D Saenz

A dose homogeneity and conformity evaluation between ViewRay and pinnacle-based linear accelerator IMRT treatment plans

ViewRay, a novel technology providing soft-tissue imaging during radiotherapy is investigated for treatment planning capabilities assessing treatment plan dose homogeneity and conformity compared with linear accelerator plans. ViewRay offers both adaptive radiotherapy and image guidance. The combination of cobalt-60 (Co-60) with 0.35 Tesla magnetic resonance imaging (MRI) allows for magnetic resonance (MR)-guided intensity-modulated radiation therapy (IMRT) delivery with multiple beams. This study investigated head and neck, lung, and prostate treatment plans to understand what is possible on ViewRay to narrow focus toward sites with optimal dosimetry. The goal is not to provide a rigorous assessment of planning capabilities, but rather a first order demonstration of ViewRay planning abilities. Images, structure sets, points, and dose from treatment plans created in Pinnacle for patients in our clinic were imported into ViewRay. The same objectives were used to assess plan quality and all critical structures were treated as similarly as possible. Homogeneity index (HI), conformity index (CI), and volume receiving <20% of prescription dose (DRx) were calculated to assess the plans. The 95% confidence intervals were recorded for all measurements and presented with the associated bars in graphs. The homogeneity index (D5/D95) had a 1-5% inhomogeneity increase for head and neck, 3-8% for lung, and 4-16% for prostate. CI revealed a modest conformity increase for lung. The volume receiving 20% of the prescription dose increased 2-8% for head and neck and up to 4% for lung and prostate. Overall, for head and neck Co-60 ViewRay treatments planned with its Monte Carlo treatment planning software were comparable with 6 MV plans computed with convolution superposition algorithm on Pinnacle treatment planning system.

Commissioning an Elekta Versa HD linear accelerator

The purpose of this study is to report the dosimetric aspects of commissioning performed on an Elekta Versa HD linear accelerator (linac) with high‐dose‐rate flattening filter‐free (FFF) photon modes and electron modes. Acceptance and commissioning was performed on the Elekta Versa HD linac with five photon energies (6 MV, 10 MV, 18 MV, 6 MV FFF, 10 MV FFF), four electron energies (6 MeV, 9 MeV, 12 MeV, 15 MeV) and 160‐leaf (5 mm wide) multileaf collimators (MLCs). Mechanical and dosimetric data were measured and evaluated. The measurements include percent depth doses (PDDs), in‐plane and cross‐plane profiles, head scatter factor (Sc), relative photon output factors (Scp), universal wedge transmission factor, MLC transmission factors, and electron cone factors. Gantry, collimator, and couch isocentricity measurements were within 1 mm, 0.7 mm, and 0.7 mm diameter, respectively. The PDDs of 6 MV FFF and 10 MV FFF beams show deeper dmax and steeper falloff with depth than the corresponding flattened beams. While flatness values of 6 MV FFF and 10 MV FFF normalized profiles were expectedly higher than the corresponding flattened beams, the symmetry values were almost identical. The cross‐plane penumbra values were higher than the in‐plane penumbra values for all the energies. The MLC transmission values were 0.5%, 0.6%, and 0.6% for 6 MV, 10 MV, and 18 MV photon beams, respectively. The electron PDDs, profiles, and cone factors agree well with the literature. The outcome of radiation treatment is directly related to the accuracy in the dose modeled in the treatment planning system, which is based on the commissioned data. Commissioning data provided us a valuable insight into the dosimetric characteristics of the beam. This set of commissioning data can provide comparison data to others performing Versa HD commissioning, thereby improving patient safety. PACS number(s): 87.56.bdThe purpose of this study is to report the dosimetric aspects of commissioning performed on an Elekta Versa HD linear accelerator (linac) with high-dose-rate flattening filter-free (FFF) photon modes and electron modes. Acceptance and commissioning was performed on the Elekta Versa HD linac with five photon energies (6 MV, 10 MV, 18 MV, 6 MV FFF, 10 MV FFF), four electron energies (6 MeV, 9 MeV, 12 MeV, 15 MeV) and 160-leaf (5 mm wide) multileaf collimators (MLCs). Mechanical and dosimetric data were measured and evaluated. The measurements include percent depth doses (PDDs), in-plane and cross-plane profiles, head scatter factor (Sc), relative photon output factors (Scp), universal wedge transmission factor, MLC transmission factors, and electron cone factors. Gantry, collimator, and couch isocentricity measurements were within 1 mm, 0.7 mm, and 0.7 mm diameter, respectively. The PDDs of 6 MV FFF and 10 MV FFF beams show deeper dmax and steeper falloff with depth than the corresponding flattened beams. While flatness values of 6 MV FFF and 10 MV FFF normalized profiles were expectedly higher than the corresponding flattened beams, the symmetry values were almost identical. The cross-plane penumbra values were higher than the in-plane penumbra values for all the energies. The MLC transmission values were 0.5%, 0.6%, and 0.6% for 6 MV, 10 MV, and 18 MV photon beams, respectively. The electron PDDs, profiles, and cone factors agree well with the literature. The outcome of radiation treatment is directly related to the accuracy in the dose modeled in the treatment planning system, which is based on the commissioned data. Commissioning data provided us a valuable insight into the dosimetric characteristics of the beam. This set of commissioning data can provide comparison data to others performing Versa HD commissioning, thereby improving patient safety. PACS number(s): 87.56.bd.

Characterization of a 0.35T MR system for phantom image quality stability and in vivo assessment of motion quantification

ViewRay is a novel MR‐guided radiotherapy system capable of imaging in near real‐time at four frames per second during treatment using 0.35T field strength. It allows for improved gating techniques and adaptive radiotherapy. Three cobalt‐60 sources (∼15,000 Curies) permit multiple‐beam, intensity‐modulated radiation therapy. The primary aim of this study is to assess the imaging stability, accuracy, and automatic segmentation algorithm capability to track motion in simulated and in vivo targets. Magnetic resonance imaging (MRI) characteristics of the system were assessed using the American College of Radiology (ACR)‐recommended phantom and accreditation protocol. Images of the ACR phantom were acquired using a head coil following the ACR scanning instructions. ACR recommended T1‐ and T2‐weighted sequences were evaluated. Nine measurements were performed over a period of seven months, on just over a monthly basis, to establish consistency. A silicon dielectric gel target was attached to the motor via a rod. 40 mm total amplitude was used with cycles of 3 to 9 s in length in a sinusoidal trajectory. Trajectories of six moving clinical targets in four canine patients were quantified and tracked. ACR phantom images were analyzed, and the results were compared with the ACR acceptance levels. Measured slice thickness accuracies were within the acceptance limits. In the 0.35 T system, the image intensity uniformity was also within the ACR acceptance limit. Over the range of cycle lengths, representing a wide range of breathing rates in patients imaged at four frames/s, excellent agreement was observed between the expected and measured target trajectories. In vivo canine targets, including the gross target volume (GTV), as well as other abdominal soft tissue structures, were visualized with inherent MR contrast, allowing for preliminary results of target tracking. PACS number: 87.61.TgViewRay is a novel MR-guided radiotherapy system capable of imaging in near real-time at four frames per second during treatment using 0.35T field strength. It allows for improved gating techniques and adaptive radiotherapy. Three cobalt-60 sources (∼15,000 Curies) permit multiple-beam, intensity-modulated radiation therapy. The primary aim of this study is to assess the imaging stability, accuracy, and automatic segmentation algorithm capability to track motion in simulated and in vivo targets. Magnetic resonance imaging (MRI) characteristics of the system were assessed using the American College of Radiology (ACR)-recommended phantom and accreditation protocol. Images of the ACR phantom were acquired using a head coil following the ACR scanning instructions. ACR recommended T1- and T2-weighted sequences were evaluated. Nine measurements were performed over a period of seven months, on just over a monthly basis, to establish consistency. A silicon dielectric gel target was attached to the motor via a rod. 40 mm total amplitude was used with cycles of 3 to 9 s in length in a sinusoidal trajectory. Trajectories of six moving clinical targets in four canine patients were quantified and tracked. ACR phantom images were analyzed, and the results were compared with the ACR acceptance levels. Measured slice thickness accuracies were within the acceptance limits. In the 0.35 T system, the image intensity uniformity was also within the ACR acceptance limit. Over the range of cycle lengths, representing a wide range of breathing rates in patients imaged at four frames/s, excellent agreement was observed between the expected and measured target trajectories. In vivo canine targets, including the gross target volume (GTV), as well as other abdominal soft tissue structures, were visualized with inherent MR contrast, allowing for preliminary results of target tracking. PACS number: 87.61.Tg.

Dosimetric differences in flattened and flattening filter-free beam treatment plans

This study investigated the dosimetric differences in treatment plans from flattened and flattening filter-free (FFF) beams from the TrueBeam System. A total of 104 treatment plans with static (sliding window) intensity-modulated radiotherapy beams and volumetric-modulated arc therapy (VMAT) beams were generated for 15 patients involving three cancer sites. In general, the FFF beam provides similar target coverage as the flattened beam with improved dose sparing to organ-at-risk (OAR). Among all three cancer sites, the head and neck showed more important differences between the flattened beam and FFF beam. The maximum reduction of the FFF beam in the mean dose reached up to 2.82 Gy for larynx in head and neck case. Compared to the 6 MV flattened beam, the 10 MV FFF beam provided improved dose sparing to certain OARs, especially for VMAT cases. Thus, 10 MV FFF beam could be used to improve the treatment plan.

Pinnacle3 modeling and end-to-end dosimetric testing of a Versa HD linear accelerator with the Agility head and flattening filter-free modes.

The Elekta Versa HD incorporates a variety of upgrades to the line of Elekta linear accelerators, primarily including the Agility head and flattening filter-free (FFF) photon beam delivery. The completely distinct dosimetric output of the head from its predecessors, combined with the FFF beams, requires a new investigation of modeling in treatment planning systems. A model was created in Pinnacle3 v9.8 with the commissioned beam data. A phantom consisting of several plastic water and Styrofoam slabs was scanned and imported into Pinnacle3 , where beams of different field sizes, source-to-surface distances (SSDs), wedges, and gantry angles were devised. Beams included all of the available photon energies (6, 10, 18, 6 FFF, and 10 FFF MV), as well as the four electron energies commissioned for clinical use (6, 9, 12, and 15 MeV). The plans were verified at calculation points by measurement with a calibrated ionization chamber. Homogeneous and heterogeneous point-dose measurements agreed within 2% relative to maximum dose for all photon and electron beams. AP photon open field measurements along the central axis at 100 cm SSD passed within 1%. In addition, IMRT testing was also performed with three standard plans (step and shoot IMRT, as well as a small- and large-field VMAT plan). The IMRT plans were delivered on the Delta4 IMRT QA phantom, for which a gamma passing rate was >99.5% for all plans with a 3% dose deviation, 3 mm distance-to-agreement, and 10% dose threshold. The IMRT QA results for the first 23 patients yielded gamma passing rates of 97.4%±2.3%. Such testing ensures confidence in the ability of Pinnacle3 to model photon and electron beams with the Agility head. PACS numbers: 87.55.D, 87.56.bd.The Elekta Versa HD incorporates a variety of upgrades to the line of Elekta linear accelerators, primarily including the Agility head and flattening filter‐free (FFF) photon beam delivery. The completely distinct dosimetric output of the head from its predecessors, combined with the FFF beams, requires a new investigation of modeling in treatment planning systems. A model was created in Pinnacle3 v9.8 with the commissioned beam data. A phantom consisting of several plastic water and Styrofoam slabs was scanned and imported into Pinnacle3, where beams of different field sizes, source‐to‐surface distances (SSDs), wedges, and gantry angles were devised. Beams included all of the available photon energies (6, 10, 18, 6 FFF, and 10 FFF MV), as well as the four electron energies commissioned for clinical use (6, 9, 12, and 15 MeV). The plans were verified at calculation points by measurement with a calibrated ionization chamber. Homogeneous and heterogeneous point‐dose measurements agreed within 2% relative to maximum dose for all photon and electron beams. AP photon open field measurements along the central axis at 100 cm SSD passed within 1%. In addition, IMRT testing was also performed with three standard plans (step and shoot IMRT, as well as a small‐ and large‐field VMAT plan). The IMRT plans were delivered on the Delta4 IMRT QA phantom, for which a gamma passing rate was >99.5% for all plans with a 3% dose deviation, 3 mm distance‐to‐agreement, and 10% dose threshold. The IMRT QA results for the first 23 patients yielded gamma passing rates of 97.4%±2.3%. Such testing ensures confidence in the ability of Pinnacle3 to model photon and electron beams with the Agility head. PACS numbers: 87.55.D, 87.56.bd

Rapid Automated Target Segmentation and Tracking on 4D Data without Initial Contours

Purpose. To achieve rapid automated delineation of gross target volume (GTV) and to quantify changes in volume/position of the target for radiotherapy planning using four-dimensional (4D) CT. Methods and Materials. Novel morphological processing and successive localization (MPSL) algorithms were designed and implemented for achieving autosegmentation. Contours automatically generated using MPSL method were compared with contours generated using state-of-the-art deformable registration methods (using Elastix© and MIMVista software). Metrics such as the Dice similarity coefficient, sensitivity, and positive predictive value (PPV) were analyzed. The target motion tracked using the centroid of the GTV estimated using MPSL method was compared with motion tracked using deformable registration methods. Results. MPSL algorithm segmented the GTV in 4DCT images in 27.0 ± 11.1 seconds per phase (512 × 512 resolution) as compared to 142.3 ± 11.3 seconds per phase for deformable registration based methods in 9 cases. Dice coefficients between MPSL generated GTV contours and manual contours (considered as ground-truth) were 0.865 ± 0.037. In comparison, the Dice coefficients between ground-truth and contours generated using deformable registration based methods were 0.909 ± 0.051. Conclusions. The MPSL method achieved similar segmentation accuracy as compared to state-of-the-art deformable registration based segmentation methods, but with significant reduction in time required for GTV segmentation.

The level of detail required in a deformable phantom to accurately perform quality assurance of deformable image registration.

The primary purpose of the study was to determine how detailed deformable image registration (DIR) phantoms need to adequately simulate human anatomy and accurately assess the quality of DIR algorithms. In particular, how many distinct tissues are required in a phantom to simulate complex human anatomy? Pelvis and head-and-neck patient CT images were used for this study as virtual phantoms. Two data sets from each site were analyzed. The virtual phantoms were warped to create two pairs consisting of undeformed and deformed images. Otsus method was employed to create additional segmented image pairs of n distinct soft tissue CT number ranges (fat, muscle, etc). A realistic noise image was added to each image. Deformations were applied in MIM Software (MIM) and Velocity deformable multi-pass (DMP) and compared with the known warping. Images with more simulated tissue levels exhibit more contrast, enabling more accurate results. Deformation error (magnitude of the vector difference between known and predicted deformation) was used as a metric to evaluate how many CT number gray levels are needed for a phantom to serve as a realistic patient proxy. Stabilization of the mean deformation error was reached by three soft tissue levels for Velocity DMP and MIM, though MIM exhibited a persisting difference in accuracy between the discrete images and the unprocessed image pair. A minimum detail of three levels allows a realistic patient proxy for use with Velocity and MIM deformation algorithms.

Characterization of air temperature in modern ion chambers due to phantom geometry and ambient temperature changes

PURPOSE Temperature and pressure corrections are necessary to account for the varying mass of air in the sensitive volume of a vented ionization chamber (IC) when performing absolute dose measurements. Locations commonly used to measure the presumed IC air temperature may not accurately represent the chamber cavity air temperature, and phantoms undergoing temperature changes further compound the problem. Prior studies have characterized thermal equilibrium in separate phantoms for Farmer chambers alone. However, the purpose of this study was to characterize the cavity air temperature dependence on changes in the ambient temperature and phantom geometry configuration for a wider and more modern variety of chambers to determine if previously published wait times apply to these chambers as well. METHODS Thermal conduction properties were experimentally investigated by modifying a PTW 0.3 cm(3) Semiflex IC with a thermocouple replacing the central electrode. Air cavity temperature versus time was recorded in three phantom geometries characteristic of common absolute dose measurements. The phantoms were (15 ± 1) °C before measurement with an IC at the treatment vault temperature of (21 ± 1) °C. Simulations were conducted to provide a theoretical basis for the measurements and to simulate temperature response of a PTW PinPoint® and Farmer chamber. The simulation methods were first validated by comparison with measured Semiflex chamber thermal response curves before extension to the other chambers. RESULTS Two thermal equilibria curves were recorded on different time scales. IC temperature initially dropped to the colder phantom temperature but subsequently increased as the phantom itself equilibrated with the warmer room temperature. In a large phantom of dimensions (25.5 × 25.5 × 23.4) cm(3), 3 min was required before the IC temperature reached within 0.5 °C of its equilibrium within the phantom. Similarly, wait times of 2 min were needed for 7.5 and 2 cm slab phantoms. CONCLUSIONS Recording of temperature in the phantom was deemed far more accurate than measurement in ambient air due to the air cavity thermally equilibrating with phantom temperature instead of the vented ambient air. Wait times of 3 and 2 min are needed for a cube and 7.5 cm slab phantom, respectively, to achieve 0.2% dosimetric accuracy (temperature accuracy of 0.5 °C). Chamber volume alone did not determine wait times, as a 0.3 cm(3) IC required a longer wait time than a Farmer chamber, suggesting wall thickness as an important variable as well.

TH‐A‐220‐06: Four‐Dimensional MRI/CT Based Auto‐Adaptive Segmentation for Real‐Time Radiotherapy in Lung Cancer Treatment

Purpose: To enable real‐time magnetic resonance (MR)/computer tomography(CT)image‐guidedradiotherapy (IGRT) and to reduce treatment planning durations in the radiotherapy of lungcancer through the design and implementation of computationally efficient four‐dimensional (4D) MR/CT based automated segmentation algorithms. Methods: Hyperpolarized helium‐3 (HP3He) and proton‐density 4DMRI lung data was acquired for six subjects. Thoracic 4DCT data often subjects with lungcancer was also acquired. Automated segmentation was performed using a novel Morphological Processing and Successive Localization (MPSL) approach. Three different MPSL segmentation algorithms were developed to segment the regions of tumor, body and lung respectively. MPSL segmentation: A mask that includes the intensity range of the region‐of‐interest was generated. Then morphological processing (and/or reconstruction) was performed to separate the target volume(s) from other regions. Then the different connected regions were labeled using the union‐find algorithm and their areas/volumes were calculated. A limit on the maximum/minimum possible area/volume of the target volume was used as a filter to segment the target volume. Morphological processing/reconstruction were performed again to create the final contours. Results: MPSL was shown to successfully segment the regions‐of‐interest (tumors,lung and body) from the images with both high and low signal‐to‐ noise ratio. With the use of 3D processing and successive localization MPSL was shown to separately classify tumor and diaphragm (that may appear within the 2D contours of the lung). MPSL segmentation was compared with manual segmentation. Average computational time for achieving automated lung segmentation using MPSL on one phase volume (128×128×128) of 4D HP3He MR data was 0.5s. For proton‐density 4DMRI data, the average time for automated segmentation of body and lung on one phase volume (128×128×128) was 2s. Conclusions: With the computation speed of the order of seconds for achieving automated segmentation, MPSL has realistic potential for application in real‐time image‐guidance for adaptive radiotherapy.

Dosimetric and localization accuracy of Elekta high definition dynamic radiosurgery

BACKGROUND AND PURPOSE With the increasingly prominent role of stereotactic radiosurgery in radiation therapy, there is a clinical need for robust, efficient, and accurate solutions for targeting multiple sites with one patient setup. The end-to-end accuracy of high definition dynamic radiosurgery with Elekta treatment planning and delivery systems was investigated in this study. MATERIALS AND METHODS A patient-derived CT scan was used to create a radiosurgery plan to seven targets in the brain. Monaco was used for treatment planning using 5 VMAT non-coplanar arcs. Prior to delivery, 3D-printed phantoms from RTsafe were ordered including a gel phantom for 3D dosimetry, phantom with 2D film insert, and an ion chamber phantom for point dose measurement. Delivery was performed using the Elekta VersaHD, XVI cone-beam CT, and HexaPOD six degree of freedom tabletop. RESULTS Absolute dose accuracy was verified within 2%. 3D global gamma analysis in the film measurement revealed 3%/2 mm passing rates >95%. Gel dosimetry 3D global gamma analysis (3%/2 mm) were above 90% for all targets with the exception of one. Results were indicative of typical end-to-end accuracies (<1 mm spatial uncertainty, 2% dose accuracy) within 4 cm of isocenter. Beyond 4 cm, 2 mm accuracy was found. CONCLUSIONS High definition dynamic radiosurgery expands clinically acceptable stereotactic accuracy to a sphere around isocenter allowing for radiosurgery of several targets with one setup with a high degree of dosimetric precision. Gel dosimetry proved to be an essential tool for the validation of the 3D dose distributions in this technique.