M Speiser

Commissioning of the Varian TrueBeam linear accelerator: a multi-institutional study.

PURPOSE Latest generation linear accelerators (linacs), i.e., TrueBeam (Varian Medical Systems, Palo Alto, CA) and its stereotactic counterpart, TrueBeam STx, have several unique features, including high-dose-rate flattening-filter-free (FFF) photon modes, reengineered electron modes with new scattering foil geometries, updated imaging hardware/software, and a novel control system. An evaluation of five TrueBeam linacs at three different institutions has been performed and this work reports on the commissioning experience. METHODS Acceptance and commissioning data were analyzed for five TrueBeam linacs equipped with 120 leaf (5 mm width) MLCs at three different institutions. Dosimetric data and mechanical parameters were compared. These included measurements of photon beam profiles (6X, 6XFFF, 10X, 10XFFF, 15X), photon and electron percent depth dose (PDD) curves (6, 9, 12 MeV), relative photon output factors (Scp), electron cone factors, mechanical isocenter accuracy, MLC transmission, and dosimetric leaf gap (DLG). End-to-end testing and IMRT commissioning were also conducted. RESULTS Gantry/collimator isocentricity measurements were similar (0.27-0.28 mm), with overall couch/gantry/collimator values of 0.46-0.68 mm across the three institutions. Dosimetric data showed good agreement between machines. The average MLC DLGs for 6, 10, and 15 MV photons were 1.33 ± 0.23, 1.57 ± 0.24, and 1.61 ± 0.26 mm, respectively. 6XFFF and 10XFFF modes had average DLGs of 1.16 ± 0.22 and 1.44 ± 0.30 mm, respectively. MLC transmission showed minimal variation across the three institutions, with the standard deviation <0.2% for all linacs. Photon and electron PDDs were comparable for all energies. 6, 10, and 15 MV photon beam quality, %dd(10)x varied less than 0.3% for all linacs. Output factors (Scp) and electron cone factors agreed within 0.27%, on average; largest variations were observed for small field sizes (1.2% coefficient of variation, 10 MV, 2 × 2 cm(2)) and small cone sizes (<1% coefficient of variation, 6 × 6 cm(2) cone), respectively. CONCLUSIONS Overall, excellent agreement was observed in TrueBeam commissioning data. This set of multi-institutional data can provide comparison data to others embarking on TrueBeam commissioning, ultimately improving the safety and quality of beam commissioning.

Dosimetric characterization of an image-guided stereotactic small animal irradiator.

Small animal irradiation provides an important tool used by preclinical studies to assess and optimize new treatment strategies such as stereotactic ablative radiotherapy. Characterization of radiation beams that are clinically and geometrically scaled for the small animal model is uniquely challenging for orthovoltage energies and minute field sizes. The irradiator employs a commercial x-ray device (XRAD 320, Precision x-ray, Inc.) with a custom collimation system to produce 1-10 mm diameter beams and a 50 mm reference beam. Absolute calibrations were performed using the AAPM TG-61 methodology. Beams half-value layer (HVL) and timer error were measured with an ionization chamber. Percent depth dose (PDD), output factors (OFs) and off-axis ratios were measured using radiochromic film, a diode and a pinpoint ionization chamber at 19.76 and 24.76 cm source-to-surface distance (SSD). PDD measurements were also compared with Monte Carlo (MC) simulations. In-air and in-water absolute calibrations for the reference 50 mm diameter collimator at 19.76 cm SSD were measured as 20.96 and 20.79 Gy min(-1), respectively, agreeing within 0.8%. The HVL at 250 kVp and 15 mAs was measured to be 0.45 mm Cu. The reference field PDD MC simulation results agree with measured data within 3.5%. PDD data demonstrate typical increased penetration with increasing field size and SSD. For collimators larger than 5 mm in diameter, OFs measured using film, an ion chamber and a diode were within 3% agreement.

An x-ray image guidance system for small animal stereotactic irradiation

An x-ray image-guided small animal stereotactic irradiator was developed and characterized to enable tumor visualization and accurate target localization for small field, high dose irradiation. The system utilizes a custom collimation system, a motorized positioning system (x, y, θ), a digital imaging panel and operating software, and is integrated with a commercial x-ray unit. The essential characteristics of the irradiator include small radiation fields (1-10 mm), high dose rate (>10 Gy min(-1)) and submillimeter target localization. The software enables computer-controlled image acquisition, stage motion and target localization providing simple and precise automated target localization. The imaging panel was characterized in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and spatial resolution. Overall localization accuracy and precision were assessed. SNR, CNR and spatial resolution are 24 dB, 21 dB and 2.8 lp mm(-1), respectively, and localization accuracy is approximately 65 µm with 6 µm precision. With the aid of image guidance, system performance was subsequently used to evaluate radiation response in a rat orthotopic lung tumor effectively sparing normal tissues and in a mouse normal lung. The capabilities of 3D treatment and cone-beam computed tomography are presented for 3D localization and delivery as a work in progress.

Initial application of a geometric QA tool for integrated MV and kV imaging systems on three image guided radiotherapy systems.

PURPOSE Several linacs with integrated kilovoltage (kV) imaging have been developed for delivery of image guided radiation therapy (IGRT). High geometric accuracy and coincidence of kV imaging systems and megavoltage (MV) beam delivery are essential for successful image guidance. A geometric QA tool has been adapted for routine QA for evaluating and characterizing the geometric accuracy of kV and MV cone-beam imaging systems. The purpose of this work is to demonstrate the application of methodology to routine QA across three IGRT-dedicated linac platforms. METHODS It has been applied to a Varian Trilogy (Varian Medical Systems, Palo Alto, CA), an Elekta SynergyS (Elekta, Stockholm, Sweden), and a Brainlab Vero (Brainlab AG, Feldkirchen, Germany). Both the Trilogy and SynergyS linacs are equipped with a retractable kV x-ray tube and a flat panel detector. The Vero utilizes a rotating, rigid ring structure integrating a MV x-ray head mounted on orthogonal gimbals, an electronic portal imaging device (EPID), two kV x-ray tubes, and two fixed flat panel detectors. This dual kV imaging system provides orthogonal radiographs, CBCT images, and real-time fluoroscopic monitoring. Two QA phantoms were built to suit different field sizes. Projection images of a QA phantom were acquired using MV and kV imaging systems at a series of gantry angles. Software developed for this study was used to analyze the projection images and calculate nine geometric parameters for each projection. The Trilogy was characterized five times over one year, while the SynergyS was characterized four times and the Vero once. Over 6500 individual projections were acquired and analyzed. Quantitative geometric parameters of both MV and kV imaging systems, as well as the isocenter consistency of the imaging systems, were successfully evaluated. RESULTS A geometric tool has been successfully implemented for calibration and QA of integrated kV and MV across a variety of radiotherapy platforms. X-ray source angle deviations up to 0.8 degrees, and detector center offsets up to 3 mm, were observed for three linacs, with the exception of the Vero, for which a significant center offset of one kV detector (prior to machine commissioning) was observed. In contrast, the gimbal-based MV source positioning of the Vero demonstrated differences between observed and expected source positions of less than 0.2 mm, both with and without gimbal rotation. CONCLUSIONS This initial application of this geometric QA tool shows promise as a universal, independent tool for quantitative evaluation of geometric accuracies of both MV and integrated kV imaging systems across a range of platforms. It provides nine geometric parameters of any imaging system at every gantry angle as well as the isocenter coincidence of the MV and kV image systems.

An athymic rat model of cutaneous radiation injury designed to study human tissue-based wound therapy

PurposeTo describe a pilot study for a novel preclinical model used to test human tissue-based therapies in the setting of cutaneous radiation injury.MethodsA protocol was designed to irradiate the skin of athymic rats while sparing the body and internal organs by utilizing a non-occlusive skin clamp along with an x-ray image guided stereotactic irradiator. Each rat was irradiated both on the right and the left flank with a circular field at a 20 cm source-to-surface distance (SSD). Single fractions of 30.4 Gy, 41.5 Gy, 52.6 Gy, 65.5 Gy, and 76.5 Gy were applied in a dose-finding trial. Eight additional wounds were created using the 41.5 Gy dose level. Each wound was photographed and the percentage of the irradiated area ulcerated at given time points was analyzed using ImageJ software.ResultsNo systemic or lethal sequelae occurred in any animals, and all irradiated skin areas in the multi-dose trial underwent ulceration. Greater than 60% of skin within each irradiated zone underwent ulceration within ten days, with peak ulceration ranging from 62.1% to 79.8%. Peak ulceration showed a weak correlation with radiation dose (r = 0.664). Mean ulceration rate over the study period is more closely correlated to dose (r = 0.753). With the highest dose excluded due to contraction-related distortions, correlation between dose and average ulceration showed a stronger relationship (r = 0.895). Eight additional wounds created using 41.5 Gy all reached peak ulceration above 50%, with all healing significantly but incompletely by the 65-day endpoint.ConclusionsWe developed a functional preclinical model which is currently used to evaluate human tissue-based therapies in the setting of cutaneous radiation injury. Similar models may be widely applicable and useful the development of novel therapies which may improve radiotherapy management over a broad clinical spectrum.

Commissioning and verification of the collapsed cone convolution superposition algorithm for SBRT delivery using flattening filter-free beams

Linacs equipped with flattening filter‐free (FFF) megavoltage photon beams are now commercially available. However, the commissioning of FFF beams poses challenges that are not shared with traditional flattened megavoltage X‐ray beams. The planning system must model a beam that is peaked in the center and has an energy spectrum that is softer than the flattened beam. Removing the flattening filter also increases the maximum possible dose rates from 600 MU/min up to 2400 MU/min in some cases; this increase in dose rate affects the recombination correction factor, Pion, used during absolute dose calibration with ionization chambers. We present the first‐reported experience of commissioning, verification, and clinical use of the collapsed cone convolution superposition (CCCS) dose calculation algorithm for commercially available flattening filter‐free beams. Our commissioning data are compared to previously reported measurements and Monte Carlo studies of FFF beams. Commissioning was verified by making point‐dose measurement of test plans, irradiating the RPC lung phantom, and performing patient‐specific QA. The average point‐dose difference between calculations and measurements of all test plans and all patient specific QA measurements is 0.80%, and the RPC phantom absolute dose differences for the two thermoluminescent dosimeters (TLDs) in the phantom planning target volume (PTV) were 1% and 2%, respectively. One hundred percent (100%) of points in the RPC phantom films passed the RPC gamma criteria of 5% and 5 mm. Our results show that the CCCS algorithm can accurately model FFF beams and calculate SBRT dose distributions using those beams. PACS number: 87.55.kh

WE‐E‐108‐06: Demonstration of a CBCT Based Monte Carlo Model for Small Animal Treatment Planning

PURPOSE To demonstrate feasibility of planning small animal irradiation using cone beam computed tomography (CBCT) data as input to a Monte Carlo (MC) based treatment planning system (TPS). METHODS The BEAMnrc/EGSnrc code was used to model 225 kVp photon beams produced by a small animal irradiator (XRAD 225Cx, Precision X-ray). Homogenous phantom plugs (Gammex RMI 467, Middleton, WI) of known material and physical density were scanned using the XRAD 225Cx CBCT system to associate unique CBCT units to the corresponding material density. Additionally, the known physical densities were then correlated to appropriate atomic compositions to create a material data set library. This enabled correlating phantom CBCT units to a material/density matrix used as input to DOSXYZnrc MC computations. A 4×4×4 cm3 multi slab phantom in homogeneous (all solid water) and heterogeneous (lung block between water slabs) configurations were CBCT scanned using 80 kVp and 0.3 mAs setting. MC dose calculation was performed for 10 mm diameter circular field. Ultimately, 10 mm circular fields in homogenous and heterogeneous phantom configuration were used to compare absolute dose computed by MCTPS and film measurement. The CBCT based MC model was finally demonstrated using an animal CBCT scan. RESULTS The gamma map of CBCT based MC calculation and film measurement at 1cm depth plane in solid water medium shows excellent agreement of 98.7%within selected region of interest using 3%/0.5 mm gamma criteria. The absolute dose comparison between measurement and simulation in homogenous and heterogeneous phantom is within 1%. In a small animal the CBCT based MC model qualitatively demonstrated higher dose to bone relative to dose deposited to soft tissues. CONCLUSION The CBCT based MC model is a valuable tool for improved dose calculation accuracy for small animal treatment planning in orthovoltage energy range.

WE-E-108-05: Evaluation of the XRAD 225Cx MC Source Model in Heterogeneous Mediums

PURPOSE This study was performed to benchmark a Monte Carlo source model of the XRAD 225Cx irradiator under heterogeneous conditions. METHODS The BEAM/EGS was used to model 225 kV photon beams from a small animal irradiator (Precision XRAD225Cx). Benchmarking the model in heterogeneous media was performed in a heterogeneous phantom (4×4×4 cm3) composed of solid water and cortical bone. The 10 mm field size virtual source from BEAMnrc was used to compute the 3D dose distribution in DOSXYZnrc. Benchmarking the simulation against measurements was performed using EBT2 film. Heterogeneous conditions were further validated using the phase space file for the 20 mm field size. Gamma analysis was performed to further validate the beam model for the heterogeneous configuration phantom. RESULTS The MC simulation indicates that there is a 2.4 times increased dose deposition in cortical bone, in agreement with published data. This increased dose deposition, however, was not observed in film measurements and it is investigated further. Gamma analysis was performed for 20 mm field size applicator and the Result of the analysis for a confined region. The single beam with 20 mm diameter circular field irradiation demonstrates a good agreement within the region of interest, with 94% of the calculated data meeting the 5%/0.5 mm criterion. CONCLUSION Our kV Monte Carlo source model demonstrates excellent agreement with measurement in heterogeneous conditions.

SU‐E‐T‐84: TrueBeam Commissioning: A Multi‐Institutional Experience

Purpose: Latest generation linear accelerators(linacs), TrueBeam (Varian Medical Systems, Palo Alto, CA) and its stereotactic counterpart, TrueBeam STx, have several unique features, including high‐dose‐rate flattening‐filter‐free (FFF) photon modes, reengineered electron modes with new scattering foil geometries, updated imaging hardware/software, and a novel control system. We have performed a comprehensive evaluation of three TrueBeam linacs at three different institutions and report on our commissioning experience. Methods: Acceptance and commissioning data were analyzed for three TrueBeam linacs equipped with 120 leaf (5 mm width) MLCs at three different institutions. Dosimetric data and mechanical parameters were compared. These included measurements of photon beam profiles (6X, 6XFFF, 10X, 10XFFF, 15X), photon and electron percent depth dose (PDD)curves (6MeV, 9MeV, 12MeV), relative photon output factors (Scp),electron cone factors, mechanical isocenter accuracy, MLC transmission, and dosimetric leaf gap (DLG). Results: Gantry/collimator isocentricity measurements were similar (0.27–0.28mm), with overall couch/gantry/collimator values of 0.46–0.68mm across the three institutions. Dosimetric data showed good agreement between machines. The largestdiscrepancy was observed with measured MLC DLG (for 6, 10 and 15 MV photons, average DLGs were 1.95 ± 1.15mm, 2.22 ± 1.30 mm, and 2.20 ± 1.24mm, respectively). Photon and electron PDDs were comparable for all energies. 6, 15 and 10 MV photon beam quality, %dd(10)x varied less than 0.3% for all machines. Electron beam quality specifier (R50) showed less than 1.7% variation for all energies. Output factors (Scp) and electron cone factors agreed within 0.27%, on average; largest variations were observed for small field sizes (0.77% variation, 2×2cm2) and small cone sizes (0.39% variation, 6×6 cm2 cone), respectively. Conclusions: Overall, strong agreement was observed in TrueBeam commissioning data. This comprehensive set of multi‐institutional data may serve as a benchmark for other institutions embarking on TrueBeam commissioning, ultimately improving the safety/quality of beam commissioning

SU‐E‐T‐274: Monte Carlo Simulations of Output Factors for a Small Animal Irradiator

PURPOSE Measurement of dosimetric parameters of small photon beams, with field sizes as small 1 mm in diameter, is particularly challenging. This work utilizes Monte Carlo techniques to calculate percent depth dose (PDD) and output factors for small photon fields from a kV x-ray based small animal irradiator. METHODS Absolute dose calibration of a commercial small animal stereotactic irradiator (XRAD225, Precision X-ray) was performed in accordance with the recommendations of AAPM TG-61 protocol. Both in-air and in-water calibrations were performed at a 30.4 cm source-to-surface distance (SSD) for a reference collimator 50 mm in diameter. The BEAM/EGS was used to model 225 kV photon beams used for most therapeutic applications. The Monte Carlo model was provided good agreement with measured beam characteristics, e.g. PDD and off-axis ratios. Subsequently, output factors for various square and circular applicators were measured using an ionization chamber and radiochromic film, and compared with MC simulations. Directional Bremsstrahlung splitting (DBS) was utilized for variance reduction to improve efficiency of the output factor simulations. The statistical uncertainty on the MC- calculated results is between 0.5% and 1% for most points. RESULTS The absolute dose measured for reference collimator at 30.4 cm SSD in water and in air is 4.1 and 4.12 Gy/min. The agreement between simulated and measured output factors was excellent, ranging from 1% to 2.84%. The MC- simulated and measured depth dose data, normalized at the surface, show excellent agreement, with a maximum deviation is approximately 2.5 %. CONCLUSIONS Monte Carlo simulation provides an indispensible tool for validating measurements of the smallest field sizes used in preclinical small animal irradiation.