VMAT plans passed the 3%/3 mm gamma criteria by more than 95%. This method was found to be sensitive to a range of potential errors. The presented approach provides fast and accurate in-vivo EPID dosimetry for IMRT and VMAT treatments and can potentially replace many pre-treatment verifications.

A Simple Method for 2-D In Vivo Dosimetry by Portal Imaging.

Purpose:To improve patient safety and treatment quality, verification of dose delivery in radiotherapy is desirable. We present a simple, easy-to-implement, open-source method for in vivo planar dosimetry of conformal radiotherapy by electronic portal imaging device (EPID).Methods:Correlation ratios, which relate dose in the mid-depth of slab phantoms to transit EPID signal, were determined for multiple phantom thicknesses and field sizes. Off-axis dose is corrected for by means of model-based convolution. We tested efficacy of dose reconstruction through measurements with off-reference values of attenuator thickness, field size, and monitor units. We quantified the dose calculation error in the presence of thickness changes to simulate anatomical or setup variations. An example of dose calculation on patient data is provided.Results:With varying phantom thickness, field size, and monitor units, dose reconstruction was almost always within 3% of planned dose. In the presence of thickness changes from planning CT, the dose discrepancy is exaggerated by up to approximately 1.5% for 1 cm changes upstream of the isocenter plane and 4% for 1 cm changes downstream.Conclusion:Our novel electronic portal imaging device in vivo dosimetry allows clinically accurate 2-dimensional reconstruction of dose inside a phantom/patient at isocenter depth. Due to its simplicity, commissioning can be performed in a few hours per energy and may be modified to the user’s needs. It may provide useful dose delivery information to detect harmful errors, guide adaptive radiotherapy, and assure quality of treatment.

SU‐F‐T‐475: An Evaluation of the Overlap Between the Acceptance Testing and Commissioning Processes for Conventional Medical Linear Accelerators

Purpose:This work's objective is to determine the overlap of processes, in terms of sub‐processes and time, between acceptance testing and commissioning of a conventional medical linear accelerator and to evaluate the time saved by consolidating the two processes.Method:A process map for acceptance testing for medical linear accelerators was created from vendor documentation (Varian and Elekta). Using AAPM TG‐106 and inhouse commissioning procedures, a process map was created for commissioning of said accelerators. The time to complete each sub‐process in each process map was evaluated. Redundancies in the processes were found and the time spent on each were calculated.Results:Mechanical testing significantly overlaps between the two processes ‐ redundant work here amounts to 9.5 hours. Many beam non‐scanning dosimetry tests overlap resulting in another 6 hours of overlap. Beam scanning overlaps somewhat ‐ acceptance tests include evaluating PDDs and multiple profiles but for only one field size while commissioning beam scanning includes multiple field sizes and depths of profiles. This overlap results in another 6 hours of rework. Absolute dosimetry, field outputs, and end to end tests are not done at all in acceptance testing. Finally, all imaging tests done in acceptance are repeated in commissioning, resulting in about 8 hours of rework. The total time overlap between the two processes is about 30 hours.Conclusion:The process mapping done in this study shows that there are no tests done in acceptance testing that are not also recommended to do for commissioning. This results in about 30 hours of redundant work when preparing a conventional linear accelerator for clinical use. Considering these findings in the context of the 5000 linacs in the United states, consolidating acceptance testing and commissioning would have allowed for the treatment of an additional 25000 patients using no additional resources.

SU‐E‐T‐249: Determining the Sensitivity of Beam Profile Parameters for Detecting Energy Changes in Flattening Filter‐Free Beams

Purpose:To identify the beam profile parameters that can be used to detect energy changes in a flattening filter‐free photon beams.Methods:Flattening filter‐free beam profiles (inline, crossline, and diagonals) were measured for multiple field sizes (25×25cm and 10×10cm) at 6MV on a clinical system (Truebeam, Varian Medical Systems Palo Alto CA). Profiles were acquired for baseline energy and detuned beams by changing the bending magnet current (BMC), above and below baseline. The following profile parameters were measured: flatness (off‐axis ratio at 80% of field size), symmetry, uniformity, slope, and the off‐axis ratio (OAR) at several off‐axis distances. Tolerance values were determined from repeated measurements. Each parameter was evaluated for sensitivity to the induced beam changes, and the minimum detectable BMC change was calculated for each parameter by calculating the change in BMC that would Result in a change in the parameter above the measurement tolerance.Results:Tolerance values for the parameters were‐Flatness≤0.1%; Symmetry≤0.4%; Uniformity≤0.01%; Slope≤ 0.001%/mm. The measurements made with a field size of 25cm and a depth of d=1.5cm showed the greatest sensitivity to bending magnet current variations. Uniformity had the highest sensitivity, able to detect a change in BMC of BMC=0.02A. The OARs and slope were sensitive to the magnitude and direction of BMC change. The sensitivity in the flatness parameter was BMC=0.04A; slope was sensitive to BMC=0.05A. The sensitivity decreased for OARs measured closer to central axis‐BMC(8cm)=0.23A; BMC(5cm)=0.47A; BMC(2cm)=1.35A. Symmetry was not sensitive to changes in BMC.Conclusion:These tests allow for better QA of FFF beams by setting tolerance levels to beam parameter baseline values which reflect variations in machine calibration. Uniformity is most sensitive to BMC changes, while OARs provide information about magnitude and direction of miscalibration.Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

SU‐E‐T‐361: Energy Dependent Radiation/light‐Field Misalignment On Truebeam Linear Accelerator

Purpose:Verifying the co‐incidence of the radiation and light field is recommended by TG‐142 for monthly and annual checks. On a digital accelerator, it is simple to verify that beam steering settings are consistent with accepted and commissioned values. This fact should allow for physicists to verify radiation‐light‐field co‐incidence for a single energy and accept that Result for all energies. We present a case where the radiation isocenter deviated for a single energy without any apparent modification to the beam steering parameters.Methods:The radiation isocenter was determined using multipleMethods:Gafchromic film, a BB test, and radiation profiles measured with a diode. Light‐field borders were marked on Gafchromic film and then irradiated for all photon energies. Images of acceptance films were compared with films taken four months later. A phantom with a radio‐opaque BB was aligned to isocenter using the light‐field and imaged using the EPID for all photon energies. An unshielded diode was aligned using the crosshairs and then beam profiles of multiple field sizes were obtained. Field centers were determined using Omni‐Pro v7.4 software, and compared to similar scans taken during commissioning. Beam steering parameter files were checked against backups to determine that the steering parameters were unchanged.Results:There were no differences between the configuration files from acceptance. All three tests demonstrated that a single energy had deviated from accepted values by 0.8 mm in the inline direction. The other two energies remained consistent with previous measurements. The deviated energy was re‐steered to be within our clinical tolerance.Conclusions:Our study demonstrates that radiation‐light‐field coincidence is an energy dependent effect for modern linacs. We recommend that radiation‐light‐field coincidence be verified for all energies on a monthly basis, particularly for modes used to treat small fields, as these may drift without influencing results from other tests.

TU‐C‐108‐07: Transmission Detector Array System for RT Delivery Monitoring: Validation Testing

Purpose: To quantify the influence of the gantry‐mounted Delta4‐AT detector array on beam quality, and the detection sensitivity to machine output and MLC position deviations. Methods: A Delta4‐AT prototype was attached to the accessory holder mount of a Varian 21EX linear accelerator. The system utilizes an array of ∼4,000 transmission diodes (40 per MLC leaf, every 2.5mm along the direction of motion) to measure the fluence of photon beams during treatment deliveries. To quantify the influence from the detector array on beam quality, the percentage depth dose curves were measured with an IBA PFD3G diode in water and compared between with and without detector array. A static field with comb‐shaped MLC pattern was used to evaluate the systems ability to detect machine output and MLC position deviations. 1–5% MU deviations and 1–5 mm MLC offsets were introduced to the static plan. The modified plans were then delivered and the measurements by Delta4‐AT were compared to the original delivery. Results: For the 6MV beam tested in this study, the transmission reduction due to the presence of the detector array is ∼1% at both dmax and 10 cm depths for multiple field sizes. The skin dose measured at 1mm depth increases by &lt; 0.5% relative to dose at dmax for field sizes up to 10cm, and by 4% and 7% for 20×20cm and 34×34cm fields, respectively. For plans with introduced MU deviations, the detected MU errors matched the planned errors within 0.8%. MLC position errors of 1‐5 mm are also identifiable through measured dose profiles by Delta4‐AT. The measured leaf position deviation is within 0.5 mm from the actual offsets introduced. Conclusion: The prototype Delta4‐AT transmission detector array has small influence on the beam quality, and successfully detected machine output deviations and MLC position offsets as low as 1% and 1mm, respectively.This research was partially supported by a grant from Scandidos Inc.

SU‐E‐T‐257: A Monte Carlo Simulation Study of Photon Beam with Energies 6X, 10X, 6FFF, 10FFF From a TrueBeam Linear Accelerator

Purpose: To compare the quality of Monte Carlo simulation results of photon beams with multiple energies with the corresponding measured beam commissioning data from a Varian TrueBeam accelerator. Methods: IAEA phase space files for photon beam energies 6X, 10X, 6FFF, 10FFF from TrueBeam linear accelerator provided by Varian were implemented as source files in the GEANT4 Monte Carlo code. The simulation geometry consisted of upper and lower tungsten alloy jaws and a cubic water phantom (50 × 50 × 50 cm3). Central axis depth dose curves (PDD) for multiple field sizes (1×1, 2×2, 4×4, 6×6, 10×10, 20×20, 30×30 cm2) and radial and transverse dose profiles for identical field sizes at 5 different depths (Dmax, 5, 10, 20, 30 cm) for all four energies were recorded in the phantom. Each simulation was performed with 2.0E9 incident particles. The results of PDD and profiles from simulations were then compared with the corresponding beam commissioning data collected with a Wellhoffer Blue Phantom using a 0.13cc ion‐chamber and a 0.8 × 0.8 mm2 diode. Results: The GEANT4 simulated PDD curve for photon beam compared favorably well, within ∼2%, against the corresponding measured ion‐chamber PDD for all energies and field sizes. The simulated in‐plane and cross‐plane profiles also compared well, within 2 mm, at the 50% level against the measured profiles for all energies, field sizes and depths. Conclusion: For the commissioning of a linear accelerator, the feasibility of utilizing Monte‐Carlo simulated beam data has been demonstrated in this study. The increase of computer speed and capabilities may Result in the adoption of Monte‐Carlo techniques for future machine commissioning and dose calculations.

On the road to invariant recognition: Explaining tradeoff and morph properties of cells in inferotemporal cortex using multiple-scale task-sensitive attentive learning

Visual object recognition is an essential accomplishment of advanced brains. Object recognition needs to be tolerant, or invariant, with respect to changes in object position, size, and view. In monkeys and humans, a key area for recognition is the anterior inferotemporal cortex (ITa). Recent neurophysiological data show that ITa cells with high object selectivity often have low position tolerance. We propose a neural model whose cells learn to simulate this tradeoff, as well as ITa responses to image morphs, while explaining how invariant recognition properties may arise in stages due to processes across multiple cortical areas. These processes include the cortical magnification factor, multiple receptive field sizes, and top-down attentive matching and learning properties that may be tuned by task requirements to attend to either concrete or abstract visual features with different levels of vigilance. The model predicts that data from the tradeoff and image morph tasks emerge from different levels of vigilance in the animals performing them. This result illustrates how different vigilance requirements of a task may change the course of category learning, notably the critical features that are attended and incorporated into learned category prototypes. The model outlines a path for developing an animal model of how defective vigilance control can lead to symptoms of various mental disorders, such as autism and amnesia.

How do Task-dependent Attentional Demands Alter How Objects are Learned?

We learn to recognize objects in the world in environments whose attentional demands vary greatly. How does such learning depend upon task-dependent attentional demands? Object recognition needs to be tolerant, or invariant, with respect to position, size, and object view changes. In monkeys and humans, a key area for recognition is the anterior inferotemporal cortex (ITa). Recent neurophysiological data show that ITa cells with high object selectivity often have low position tolerance. We propose a neural model whose cells learn to simulate this tradeoff, as well as ITa responses to image morphs, while explaining how invariant recognition properties may arise gradually due to processes across multiple cortical areas, including the cortical magnification factor, multiple receptive field sizes, and top-down attentive matching and learning properties that may be tuned by task requirements to attend to either concrete or abstract visual features. The model predicts that data from the tradeoff and image morph tasks emerge from different task-dependent levels of attentive vigilance in the animals performing them. Computer simulations predict how receptive field properties would change under different task-sensitive vigilance levels. The model also predicts how vigilance may be controlled by mismatches between top-down learned expectations and bottom-up perceptual inputs, leading to acetylcholine release in neocortical circuits and an increase in vigilance. These results emphasize the importance of top-down attentional mechanisms in object learning and recognition, and of the need to carefully monitor task demands in studies of perceptual and cognitive processing.

Assessment of computerized treatment planning system accuracy in calculating wedge factors of physical wedged fields for 6 MV photon beams

Wedge filters are commonly used in external beam radiotherapy to achieve a uniform dose distribution within the target volume. The main objective of this study was to investigate the accuracy of the beam modifier algorithm of Theraplan plus (TPP version 3.8) treatment planning system and to confirm that either the beam hardening, beam softening and attenuation coefficients along with wedge geometry and measured wedge factor at single depth and multiple fields sizes can be the replacement of wedged profile and wedged cross-sectional data or not. In this regard the effect of beam hardening and beam softening was studied with physical wedges for 6 MV photons. The Normalized Wedge Factors (NWFs) were measured experimentally as well as calculated with the Theraplan plus, as a function of depth and field size in a water phantom for 15°, 30°, 45°, and 60° wedge filters. The beam hardening and softening was determined experimentally by deriving the required coefficients for all wedge angles. The TPP version 3.8 requires wedge transmission factor at single depth and multiple field sizes. Without incorporating the hardening and softening coefficients the percent difference between measured and calculated NFWs was as high as 7%. After the introduction of these parameters into the algorithm, the agreement between measured and TPP (V 3.8) calculated NWFs were improved to within 2 percent for various depths. Similar improvement was observed in TPP version 3.8 while calculating NWFs for various field sizes when the required coefficients were adjusted. In conclusion, the dose calculation algorithm of TPP version 3.8 showed good accuracy for a 6 MV photon beam provided beam hardening and softening parameters are taken into account. From the results, it is also concluded that, the beam hardening, beam softening and attenuation coefficients along with wedge geometry and measured wedge factor at single depth and multiple fields sizes can be the replacement of wedged profile and wedged cross-sectional data in the TPS. The study also indicated that by ignoring the beam softening and beam hardening will result in an inaccurate dose to the target volume of the patient.

SU‐GG‐T‐402: On Monte Carlo Modeling of Megavoltage Photon Beams: A Revisited Study on Beam Parameters Sensitivity

Purpose: To commission Monte Carlo beam models for 5 Varian megavoltage photon beams (4, 6, 10, 15 and 18 MV). The goal is to match measured dose distributions in a water phantom within 1% local difference for a wide range of field sizes. Another objective is to re‐investigate the sensitivity of dose distributions to the primary electron beam parameters. Material and Methods: The GEPTS Monte Carlo code is used for photon beam simulation and dose calculations. Linac geometry models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma (Med. Phys. Vol. 34, 2007), and (iii) recent drawings of the W/Pb shields. Measurements were performed using pinpoint and Farmer ionization chambers depending on the field size. In addition to the three commonly used primary electron beam parameters (EAV: average energy, FWHM: energy spectrum broadening, and R: beam radius), we also included the primary electron angular divergence (q). Results: We were able to match within 1% local difference measured depth dose distributions and lateral dose profiles (within 1%/1mm in penumbra), at any depth beyond 1 cm. Varying primary beam parameters away from the solution can lead to 5% differences with measurements for small (e.g. 2×2 cm2) and large (e.g. 35×35 cm2) field size WHILE a perfect agreement still stands for 10×10. Very small and very large field sizes prove to be more sensitive to FWHM, R and q variations than the 10×10 field size. Conclusion: GEPTS beam models reproduce measured dose distributions within 1% local difference and simultaneously for field sizes varying from 2×2 to 35×35. Achieving 1% agreement between measured and calculated dose distributions for the 10×10 field size is relatively easy. However, this does not guaranty a similar agreement for other field sizes. Therefore, Monte Carlo beam models need to be commissioned for multiple field sizes..

Comparison of Virtual HDR Prostate Treatment Plans Created with Fixed and Variable Aperture Collimators

We previously compared Virtual HDR prostate CyberKnife System (CK) SBRT plans versus simulated HDR brachytherapy plans, based on identical contour sets for each comparison plan pair, in 10 consecutive patients (Int J Radiat Oncol Biol Phys 2008; 70: 1588-1597). In the current analysis, for the same patient cohort, we assess the same endpoints for plans created using the recently released Iris™ Variable Aperture Collimator. The 10 Virtual HDR prostate CK cases were planned and treated using one or two fixed collimators (“Fixed”). The prescription dose was 38 Gy delivered in 4 fractions. These cases were replanned using the Iris Collimator and the recently released Sequential Optimization inverse planning algorithm (“Iris”). The Iris Collimator allows the use of any of 12 field sizes between 5 and 60 mm for each beam in a single traversal of the robotic manipulator. Replanning objectives included maintaining or improving the conformality and normal tissue sparing of the original plan, increasing extraurethral-intraprostatic dose escalation, and minimizing the number of beams and monitor units (MU). Median (range): Non-zero beams: 266 (234-318), Fixed; 191 (141-208), Iris; median difference 92, 10/10 cases lower with Iris. MU: 101.2K (76.0K-116.6K), Fixed; 76.4K (67.1K-89.2K), Iris; 10/10 cases lower with Iris. Beam-on time: 42.2 min, Fixed (600 MU/min CK platform); 23.9 min, Iris (800 mu/min CK platform). Rx isodose line: 57% (49-67%), Fixed; 53% (47-60%), Iris; 10/10 cases same or lower with Iris. V125: 43.95% (28.4-55.5%), Fixed; 45.6% (35.5-55.9%), Iris; 7/10 cases higher with Iris. V150: 8.45% (0.1-20.5%), Fixed; 18.05% (3.0-22.0%), Iris; 9/10 cases higher with Iris. nCI: 1.14 (1.10-1.23), Fixed; 1.12 (1.10-1.20), Iris; 8/10 cases same or lower with Iris. Urethra Dmax: 44.25 Gy (43.1-46.1 Gy), Fixed; 42.05 Gy (40.6-43.4 Gy), Iris; 9/10 cases lower with Iris. Rectum Dmax, Bladder Dmax, Urethra D50: No significant difference. Treatment plans generated using the Iris Collimator and Sequential Optimization consistently have considerably fewer beams and MU than plans generated using one or two fixed collimators, with a median 43% reduction of beam on time per case. Part of the time reduction is due to a higher dose rate linac (800 vs. 600 MU/min). Simultaneously, the Iris Collimator plans typically have similar or better conformality, higher maximum intraprostatic doses, lower maximum urethral doses, and similar maximum doses to the bladder and rectum. In summary, using multiple field sizes enabled by the Iris Collimator can produce treatment plans with a more tailored, HDR-like dose distribution and that can be delivered more efficiently than plans created with one or two fixed collimators.

The design, physical properties and clinical utility of an iris collimator for robotic radiosurgery

Robotic radiosurgery using more than one circular collimator can improve treatment plan quality and reduce total monitor units (MU). The rationale for an iris collimator that allows the field size to be varied during treatment delivery is to enable the benefits of multiple-field-size treatments to be realized with no increase in treatment time due to collimator exchange or multiple traversals of the robotic manipulator by allowing each beam to be delivered with any desired field size during a single traversal. This paper describes the Iris™ variable aperture collimator (Accuray Incorporated, Sunnyvale, CA, USA), which incorporates 12 tungsten–copper alloy segments in two banks of six. The banks are rotated by 30° with respect to each other, which limits the radiation leakage between the collimator segments and produces a 12-sided polygonal treatment beam. The beam is approximately circular, with a root-mean-square (rms) deviation in the 50% dose radius of <0.8% (corresponding to <0.25 mm at the 60 mm field size) and an rms variation in the 20–80% penumbra width of about 0.1 mm at the 5 mm field size increasing to about 0.5 mm at 60 mm. The maximum measured collimator leakage dose rate was 0.07%. A commissioning method is described by which the average dose profile can be obtained from four profile measurements at each depth based on the periodicity of the isodose line variations with azimuthal angle. The penumbra of averaged profiles increased with field size and was typically 0.2–0.6 mm larger than that of an equivalent fixed circular collimator. The aperture reproducibility is ⩽0.1 mm at the lower bank, diverging to ⩽0.2 mm at a nominal treatment distance of 800 mm from the beam focus. Output factors (OFs) and tissue-phantom-ratio data are identical to those used for fixed collimators, except the OFs for the two smallest field sizes (5 and 7.5 mm) are considerably lower for the Iris Collimator. If average collimator profiles are used, the assumption of circular symmetry results in dose calculation errors that are <1 mm or <1% for single beams across the full range of field sizes; errors for multiple non-coplanar beam treatment plans are expected to be smaller. Treatment plans were generated for 19 cases using the Iris Collimator (12 field sizes) and also using one and three fixed collimators. The results of the treatment planning study demonstrate that the use of multiple field sizes achieves multiple plan quality improvements, including reduction of total MU, increase of target volume coverage and improvements in conformality and homogeneity compared with using a single field size for a large proportion of the cases studied. The Iris Collimator offers the potential to greatly increase the clinical application of multiple field sizes for robotic radiosurgery.

Dosimetric characterization and application of an imaging beam line with a carbon electron target for megavoltage cone beam computed tomography

Imaging dose from megavoltage cone beam computed tomography (MVCBCT) can be significantly reduced without loss of image quality by using an imaging beam line (IBL), with no flattening filter and a carbon, rather than tungsten, electron target. The IBL produces a greater keV-range x-ray fluence than the treatment beam line (TBL), which results in a more optimal detector response. The IBL imaging dose is not necessarily negligible, however. In this work an IBL was dosimetrically modeled with the Philips Pinnacle3 treatment planning system (TPS), verified experimentally, and applied to clinical cases. The IBL acquisition dose for a 200 degrees gantry rotation was verified in a customized acrylic cylindrical phantom at multiple imaging field sizes with 196 ion chamber measurements. Agreement between the measured and calculated IBL dose was quantified with the 3D gamma index. Representative IBL and TBL imaging dose distributions were calculated for head and neck and prostate patients and included in treatment plans using the imaging dose incorporation (IDI) method. Surface dose was measured for the TBL and IBL for four head and neck cancer patients with MOSFETs. The IBL model, when compared to the percentage depth dose and profile measurements, had 97% passing gamma indices for dosimetric and distance acceptance criteria of 3%, 3 mm, and 100% passed for 5.2%, 5.2 mm. For the ion chamber measurements of phantom image acquisition dose, the IBL model had 93% passing gamma indices for acceptance criteria of 3%, 3 mm, and 100% passed for 4%, 4 mm. Differences between the IBL- and TBL-based IMRT treatment plans created with the IDI method were dosimetrically insignificant for both the prostate and head and neck cases. For IBL and TBL beams with monitor unit values that would result in the delivery of the same dose to the depth of maximum dose under standard calibration conditions, the IBL imaging surface dose was higher than the TBL imaging surface dose by an average of 18%, with a standard deviation of 8% (p = 2 x 10(-6)). The IBL can be modeled with acceptable accuracy using a standard TPS, and accounting for IBL dose in treatment plans with the IDI method is straightforward. The resulting composite dose distributions, assuming similar imaging doses, are negligibly different from those of the TBL. The increased IBL surface dose relative to the TBL is likely clinically insignificant.

SU-GG-T-105: Optimal Mix of Electron and Photon Energies Using Linear Programming

Purpose: To find an optimal mix of electron (and photon) energies for treatment of superficial lesions. Introduction: The PDD curves of available electron energies frequently do not match the prescription depths and desired depth doses for superficial lesions. A mixture of electron (and photon) energies however can be used to create a composite PDD curve that closely matches the clinically required one. Method and Materials: The desired PDD is determined either manually, defining it at several depths, or semi‐automatically, defining only the dose at 1 mm and the proximal and distal 90% depths. In this case, the 50% and the maximum depths will be estimated. An Excel table contains PDDs for all electron energies with 0, 0.5,..,2 cm bolus. The small field size dependence of the PDDs is neglected. The table also includes PDDs of photon beams with multiple field sizes. The user can select multiple available energies from the table. For optimization criteria specifying PDD values to fit, upper/lower limits and exact values are all allowed. Inclusion of electron fields with boluses and photon fields is also possible. The score function is the maximum deviation between the desired PDD values and the resulting PDD at the depths to be fitted, while requesting lower, upper or equality constraints (if defined). The optimization was performed with the linear programming code of MATLAB. Results: Tests for the correctness of the optimization and a number of optimization tests were providing good match to the desired PDD and satisfying the given constraints. Multiple examples will be presented. Conclusion: The method is simple, easy to use, and an optimal PDD can be achieved. Since the program is written in MATLAB, it is transferable.

SU‐GG‐T‐461: Beam Properties and Beam Modeling of a Radiosurgical Variable Aperture Collimator

Purpose: The CyberKnife® Robotic Radiosurgery System (Accuray, Inc.) was developed with twelve fixed secondary collimators which deliver circular beams from 5 to 60 mm in diameter at 800 mm SAD. The effective beam diameters vary as actual SADs can range from approximately 650 to 1000 mm depending on the particular treatment. Accuray recently developed the Iris™ Variable Aperture Collimator to improve the ability to use multiple field sizes in a treatment. Tests were performed to compare the beam properties of the Iris Collimator with the fixed collimators. Method and Materials: The Iris Collimator consists of two stacked banks of rotationally offset hexagonal apertures. The resulting beam has a 12‐sided shape and a field size that can be varied between 5 and 60 mm at 800 mm SAD. The standard (non‐Monte Carlo) CyberKnife dose calculation algorithm assumes a circularly symmetric beam model. Beam data acquired with stereotactic diodes was processed to generate average dose profiles that are representative of equivalent circular beams for the Iris Collimator. These beam profiles were uploaded to the treatment planning system and the calculated dose distributions were compared with radiochromic film measurements. Results: Gamma histogram film analyses and water phantom measurements show that Iris Collimator beams are substantially equivalent to fixed circular collimator beams with respect to beam quality, collimator transmission and transverse profiles. Maximum leakage dose through the Iris Collimator &lt;0.2%, RMS deviation of the 50% isodose curve from circularity &lt;2%, and the average profile 20–80% penumbra is typically a fraction of a millimeter wider than that of equivalent fixed circular collimator. Conclusion: The beam properties of the 12‐sided Iris™ Collimator are sufficiently similar to those of the fixed circular collimators that the same beam modeling approach and standard dose calculation algorithm used for fixed collimators can be used with the Iris Collimator.