Varian Trilogy Accelerator Research Articles

Реконструкция действующего спектра тормозного излучения медицинских линейных ускорителей электронов по глубинным распределениям дозы в водном фантоме

Purpose: Development of the bremsstrahlung spectrum reconstruction method of medical electron linear accelerators (ELA) with different field sizes on the base of the deep dose distributions in a water phantom and determination of photon spectra for Varian Trilogy accelerator 6 MV. 
 Material and methods: The proposed methodology is based on the use of dose kernels algorithm of point monoenergetic monodirectional source (pencil beam (PB)) for the deep dose distribution calculation, created different cross-section beams of in a water phantom, and experimental measurements of these distributions. For solving the inverse problem is applied Toolbox routines 'ptimtool knowing mathematical package MATLAB to solve.
 Results: Bremsstrahlung energy spectrum generated medical accelerator Varian Triology with different sizes of square fields from 3×3 up to 40×40 cm and average energy photons, depending on the size of the fields were received. Dose kernels for a set of defined energies PB were calculated. Depth dose distribution in a water phantom, calculated using the obtained spectra and dose kernels agree well with measurement dose distributions. 
 Conclusion: The proposed technique reconstruction of bremsstrahlung spectrum of electron linear accelerator is adequate. Average energy spectra of bremsstrahlung photons for Varian Trilogy Accelerator in regime 6 MV varies from 1.71 to 1.43 MeV depending on the field size.

A simple method to back-project isocenter dose of radiotherapy treatments using EPID transit dosimetry

The aim of this work was to implement a simple algorithm to evaluate isocenter dose in a phantom using the back-projected transmitted dose acquired using an Electronic Portal Imaging Device (EPID) available in a Varian Trilogy accelerator with two nominal 6 and 10 MV photon beams. This algorithm was developed in MATLAB language, to calibrate EPID measured dose in absolute dose, using a deconvolution process, and to incorporate all scattering and attenuation contributions due to photon interactions with phantom. Modeling process was simplified by using empirical curve adjustments to describe the contribution of scattering and attenuation effects. The implemented algorithm and method were validated employing 19 patient treatment plans with 104 clinical irradiation fields projected on the phantom used. Results for EPID absolute dose calibration by deconvolution have showed percent deviations lower than 1%. Final method validation presented average percent deviations between isocenter doses calculated by back-projection and isocenter doses determined with ionization chamber of 1,86% (SD of 1,00%) and -0,94% (SD of 0,61%) for 6 and 10 MV, respectively. Normalized field by field analysis showed deviations smaller than 2% for 89% of all data for 6 MV beams and 94% for 10 MV beams. It was concluded that the proposed algorithm possesses sufficient accuracy to be used for in vivo dosimetry, being sensitive to detect dose delivery errors bigger than 3-4% for conformal and intensity modulated radiation therapy techniques.

Real-time dosimetry with Yb-doped silica optical fibres

Over the years, many efforts have been made to develop radiation detectors to handle the complex issues of small field dosimetry and achieve the increasing accuracy, precision and in vivo dose monitoring required by the new advanced treatment modalities. In this context, interest has surged in the development of sensors based on scintillating optical fibres. In this paper, the near-infrared radioluminescence and dosimetric properties of Yb-doped silica optical fibres, coupled with a laboratory prototype based on an avalanche photodiode, were studied by irradiating the fibres with photons and electron beams generated by a Varian Trilogy accelerator. The performance of the system in standard and small field sizes has also been investigated, comparing the output factor, percentage depth dose and off-axis ratio measurements of the prototypal detector with other commercial sensors, including the Exradin W1 scintillator.The results of this study demonstrate that the drawback due to the stem effect in Yb-doped silica optical fibres can be managed in a simple but effective way by optical filtering. The robustness of the system in complex dosimetric scenarios and the accuracy and precision achieved by Yb-doped fibres in relative dose assessments suggest an effective use of the system for real-time in vivo dosimetry applications.

Characterization of ytterbium-doped silica optical fibre as scintillator dosimeter

Introduction Scintillator dosimeters (SD) have been a research topic by many groups over the last decade. The recent availability of a commercial system (Exradin W1 by Standard Imaging) represents a significant accomplishment. The attractiveness of SD would be furtherly enhanced by a scintillator free from any spectral superposition with the Cherenkov light, so to avoid any sensitive calibration procedure for the stem effect correction. Yb-doped silica optical fibres, thanks to their near-infrared (NIR) emission, proved to be a promising option. Purpose This study aims to characterize the dosimetric properties of Yb-doped fibers in radiotherapy and to compare their results with those obtained by various reference dosimeters like micro ion-chambers, the commercial SD, and diodes designed for small field dosimetry. Materials and methods Yb-doped fibres were prepared by sol–gel. The scintillation was detected with a laboratory-made photon counting system based on an avalanche photodiode (APD), using a long-pass filter at 950 nm. Irradiations were carried out with photons and electron beams generated by a Varian Trilogy accelerator. Results The NIR scintillation proved to be unaffected by the stem effect, even in unfavorable large field irradiations. The system showed a satisfactory reproducibility, good sensitivity, linear dose-rate response, independence of the signal (total counts) of dose rate and impinging beam orientation. The results were in good agreement with reference dosimeters in terms of relative dose profiles and output factors. Conclusion Findings pave the way to the engineering design of the system, which could be an interesting option also for real-time in-vivo dosimetry applications. Disclosure Authors declare no relationship that may bias the presentation.

Superficial dose evaluation of four dose calculation algorithms

Accurate superficial dose calculation is of major importance because of the skin toxicity in radiotherapy, especially within the initial 2mm depth being considered more clinically relevant. The aim of this study is to evaluate superficial dose calculation accuracy of four commonly used algorithms in commercially available treatment planning systems (TPS) by Monte Carlo (MC) simulation and film measurements. The superficial dose in a simple geometrical phantom with size of 30cm×30cm×30cm was calculated by PBC (Pencil Beam Convolution), AAA (Analytical Anisotropic Algorithm), AXB (Acuros XB) in Eclipse system and CCC (Collapsed Cone Convolution) in Raystation system under the conditions of source to surface distance (SSD) of 100cm and field size (FS) of 10×10cm2. EGSnrc (BEAMnrc/DOSXYZnrc) program was performed to simulate the central axis dose distribution of Varian Trilogy accelerator, combined with measurements of superficial dose distribution by an extrapolation method of multilayer radiochromic films, to estimate the dose calculation accuracy of four algorithms in the superficial region which was recommended in detail by the ICRU (International Commission on Radiation Units and Measurement) and the ICRP (International Commission on Radiological Protection). In superficial region, good agreement was achieved between MC simulation and film extrapolation method, with the mean differences less than 1%, 2% and 5% for 0°, 30° and 60°, respectively. The relative skin dose errors were 0.84%, 1.88% and 3.90%; the mean dose discrepancies (0°, 30° and 60°) between each of four algorithms and MC simulation were (2.41±1.55%, 3.11±2.40%, and 1.53±1.05%), (3.09±3.00%, 3.10±3.01%, and 3.77±3.59%), (3.16±1.50%, 8.70±2.84%, and 18.20±4.10%) and (14.45±4.66%, 10.74±4.54%, and 3.34±3.26%) for AXB, CCC, AAA and PBC respectively. Monte Carlo simulation verified the feasibility of the superficial dose measurements by multilayer Gafchromic films. And the rank of superficial dose calculation accuracy of four algorithms was AXB>CCC>AAA>PBC. Care should be taken when using the AAA and PBC algorithms in the superficial dose calculation.

SU‐E‐T‐433: Calibration Accuracy in Mailed High‐Resolution 3D Dosimetry Service for SRS/SBRT QA

Purpose:SRS/SBRT combines hypofractionation with excellent dose distributions. However, extremely steep gradients across the target along with dose escalation, if not administered accurately, may lead to serious complications, recurrences, or even fatalities. Existing commercial QA products either lack adequate spatial resolution or the 3D aspect. By contrast, the new CrystalBall™ mailed high‐resolution 3D dosimetry service removes the above limitations while reducing the overall workload on medical physics staff. The exposed dosimeters, which change optical density in proportion to local dose, are sent back to the manufacturer (MGS Research Inc., Madison, CT) for sub‐millimeter‐resolution laser‐CT scanning and QA data analysis. QA report is returned electronically within 24 hours. The purpose of this study was to evaluate the dose calibration accuracy in this system.Methods:Two spherical CrystalBall™ polymer gel dosimeters from the same batch, 166 mm diameter, with embedded 3D image registration markers, were mounted in a special phantom designed for reproducible positioning. For full end to end testing, the optical guidance array was mounted onto the phantom and a CT was taken. Two separate Rapid Arc SRS plans were designed. Varian Medical Systems optical guidance system was used to position the phantom and the SRS treatment plans were delivered to the two spheres on Varian's Trilogy Accelerator. Exposed dosimeters were mailed back to the manufacturer for laser CT scanning and analysis.Results:For each plan, 3D gamma passing rate was 100% for 2%/2mm distance‐to‐agreement criteria above 50% isodose level. The two calibration curves, generated using volumetric dose and optical density data, showed excellent mutual agreement (max difference 2.2%, median difference 0.75%).Conclusion:The clinical utility of new CrystalBall™ mailed QA service for SRS/SBRT and high accuracy of dose calibration have been validated. The workflow associated with the use of the CrystalBall™ in clinical setting was found to be minimal.The presenting author is the founder of and has an ownership in MGS Research Inc which manufactures the CrystalBall system for 3D dosimetry.

SU-D-17A-02: Four-Dimensional CBCT Using Conventional CBCT Dataset and Iterative Subtraction Algorithm of a Lung Patient

Purpose: The Iterative Subtraction Algorithm (ISA) method generates retrospectively a pre-selected motion phase cone-beam CT image from the full motion cone-beam CT acquired at standard rotation speed. This work evaluates ISA method with real lung patient data. Methods: The goal of the ISA algorithm is to extract motion and no- motion components form the full reconstruction CBCT. The workflow consists of subtracting from the full CBCT all of the undesired motion phases and obtain a motion de-blurred single-phase CBCT image, followed by iteration of this subtraction process. ISA is realized as follows: 1) The projections are sorted to various phases, and from all phases, a full reconstruction is performed to generate an image CTM. 2) Generate forward projections of CTM at the desired phase projection angles, the subtraction of projection and the forward projection will reconstruct a CTSub1, which diminishes the desired phase component. 3) By adding back the CTSub1 to CTm, no motion CBCT, CTS1, can be computed. 4) CTS1 still contains residual motion component. 5) This residual motion component can be further reduced by iteration. The ISA 4DCBCT technique was implemented using Varian Trilogy accelerator OBI system. To evaluate the method, a lung patient CBCT dataset was used. The reconstruction algorithm is FDK. Results: The single phase CBCT reconstruction generated via ISA successfully isolates the desired motion phase from the full motion CBCT, effectively reducing motion blur. It also shows improved image quality, with reduced streak artifacts with respect to the reconstructions from unprocessed phase-sorted projections only. Conclusion: A CBCT motion de-blurring algorithm, ISA, has been developed and evaluated with lung patient data. The algorithm allows improved visualization of a single phase motion extracted from a standard CBCT dataset. This study has been supported by National Institute of Health through R01CA133539.

Dosimetric characteristics of the small diameter BrainLab™ cones used for stereotactic radiosurgery

The purpose was to study the dosimetric characteristics of the small diameter (≤10.0 mm) BrainLAB cones used for stereotactic radiosurgery (SRS) treatments in conjunction with a Varian Trilogy accelerator. Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields is completely understood. Although a number of investigators have published the dosimetric characteristics of SRS cones, to our knowledge, there is no generally accepted value for the relative output factor (ROF) for the 5.0 mm diameter cone. Therefore, we have investigated the dosimetric properties of the small (≤10.0 mm) diameter BrainLAB SRS cones used in conjunction with the iPlan TPS and a Trilogy linear accelerator with a SRS beam mode. Percentage depth dose (PDD), off‐axis ratios (OAR), and ROF were measured using a SRS diode and verified with Monte Carlo (MC) simulations. The dependence of ROF on detector material response was studied. The dependence of PDD, OAR, and ROF on the alignment of the beam CAX with the detector motion line was also investigated using MC simulations. An agreement of 1% and 1 mm was observed between measurements and MC for PDD and OAR. The calculated ROF for the 5.0 mm diameter cone was 0.692±0.008 — in good agreement with the measured value of 0.683±0.007 after the diode response was corrected. Simulations of the misalignment between the beam axis and detector motion axis for angles between 0.5°–1.0° have shown a deviation > 2% in PDD beyond a certain depth. We have also provided a full set of dosimetric data for BrainLAB SRS cones. Monte Carlo calculated ROF values for cones with diameters less than 10.0 mm agrees with measured values to within 1.8%. Care should be exercised when measuring PDD and OAR for small cones. We recommend the use of MC to confirm the measurement under these conditions.PACS numbers: 87.53.Ly, 87.55.‐x, 87.53.Bn, 87.55.K‐

SU-E-T-536: SRS Diode and Diamond Detector Signal Response Correction Factors in Small Diameter Stereotactic Radiosurgery Fields

Purpose: To evaluate the magnitude of correction factors that need to be applied to the response of a SRS diode and a diamond detector used to measure the relative output factor of the 5.0 mm diameter BrainLab stereotactic cone in conjunction with a Varian Trilogy accelerator. Method and Materials: Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields are completely understood. We have previously reported a 3% difference between the relative output factor (ROF) for the 5.0mm diameter cone measured with uncompensated Scanditronix/IBA Si‐ diode and ROF calculated by Monte Carlo simulations using MCSIM code. However, the reported values for ROF did not account for differences in response of Si under conditions lacking lateral electronic equilibrium. A Si‐ response correction factor has been calculated using MCSIM code and was applied to the measured value for ROF. In addition PTW 60003 diamond detector was used to verify previous measurements. Corrections for dose rate dependence and volume averaging have been made using MC simulations. Results: ROF obtained for SRS diode (0.683±0.011) and diamond detector (0.685±0.008) are in excellent agreement (within 0.2%) with each other and within 1–1.3% of previously reported MC calculated value of ROF (0.692±0.011). Conclusions: We have performed an experimental and theoretical investigation of ROF for radiation field created by BrainLab 5.0 mm SRS cone in conjunction with a Trilogy accelerator equipped with SRS mode. Methods of obtaining ROF using Scanditronix/IBA Si‐diode and PTW Diamond 60003 detector have been outlined. Monte Carlo simulations were used to correct for various effects contributing to potential detector response errors. We conclude that corrections for detector signal response up to ±4% were needed for both SRS diode and diamond detectors.

An evaluation of cine-mode 3D portal image dosimetry for Volumetric Modulated Arc Therapy

We investigated cine-mode portal imaging on a Varian Trilogy accelerator and found that the linearity and other dosimetric properties are sufficient for 3D dose reconstruction as used in patient-specific quality assurance for VMAT (RapidArc) treatments. We also evaluated the gantry angle label in the portal image file header as a surrogate for the true imaged angle. The precision is only just adequate for the 3D evaluation method chosen, as discrepancies of 2° were observed.

SU‐GG‐T‐517: Dosimetric Characteristics of the Small Diameter BrainLab Cones for Stereotactic Radiosurgery

Purpose: To study the dosimetric characteristics of the small diameter (≤10.0mm) BrainLab cones used for stereotactic radiosurgery (SRS) treatments in conjunction with a Varian Trilogy accelerator. Method and Materials: Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields is completely understood. Although a number of investigators have published the dosimetric characteristics of SRS cones, to our knowledge, there is no generally accepted value for the relative output factor (ROF) for the 5.0mm diameter cone. Therefore we have investigated the dosimetric properties of the small (≤10.0mm) diameter BrainLab SRS cones used in conjunction with the iPlan TPS and a Trilogy linear accelerator with a SRS beam mode. Percentage depth dose (PDD), off‐axis ratios (OAR) and ROF were measured using a SRS diode and verified with Monte Carlo (MC) simulations. The dependence of PDD, OAR and ROF on the alignment of the beam CAX with the detector motion line was also investigated using MC simulations. Results: An agreement of 1% and 1mm was observed between measurements and MC for PDD and OAR. The calculated ROF for the 5.0mm diameter cone was 0.695±0.008 in good agreement with the measured value of 0.711. For 7.5 and 10.0mm diameter cones the agreement was within 0.5%. Simulations of the misalignment of the beam axis and detector motion axis for angles between 0.5°–1.0° show a deviation >2% in PPD beyond certain depth. Conclusions: Monte Carlo calculated ROF values for 5.0 mm diameter cone agrees with our measurement to within 2.3%. Care should be exercised when measuring PDD and OAR for small cones. We recommend the use of MC to confirm the measurement under these conditions.

Determination of output factors for stereotactic radiosurgery beams

Accurate dosimetry of the narrow beam tends to be difficult to perform due to the absence of lateral electronic equilibrium and the steep dose gradient, as well as the finite size of detectors. Thus, although the high dose rate 6 MV beam on the VARIAN Trilogy accelerator is increasingly utilized for stereotactic radiosurgery (SRS) treatment, there is no general agreement in the SRS beam output factor values among the Trilogy user community. Trilogy SRS beams are confined by cone collimators and the available collimator sizes range from 5 and 10 to 30 mm, in every 2 mm increment. A range of the relative output factors are in clinic use. This variation may impair observations of dose response and optimizations of the prescribed dose. It is necessary to investigate an accurate, easily performable, and detector independent method for the narrow beam output factor measurement. In this study, a scanning beam/scanning chamber method was proposed to overcome the limitation/difficulty of using a relatively large detector in narrow beam output factor measurement. Specifically, for the scanning beam method, multiple narrow beams are used for the dose measurement using a finite size chamber. These multiple scanning beams form an equivalent large uniform field which provides lateral electron equilibrium condition. After the measurement, the contributions from neighboring beams are deconvolved and the value is used for output factor determinations. For a Linac that cannot move a beam laterally, the scanning chamber method can be used to achieve the same result. The output factors determined in such a method were compared to chambers (a 0.015 cc PTW PinPoint ion chamber and a 0.125 cc PTW ion chamber) and film measurement, as well as with Monte Carlo simulation. Film and Monte Carlo results are found to be in excellent agreement with the measurement using the scan beam method. However, the VARIAN recommended output factors measured directly by Wellhöfer CC01 chamber and Scanditronix photon field diode are consistently higher for all the cones. Especially for the 5 mm cone, the difference is more than 10%. Overall, the results suggested that the new method can help overcoming the detector volume averaging effect and the positioning uncertainties, which constitute the major challenge in small radiosurgical beam output factor measurement, and provide reliable output factors.

Implementation and experimental validation of the high dose rate stereotactic treatment mode at Varian accelerators.

Background. Treating small tumours in the brain requires usage of small fields. However, it is a challenge for all treatment planning systems (TPS) to modulate small fields (≤3×3 cm2) for which lateral electronic equilibrium does not exits. Material and methods. Open field beam data was measured for the stereotactic mode at our Varian Trilogy accelerator and was implemented in the Eclipse TPS for both the Pencil Beam (PB) and the Anisotropic Analytical Algorithm (AAA). Additional output factors, profiles and percentage depth dose curves were measured in a water phantom for asymmetric fields and fields defined by the Varian 120 MLC and the BrainLAB m3 MLC. Dose distributions in transverse sections were measured with gafchromic films in a homogeneous phantom. These were compared to those calculated by the Eclipse TPS for 11 stereotactic treatment plans. The plans included non-coplanar, dynamic and arc fields. All comparisons for the film measurements in the phantom were made with a Gamma criterion of 2 mm in distance and 3% in dose. Results. The AAA and the PB algorithms broadened the penumbra of the profiles by approximately 1.5 mm and 2.0 mm, respectively. For static fields defined by the Varian 120 MLC, the measured field size was 1.0 mm broader than the calculated. The output factors were typically within 2% for field sizes ≥ 2×2 cm2. More than 95% of the points passed the Gamma criterion. Up to 50% of the points in the low dose regions failed the criterion, but overall the match was satisfactory. Conclusion. The accuracy of calculations performed by the Eclipse TPS for the SRS mode was shown to be within a few percent for most clinical fields. AAA was found to be superior to the PB algorithm in modelling the dose in the penumbra region.

WE‐E‐AUD B‐05: Monte Carlo Dose Verification and Quality Assurance for Multi‐Target SRT

Purpose: To provide accurate, thorough and fast dose verification for hypo‐fractionated stereotactic radiotherapy (SRT) of small and multi targets planned with a Varian Eclipse treatment planning system delivered on a Varian Trilogy accelerator. Method and Materials: Seven brain and lung hypo‐fractionated SRT plans were generated by the Eclipse system for delivery on the Trilogy accelerator with the Millenium‐120 leaf multileaf collimator (MLC). These clinical SRT plans required thorough quality assurance measurements to obtain absolute point dose and 2‐D dose distributions due to the low number of fractions and high fraction dose. For small‐field and multi‐target plans, the EGS4/MCSIM code was used to calculate the dose distribution. A 0.125 cc ion chamber, a 0.016 cc pin‐point chamber and Kodak EDR2 film were used for the measurements and the results were compared with Monte Carlo calculations. Results: The dosimetry for small‐field and multi‐target treatment plans is challenging due to the comparable ranges of secondary electron and the field sizes defined by SRT MLC segments. Our Monte Carlo simulations can accurately reproduce the Trilogy dose distributions (within 1%/1mm). For the clinical SRT plans investigated in this work, the Monte Carlo doses agreed within 3% with ion chamber measurements and within 2%/2mm with film measurements. The doses calculated by the Eclipse AAA algorithm differed by no more than 5% from Monte Carlo calculations for small (4–40 cc) PTVs. Conclusion: Monte Carlo dose calculation provides accurate, thorough and fast dose verification for hypo‐fractionated SRT for small and multi‐target treatment plans generated by a Varian Eclipse treatment planning system on a Varian Trilogy accelerator.

SU‐GG‐T‐497: Comparison of Prostate IMRT Plans From Three Commercial Treatment Planning Systems

Purpose: Different treatment planning systems (TPS) use different treatment optimization and leaf sequencing algorithms. This work compares prostate IMRT plans optimized with three TPS to investigate the plan quality in terms of target conformity and delivery efficiency. Method and Materials: Eleven prostate cases were planned with the Corvus, Xio and Eclipse TPS using appropriate optimization parameters and dose constraints to meet the acceptance criteria. Plans were normalized for at least 95% of PTV to receive the prescription dose Dp. Dose‐volume histograms and isodose distributions were compared. Other quantities such as Dmin (the minimum dose received by 99% of CTV/PTV), Dmax (the maximum dose received by 1% of CTV/PTV), the volume of CTV/PTV receiving 110%, 105% and 95% of Dp (V110%, V105%, V95%), the volume of rectum and bladder receiving 65 and 40Gy (V65, V40), and the volume of femur receiving 50Gy (V50) were evaluated. Total segments and MUs were also compared. Results: While all plans meet target dose specifications and normal tissue constraints both XiO and Eclipse plans show less target dose heterogeneity (smaller Dmax and V110%) and lower V65 and V40 for the rectum and bladder compared to the Corvus plans. The PTV Dmin is about 2Gy lower for XiO plans than Corvus and Eclipse plans while the XiO and Eclipse plans have slightly higher V50 than the Corvus plans. The Eclipse and XiO plans require significantly less MUs to deliver than the Corvus plans. Conclusions To deliver on a Varian Trilogy accelerator, the Eclipse and XiO plans have better target dose uniformity, slightly less rectal and bladder doses and faster beam delivery. The Corvus plans have less rectal volumes receiving low doses (5–20Gy) while the XiO plans have slightly lower target doses. Overall, the Eclipse TPS is favored for our prostate IMRT planning.

SU‐GG‐T‐349: Large‐Field Sensitivity Analysis of a Non‐Cylindrically Symmetric Monte Carlo Beam Model of a Varian Electron Accelerator

Purpose: To facilitate the development and commissioning of radiotherapy electron beam models by studying the dosimetric response of a Varian Trilogy electron accelerator to variations of the geometric and electron beam parameters of the treatment head, including lateral offsets of linac components and lateral and angular offsets of the incident electron beam. Method and Materials: We have performed a sensitivity study examining the effect of varying beam and geometric parameters of a Monte Carlo model of a Varian Trilogy accelerator. The accelerator was modeled using a modified BEAMnrc package that included asymmetric accelerator and beam models. The dose output was modeled using DOSXYZnrc. Electron beams were modeled for 40×40 cm2 fields with applicators removed. Profiles at dmax and in the bremsstrahlung region were calculated for 4, 12, and 20 MeV configurations. Results: The dose distributions were evaluated for flatness, symmetry, and OAR. Scattering foil thickness had a negligible effect on dmax profiles, but increased the bremsstrahlung production rate. A 3 mm shift in the interfoil distance had a 5% effect on the OAR at 12 cm, but only 2% at 5 cm radius. Profiles in the bremsstrahlung region help differentiate between similar dmax behavior for changes in incident electron angle and lateral secondary scattering foil position. A 4 mm variation in the electron spot size had a negligible effect on the 4 MeV dose distribution, but varied the flatness of the 12 and 20 MeV beams by 2% and 5%, respectively. Conclusion: Lateral and angular offsets of beam and accelerator components have strong effects on dose distributions, and need to be included in any high‐accuracy electron beam model. Large fields without applicators and bremsstrahlung profiles can give additional insight into how parameters should be adjusted when commissioning a Monte Carlo beam model.Support from NIH R01 CA104777‐01A2.

MO‐D‐AUD‐07: Determination of Output Factors for Stereotactic Radiosurgery Beams by Monte Carlo and Measurements

Purpose: The Varian Trilogy accelerator provides high dose rate (1000MU/min) photon beams for stereotactic radiosurgery (SRS). Different output factors have been reported and their use may impair observations of dose response and optimization of prescribed dose. In this work, we investigated the output factors for the Trilogy 6 MV SRS beam using Mote Carlo simulations and measurements. Method and Materials: The Trilogy SRS cone collimators are 5 mm to 30 mm in diameter. Chamber measurement of output factors is difficult for small cone sizes. In this work, the measurement was carried out using a 0.015cc pinpoint chamber and Gafchromic EBT film. The Monte Carlo simulation was performed using the MCBEAM/MCSIM codes for linac head simulation and phantom dose calculation. The Monte Carlo calculations were validated using the measurement results and compared with the Varian recommended beam data. Results and Conclusions The agreement in percent depth doses and dose profiles between the simulation results, the measurement results and the Varian data was within 2%/1mm for the 10×10 cm2 reference field and all the cones at a 100 cm source to surface distance. The output factors obtained from the Monte Carlo simulations were in excellent agreement with the values from the film measurements. Similar agreement was found between the Monte Carlo results and the pinpoint chamber values except for cones with diameter less than 20 mm where the 2 mm chamber diameter has become comparable to the field size. However, the Varian recommended output factors are consistently higher than the Monte Carlo results, especially for the 5 mm cone, where the difference reaches 10.9%. Therefore, great caution must be taken with the use of the Varian recommended beam data.

Investigation of Linac-Based Image-Guided Hypofractionated Prostate Radiotherapy

A hypofractionation treatment protocol for prostate cancer was initiated in our department in December 2003. The treatment regimen consists of a total dose of 36.25 Gy delivered at 7.25 Gy per fraction over 10 days. We discuss the rationale for such a prostate hypofractionation protocol and the need for frequent prostate imaging during treatment. The CyberKnife (Accuray Inc., Sunnyvale, CA), a linear accelerator mounted on a robotic arm, is currently being used as the radiation delivery device for this protocol, due to its incorporation of near real-time kV imaging of the prostate via 3 gold fiducial seeds. Recently introduced conventional linac kV imaging with intensity modulated planning and delivery may add a new option for these hypofractionated treatments. The purpose of this work is to investigate the use of intensity modulated radiotherapy (IMRT) and the Varian Trilogy Accelerator with on-board kV imaging (Varian Medical Systems Inc., Palo Alto, CA) for treatment of our hypofractionated prostate patients. The dose-volume histograms and dose statistics of 2 patients previously treated on the CyberKnife were compared to 7-field IMRT plans. A process of acquiring images to observe intrafraction prostate motion was achieved in an average time of about 1 minute and 40 seconds, and IMRT beam delivery takes about 40 seconds per field. A complete 7-field IMRT plan can therefore be imaged and delivered in 10 to 17 minutes. The Varian Trilogy Accelerator with on-board imaging and IMRT is well suited for image-guided hypofractionated prostate treatments. During this study, we have also uncovered opportunities for improvement of the on-board imaging hardware/software implementation that would further enhance performance in this regard.

Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods

The purpose of this investigation is to study the feasibility of using an alternative method to commission stereotactic radiosurgery beams shaped by micro multi-leaf collimators by using Monte Carlo simulations to obtain beam characteristics of small photon beams, such as incident beam particle fluence and energy distributions, scatter ratios, depth–dose curves and dose profiles where measurements are impossible or difficult. Ionization chambers and diode detectors with different sensitive volumes were used in the measurements in a water phantom and the Monte Carlo codes BEAMnrc/DOSXYZnrc were used in the simulation. The Monte Carlo calculated data were benchmarked against measured data for photon beams with energies of 6 MV and 10 MV produced from a Varian Trilogy accelerator. The measured scatter ratios and cross-beam dose profiles for very small fields are shown to be not only dependent on the size of the sensitive volume of the detector used but also on the type of detectors. It is known that the response of some detectors changes at small field sizes. Excellent agreement was seen between scatter ratios measured with a small ion chamber and those calculated from Monte Carlo simulations. The values of scatter ratios, for field sizes from 6 × 6 mm2 to 98 × 98 mm2, range from 0.67 to 1.0 and from 0.59 to 1.0 for 6 and 10 MV, respectively. The Monte Carlo calculations predicted that the incident beam particle fluence is strongly affected by the X–Y-jaw openings, especially for small fields due to the finite size of the radiation source. Our measurement confirmed this prediction. This study demonstrates that Monte Carlo calculations not only provide accurate dose distributions for small fields where measurements are difficult but also provide additional beam characteristics that cannot be obtained from experimental methods. Detailed beam characteristics such as incident photon fluence distribution, energy spectra, including composition of primary and scattered photons, can be independently used in dose calculation models and to improve the accuracy of measurements with detectors with an energy-dependent response. Furthermore, when there are discrepancies between results measured with different detectors, the Monte Carlo calculated values can indicate the most correct result. The data set presented in this study can be used as a reference in commissioning stereotactic radiosurgery beams shaped by a BrainLAB m3 on a Varian 2100EX or 600C accelerator.

Investigation of Linac-based Image-guided Hypofractionated Prostate Treatment

Purpose/Objective: A hypofractionation protocol for prostate treatment was initiated in our department on December 2003. This short course high dose per fraction treatment regimen requires special attention to any potential geographical miss during treatment, such as inter- and/or intra-fraction prostate motion. The CyberKnife has been used as the radiation delivery device for this protocol. The choice of the CyberKnife is due to its incorporation of near real-time imaging and localization of the prostate based on three gold fiducial seeds placed in the prostate prior to CT. The purpose of this work is to investigate the use of IMRT and the Varian Trilogy Accelerator with on-board (kV) imaging for treatment of our hypofractionated prostate patients. Materials/Methods: Patients eligible for this protocol are those with newly diagnosed low risk prostate cancer (T1c T2a, iPSA ≤ 10 ng/ml, bGS ≤ 3+3). A total dose of 36.25Gy is delivered with 7.25Gy per fraction for 5 fractions over 5 days. An anthropomorphic pelvis phantom was used as the test patient. Three gold fiducial seeds were placed in the phantom in the region of the prostate. A 5 field IMRT plan was created for a typical prostate volume using the Varian Eclipse planning system. The on-board imager (OBI) of the Trilogy Accelerator is mounted on the treatment machine gantry via robotically controlled arms which operate along three axes of motion. A 150 kV X-ray tube is opposed to the amorphous silicon flat-panel detector and the source-detector pair are oriented orthogonal to the direction of the MV treatment beam. The IMRT plan was imaged and delivered on a 23EX Trilogy Accelerator in our department. Orthogonal image sets (one MV image and one kV image) were obtained prior to beam-on at each treatment gantry angle. This was to simulate determination and correction for inter-fraction setup errors (first gantry angle) and intra-fraction prostate motion (subsequent gantry angles). The dose distribution and DVHs for a patient previously treated on the CyberKnife was compared to an IMRT plan. Results: Based on our study, we estimate the time for patient setup and imaging of the first field is 4 minutes. This is essentially the time it takes to get the patient in treatment position and take images to correct for any inter-fraction setup error. Orthogonal MV-kV imaging at each subsequent treatment field beam angle requires about 1 minute per field. Moving the table to correct for any detected intra-fraction prostate motion at this step adds approximately 15 to 30 seconds. The total time to deliver each treatment field including gantry motion, mode up, beam-on, and leaf movement for IMRT delivery was about 1.4 minutes per field. Total treatment time from patient on the table to patient leaving the room would be approximately 16 to 22 minutes for a 5 to 7 field IMRT plan. Time between successive images for intra-fraction prostate localization is 40 to 90 seconds. These times are in contrast to the total treatment time per fraction on the CyberKnife of 40 to 45 minutes with imaging for prostate localization every 30 to 75 seconds. A 7 field Eclipse IMRT dose distribution compared favorably to that produced by the CyberKnife. After both plans were normalized to deliver 36.25Gy to 95% of the PTV, the maximum dose in the PTV was 39.2Gy for the IMRT plan and 39.6Gy for the CyberKnife plan. Maximum dose to the rectum was 2% greater in the CyberKnife plan but the dose to 20% of the rectum was 27.7Gy in the IMRT plan and 30.7Gy in the CyberKnife plan. Conclusions: Orthogonal MV and kV image sets provide a time savings when acquiring multiple sets of images during a treatment fraction. The Varian Trilogy Accelerator with on-board kV imaging is well suited for image guided prostate treatments.