Varian Linac Research Articles

SU‐E‐T‐460: Impact of the LINAC Repetition Rate On a High‐Resolution Liquid Ionization Chamber Array for Patient‐Specific QA

Purpose:The purpose of this study is to investigate the LINAC repetition‐rate (dose‐rate) dependence of OCTAVIUS 1000SRS liquid ionization chamber (LIC) array for patient specific QA of SRT plans delivered with flattening‐filter‐free (FFF) beams.Methods:1) The repetition‐rate dependence of 1000SRS was measured in a phantom constructed with 5‐cm solid water above and below the array for build‐up and backscatter. A 0.3cc calibrated ion chamber was also placed along the central axis 2.3cm below the center chamber of the array for normalizing LINAC output fluctuation. The signals from the center chamber of the array under different repetition rates in the range of 400–2400 MU/min for 6xFFF and 10xFFF beams on a Varian TrueBeamSTx LINAC, normalized by the independent chamber readings, were analyzed for the array response dependence on repetition rates. 2) Twelve Step‐and‐shoot IMRS QA plans (6xFFF and 10xFFF) were delivered to the array under different repetition rates for analysis and comparison. 3) The absolute doses measured by the center chamber were compared to measurements using an independent ionization chamber with the identical setup, taken as the gold standard. 4) The correction factors based on the actual delivery repetition rate were applied to the measurements, and the results were compared again to the gold standard.Results:1) The 1000SRS array exhibited repetition‐rate dependence for FFF beams, up to 5% for 6xFFF and 10% for 10xFFF; 2) The array showed clinically‐acceptable repetition‐rate dependence for regular flattened beams; 3) This repetition‐rate dependence significantly affected the measurement accuracy, thereby affecting IMRS QA results; 4) By applying an empirical repetition‐rate correction, the corrected measurements agreed better with the gold standard ion chamber measurements.Conclusion:OCTAVIUS 1000SRS LIC array exhibited considerable repetition‐rate dependence for FFF beams, which will affect the accuracy of the absolute QA measurements, especially for IMRS plans with the step‐and‐shoot technique.

SU‐E‐T‐92: Achieving Desirable Lung Doses in Total Body Irradiation Based On in Vivo Dosimetry and Custom Tissue Compensation

Purpose:To achieve desirable lung doses in total body irradiation (TBI) based on in vivo dosimetry and custom tissue compensation.Methods:The 15 MV photon beam of a Varian TrueBeam STx linac was used for TBI. Patients were positioned in the lateral decubitus position for AP/PA treatment delivery. Dose was calculated using the midpoint of the separation distance across the patient's umbilicus. Patients received 200 cGy twice daily for 3 days. The dose rate at the patient's midplane was approximately 10 cGy/min. Cerrobend blocks with a 5‐HVL thickness were used for the primary lung shielding. A custom styrofoam holder for rice‐flour filled bags was created based on the lung block cutouts. This was used to provide further lung shielding based on in vivo dose measurements. Lucite plates and rice‐flour bags were placed in the head, neck, chest, and lower extremity regions during the treatment to compensate for the beam off‐axis output variations. Two patients were included in the study. Patients 1 and 2 received a craniospinal treatment (1080 cGy) and a mediastinum treatment (2520 cGy), respectively, before the TBI. During the TBI nanoDot dosimeters were placed on the patient skin in the forehead, neck, umbilicus, and lung regions for dose monitoring. The doses were readout immediately after the treatment. Based on the readings, fine tuning of the thickness of the rice‐flour filled bags was exploited to achieve the desirable lung doses.Results:For both patients the mean lung doses, which took into consideration all treatments, were controlled within 900 +/−10% cGy, as desired. Doses to the forehead, neck, and umbilicus were achieved within +/−10% of the prescribed dose (1200 cGy).Conclusion:A reliable and robust method was developed to achieve desirable lung doses and uniform body dose in TBI based on in vivo dosimetry and custom tissue compensator.

SU‐E‐I‐06: A Dose Calculation Algorithm for KV Diagnostic Imaging Beams by Empirical Modeling

Purpose:To develop accurate three‐dimensional (3D) empirical dose calculation model for kV diagnostic beams for different radiographic and CT imaging techniques.Methods:Dose was modeled using photon attenuation measured using depth dose (DD), scatter radiation of the source and medium, and off‐axis ratio (OAR) profiles. Measurements were performed using single‐diode in water and a diode‐array detector (MapCHECK2) with kV on‐board imagers (OBI) integrated with Varian TrueBeam and Trilogy linacs. The dose parameters were measured for three energies: 80, 100, and 125 kVp with and without bowtie filters using field sizes 1×1–40×40 cm2 and depths 0–20 cm in water tank.Results:The measured DD decreased with depth in water because of photon attenuation, while it increased with field size due to increased scatter radiation from medium. DD curves varied with energy and filters where they increased with higher energies and beam hardening from half‐fan and full‐fan bowtie filters. Scatter radiation factors increased with field sizes and higher energies. The OAR was with 3% for beam profiles within the flat dose regions. The heal effect of this kV OBI system was within 6% from the central axis value at different depths. The presence of bowtie filters attenuated measured dose off‐axis by as much as 80% at the edges of large beams. The model dose predictions were verified with measured doses using single point diode and ionization chamber or two‐dimensional diode‐array detectors inserted in solid water phantoms.Conclusion:This empirical model enables fast and accurate 3D dose calculation in water within 5% in regions with near charge‐particle equilibrium conditions outside buildup region and penumbra. It considers accurately scatter radiation contribution in water which is superior to air‐kerma or CTDI dose measurements used usually in dose calculation for diagnostic imaging beams. Considering heterogeneity corrections in this model will enable patient specific dose calculation.

SU‐E‐T‐51: A National QA Audit of QA Systems Used for IMRT and VMAT Patient QA

Purpose:To independently validate patient‐specific quality assurance for IMRT and VMAT plans using the same set of treatment plans for all institutes.Methods:In February 2014 we devised a set of treatment plans: simple IMRT/VMAT plans; more complex IMRT/VMAT plans and a stereotactic VMAT plan, all 6MV for both Varian and Elekta linacs. In total we used 5 Varian and 8 Elekta plans. The plans were imported in the institute's treatment planning system for dose computation on the phantom of the audit team and the institute's phantom. Additionally, 10x10 cm2 fields were made and computed on both phantoms. Next, the audit team performed measurements using the audit equipment. So far, 18 audits have been performed and we expect to have concluded the audits by June 2015. The measurements were performed using an ionization chamber (PinPoint, PTW), Gafchromic film and a 2D ionization chamber array, all in an octagonal phantom (Octavius, PTW, see Figure). Differences between the measured and computed 2D dose distributions were investigated using a gamma analysis with a 5%/1mm criterion for the stereotactic treatment plan and 3%/3mm for the other plans. Additionally, the participating centres performed QA measurements of the same treatment plans according to their local protocol and equipment.Results:For the 10x10 field on the phantom, the first 18 audits showed differences with respect to the planning of −0.21 (range: −1.8; 2.2)%. See Table 1 for more results. The findings compared well with the QA measurement results reported by the institutions according to their local protocols.Conclusion:These preliminary results demonstrate that such a national QA audit is feasible. Importing and computing the prepared treatment plans in the planning systems in use in the country is achievable. The local QA systems provided similar results as found with the audit.

SU‐E‐T‐775: Use of Electronic Portal Imaging Device (EPID) for Quality Assurance (QA) of Electron Beams On Varian Truebeam System

Purpose:In a previous study we have demonstrated the feasibility of using EPID to QA electron beam parameters on a single Varian TrueBeam LINAC. This study aims to provide further investigation on (1) reproducibility of using EPID to detect electron beam energy changes on multiple machines and (2) evaluation of appropriate calibration methods to compare results from different EPIDs.Methods:Ad‐hoc mode electron beam images were acquired in developer mode with XML code. Electron beam data were collected on a total of six machines from four institutions. A custom‐designed double‐wedge phantom was placed on the EPID detector. Two calibration methods ‐ Pixel Sensitivity Map (PSM) and Large Source‐to‐Imager Distance Flood Field (LSID‐FF) ‐ were used. To test the sensitivity of EPID in detecting energy drifts, Bending Magnet Current (BMC) was detuned to invoke energy changes corresponding to ∼±1.5 mm change in R50% of PDD on two machines from two institutions. Percent depth ionization (PDI) curves were then analyzed and compared with the respective baseline images using LSID‐FF calibration. For reproducibility testing, open field EPID images and images with a standard testing phantom were collected on multiple machines. Images with and without PSM correction for same energies on different machines were overlaid and compared.Results:Two pixel shifts were observed in PDI curve when energy changes exceeded the TG142 tolerance. PSM showed the potential to correct the differences in pixel response of different imagers. With PSM correction, the histogram of images differences obtained from different machines showed narrower distributions than those images without PSM correction.Conclusion:EPID is sensitive for electron energy changes and the results are reproducible on different machines. When overlaying images from different machines, PSM showed the ability to partially eliminate the intrinsic variation of various imagers.Research Funding from Varian Medical Systems Inc.Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

SU‐E‐T‐481: In Vivo and Post Mortem Animal Irradiation: Measured Vs. Calculated Doses

Purpose:Computerized radiation therapy treatment planning is performed on almost all patients today. However it is seldom used for laboratory irradiations. The first objective is to assess whether modern radiation therapy treatment planning (RTP) systems accurately predict the subject dose by comparing in vivo and decedent dose measurements to calculated doses. The other objective is determine the importance of using a RTP system for laboratory irradiations.Methods:5 MOSFET radiation dosimeters were placed enterically in each subject (2 sedated Rhesus Macaques) to measure the absorbed dose at 5 levels (carina, lung, heart, liver and rectum) during whole body irradiation. The subjects were treated with large opposed lateral fields and extended distances to cover the entire subject using a Varian 600C linac. CT simulation was performed ante‐mortem (AM) and post‐mortem (PM). To compare AM and PM doses, calculation points were placed at the location of each dosimeter in the treatment plan. The measured results were compared to the results using Varian Eclipse and Prowess Panther RTP systems.Results:The Varian and Prowess treatment planning system agreed to within in +1.5% for both subjects. However there were significant differences between the measured and calculated doses. For both animals the calculated central axis dose was higher than prescribed by 3–5%. This was caused in part by inaccurate measurement of animal thickness at the time of irradiation. For one subject the doses ranged from 4% to 7% high and the other subject the doses ranged 7% to 14% high when compared to the RTP doses.Conclusions:Our results suggest that using proper CT RTP system can more accurately deliver the prescribed dose to laboratory subjects. It also shows that there is significant dose variation in such subjects when inhomogeneities are not considered in the planning process.

SU‐E‐T‐781: Using An Electronic Portal Imaging Device (EPID) for Correlating Linac Photon Beam Energies

Purpose:Electronic portal imaging devices (EPIDs) have proven to be useful for measuring several parameters of interest in linear accelerator (linac) quality assurance (QA). The purpose of this project was to evaluate the feasibility of using EPIDs for determining linac photon beam energies.Methods:Two non‐clinical Varian TrueBeam linacs (Varian Medical Systems, Palo Alto, CA) with 6MV and 10MV photon beams were used to perform the measurements. The linacs were equipped with an amorphous silicon based EPIDs (aSi1000) that were used for the measurements. We compared the use of flatness versus percent depth dose (PDD) for predicting changes in linac photon beam energy. PDD was measured in 1D water tank (Sun Nuclear Corporation, Melbourne FL) and the profiles were measured using 2D ion‐chamber array (IC‐Profiler, Sun Nuclear) and the EPID. Energy changes were accomplished by varying the bending magnet current (BMC). The evaluated energies conformed with the AAPM TG142 tolerance of ±1% change in PDD.Results:BMC changes correlating with a ±1% change in PDD corresponded with a change in flatness of ∼1% to 2% from baseline values on the EPID. IC Profiler flatness values had the same correlation. We observed a similar trend for the 10MV beam energy changes. Our measurements indicated a strong correlation between changes in linac photon beam energy and changes in flatness. For all machines and energies, beam energy changes produced change in the uniformity (AAPM TG‐142), varying from ∼1% to 2.5%.Conclusions:EPID image analysis of beam profiles can be used to determine linac photon beam energy changes. Flatness‐based metrics or uniformity as defined by AAPM TG‐142 were found to be more sensitive to linac photon beam energy changes than PDD.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‐90: Accuracy of Calibration of Lithium‐6 and ‐7 Enriched LiF TLDs for Neutron Measurements in High Energy Radiotherapy

Purpose:To determine the potential error involved in the interpretation of neutron measurements from medical linear accelerators (linacs) using TLD‐600H and TLD‐700H if standard AmBe and 2⁵2Cf neutron sources are used for calibration without proper inclusion of neutron energy spectrum information.Methods:The Kerma due to neutrons can be calculated from the energy released by various nuclear interactions (elastic and inelastic scatter, (n,α), (n,p), (n,d), (n,t), (n,2n), etc.). The response of each TLD can be considered the sum of the neutron and gamma components; each proportional to the Kerma. Using the difference between the measured TLD responses and the ratio of the calculated Kerma for each material, the neutron component of the response can be calculated. The Monte Carlo code MCNP6 has been used to calculate the neutron energy spectra resulting from photonuclear interactions in a Varian 21EX linac. TLDs have been exposed to the mixed (γ‐n) field produced by a linac and AmBe and 2⁵2Cf standard neutron sources.Results:For dosimetry of neutrons from AmBe or 2⁵2Cf sources, assuming TLD‐700H insensitivity to neutrons will Result in 10% or 20% overestimation of neutron doses respectively.For dosimetry of neutrons produced in a Varian 21EX, applying a calibration factor derived from a standard AmBe or 2⁵2Cf source will Result in an overestimation of neutron fluence, by as much as a factor of 47.The assumption of TLD‐700H insensitivity to neutrons produced by linacs leads to a negligible error due to the extremely high Kerma ratio (600H/700H) of 3000 for the assumed neutron spectrum.Conclusion:Lithium‐enriched TLDs calibrated with AmBe and/or 2⁵2Cf neutron sources are not accurate for use under the neutron energy spectrum produced by a medical linear accelerator.

SU‐E‐T‐757: TMRs Calculated From PDDs Versus the Direct Measurements for Small Field SRS Cones

Purpose:To investigate the variation of TMR for SRS cones obtained by TMR scanning, calculation from PDDs, and point measurements. The obtained TMRs were also compared to the representative data from the vendor.Methods:TMRs for conical cones of 4, 5, 7.5, 10, 12.5, 15, and 17.5 mm diameter (jaws set to 5×5 cm) were obtained for 6X FFF and 10X FFF energies on a Varian Edge linac. TMR scanning was performed with a Sun Nuclear 3D scanner and Edge detector at 100 cm SDD. TMR point measurements were measured with a Wellhofer tank and Edge detector, at multiple depths from 0.5 to 20 cm and 100 cm SDD. PDDs for converting to TMR were scanned with a Wellhofer system and SFD detector. The formalism of converting PDD to TMR, given in Khan's book (4th Edition, p.161) was applied. Sp values at dmax were obtained by measuring Scp and Sc of the cones (jaws set to 5×5 cm) using the Edge detector, and normalized to the 10×10 cm field.Results:Along the central axis beyond dmax, the RMS and maximum percent difference of TMRs obtained with different methods were as follows: (a) 1.3% (max=3.5%) for the calculated TMRs from PDDs versus direct scanning; (b) 1.2% (max=3.3%) for direct scanning versus point measurement; (c) 1.8% (max=5.1%) for the calculated versus point measurements; (d) 1.0% (max=3.6%) for direct scanning versus vendor data; (e) 1.6% (max=7.2%) for the calculated versus vendor data.Conclusion:The overall accuracy of TMRs calculated from PDDs was comparable with that of direct scanning. However, the uncertainty at depths greater than 20 cm, increased up to 5% when compared to point measurements. This issue must be considered when developing a beam model for small field SRS planning using cones.

TH‐EF‐BRB‐02: Combination of Multiple EPID Imager Layers Improves Image Quality and Tracking Performance of Low Contrast Objects

Purpose:We hypothesize that an electronic portal imaging device (EPID) consisting of multiple layers, each containing a scintillator coupled to amorphous silicon flat panel imager (AMFPI), increases photon detection efficiency, improving image quality and tracking accuracy of low contrast targets during radiotherapy.Methods:The prototype 4‐layer imager evaluated in this study contained four individually programmable layers each with a copper‐Gd₂O₂S scintillator coupled to an AMFPI with a complete set of readout electronics. The imager was placed on a Varian TrueBeam linac. A Las‐Vegas phantom programmed with sinusoidal motion (Peak‐to‐peak amplitude=20cm, Period=3.5sec) was imaged at a frame rate of 10Hz with one to four layers activated (activation of only the top layer is equivalent to using a conventional EPID). Image quality was assessed by counting the number of visible circles, and computing the contrast‐to‐noise ratio (CNR) of the least visible circle (depth=0.5mm, diameter=7mm). A previously validated tracking algorithm was employed for auto‐tracking. Tracking error was defined as the difference between the programmed and tracked positions of the circle. The Pearson correlation coefficient of the CNR and tracking error was also computed.Results:Motion‐induced EPID image blurring significantly reduced circle visibility. During one cycle of phantom motion, the number of visible circles ranged from 11–23, 13–24, 15–25, and 16–26 when imaged with one, two, three and four layer imagers, respectively. Compared with using only a single layer, combining 2, 3, and 4 layers increased the median CNR by factors of 1.19, 1.42, and 1.71, respectively and reduced the average tracking error from 3.32mm to 1.67mm to 1.47mm, and 0.74mm, respectively. Significant correlations (p∼10−9) were found between the tracking error and CNR.Conclusion:Combination of four amorphous silicon layers, instead of single layer, significantly improves the EPID image quality and tracking accuracy for a poorly visible object that replicates lung tumor radiotherapy conditions.The project described was supported, in part, by a grant from Varian Medical Systems, Inc., and Award No. R01CA188446‐01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

SU‐E‐T‐118: Analysis of Variability and Stability Between Two Water Tank Phantoms Utilizing Water Tank Commissioning Procedures

Purpose:The commissioning criteria of water tank phantoms are essential for proper accuracy and reproducibility in a clinical setting. This study outlines the results of mechanical and dosimetric testing between PTW MP3‐M water tank system and the Standard Imaging Doseview 3D water tank system.Methods:Measurements were taken of each axis of movement on the tank using 30 cm calipers at 1, 5, 10, 50, 100, and 200 mm for accuracy and reproducibility of tank movement. Dosimetric quantities such as percent depth dose and dose profiles were compared between tanks using a 6 MV beam from a Varian 23EX LINAC. Properties such as scanning speed effects, central axis depth dose agreement with static measurements, reproducibility of measurements, symmetry and flatness, and scan time between tanks were also investigated.Results:Results showed high geometric accuracy within 0.2 mm. Central axis PDD and in‐field profiles agreed within 0.75% between the tanks. These outcomes test many possible discrepancies in dose measurements across the two tanks and form a basis for comparison on a broader range of tanks in the future.Conclusion:Both 3D water scanning phantoms possess a high degree of spatial accuracy, allowing for equivalence in measurements regardless of the phantom used. A commissioning procedure when changing water tanks or upon receipt of a new tank is nevertheless critical to ensure consistent operation before and after the arrival of new hardware.

SU‐E‐J‐196: Implementation of An In‐House Visual Feedback System for Motion Management During Radiation Therapy

Purpose:To describe an in‐house video goggle feedback system for motion management during simulation and treatment of radiation therapy patients.Methods:This video goggle system works by splitting and amplifying the video output signal directly from the Varian Real‐Time Position Management (RPM) workstation or TrueBeam imaging workstation into two signals using a Distribution Amplifier. The first signal S[1] gets reconnected back to the monitor. The second signal S[2] gets connected to the input of a Video Scaler. The S[2] signal can be scaled, cropped and panned in real time to display only the relevant information to the patient. The output signal from the Video Scaler gets connected to an HDMI Extender Transmitter via a DVI‐D to HDMI converter cable. The S[2] signal can be transported from the HDMI Extender Transmitter to the HDMI Extender Receiver located inside the treatment room via a Cat5e/6 cable. Inside the treatment room, the HDMI Extender Receiver is permanently mounted on the wall near the conduit where the Cat5e/6 cable is located. An HDMI cable is used to connect from the output of the HDMI Receiver to the video goggles.Results:This video goggle feedback system is currently being used at two institutions. At one institution, the system was just recently implemented for simulation and treatments on two breath‐hold gated patients with 8+ total fractions over a two month period. At the other institution, the system was used to treat 100+ breath‐hold gated patients on three Varian TrueBeam linacs and has been operational for twelve months. The average time to prepare the video goggle system for treatment is less than 1 minute.Conclusion:The video goggle system provides an efficient and reliable method to set up a video feedback signal for radiotherapy patients with motion management.

SU‐E‐T‐429: Feasibility Study On Three‐Dimensional GRID Therapy in Conventional Linacs

Purpose:Two‐dimensional GRID therapy, traditionally planned and delivered using a dedicated GRID block or MLC modulation, has shown clinical efficacy in treating bulky tumors. However, the large dose to normal tissues outside target can be limiting. We hypothesize that modulation in the third dimension will improve dose sparing of normal tissues, maximize the bystander effect within the target, and ultimately improve the therapy effectiveness. This study aims to investigate the feasibility of a three‐dimensional GRID technique using conventional LINACs to achieve a 3D lattice of high dose volumes within a target.Methods:Datasets of patient's having large tumor sizes were used to investigate the planning and delivering of 3D GRID using a Varian TrueBeam linac. Original patient contours of PTV are exported from a TPS to DICOManTX where 3D GRID targets are generated in programmable configurations. A structure of avoidance (SOA), i.e., PTV minus GRID targets, is also generated to facilitate inverse planning to achieve the desired pattern. The artificial structures were sent back to the TPS where an IMRT or VMAT plan is designed to deliver a desired high dose to GRID targets while minimizing the dose to the SOA as much as possible.Results:The programmable GRID target generator enables us to modify the target geometry to maximize the peak‐to‐valley ratio. Preliminary results show that plans based on spherical GRID targets achieve a higher peak‐to‐valley dose ratio compared with cylindrical targets. High dose spillage outside the target was eliminated. IMRT planning requires the number of beams to be larger than 16, while for VMAT the number of arcs should be at least 4 in order to achieve dosimetric goals.Conclusion:Planning and delivering 3D GRID therapy using conventional LINACs was shown to be feasible. More research and development are required before this new modality can be implemented clinically.

SU‐E‐T‐211: Comparison of Seven New TrueBeam Linacs with Enhanced Beam Data Conformance Using a Beam Comparison Software Tool

Purpose:To develop an independent software tool to assist in commissioning linacs with enhanced beam conformance, as well as perform ongoing QA for dosimetrically equivalent linacs.Methods:Linac manufacturers offer enhanced beam conformance as an option to allow for clinics to complete commissioning efficiently, as well as implement dosimetrically equivalent linacs. The specification for enhanced conformance includes PDD as well as profiles within 80% FWHM. Recently, we commissioned seven Varian TrueBeam linacs with enhanced beam conformance. We developed a software tool in Visual Basic to allow us to load the reference beam data and compare our beam data during commissioning to evaluate enhanced beam conformance. This tool also allowed us to upload our beam data used for commissioning our dosimetrically equivalent beam models to compare and tweak each of our linac beams to match our modelled data in Varian's Eclipse TPS. This tool will also be used during annual QA of the linacs to compare our beam data to our baseline data, as required by TG‐142.Results:Our software tool was used to check beam conformance for seven TrueBeam linacs that we commissioned in the past six months. Using our tool we found that the factory conformed linacs showed up to 3.82% difference in their beam profile data upon installation. Using our beam comparison tool, we were able to adjust the energy and profiles of our beams to accomplish a better than 1.00% point by point data conformance.Conclusion:The availability of quantitative comparison tools is essential to accept and commission linacs with enhanced beam conformance, as well as to beam match multiple linacs. We further intend to use the same tool to ensure our beam data conforms to the commissioning beam data during our annual QA in keeping with the requirements of TG‐142.

Field size consistency of nominally matched linacs.

Given that there is increasing recognition of the effect that sub-millimetre changes in collimator position can have on radiotherapy beam dosimetry, this study aimed to evaluate the potential variability in small field collimation that may exist between otherwise matched linacs. Field sizes and field output factors were measured using radiochromic film and an electron diode, for jaw- and MLC-collimated fields produced by eight dosimetrically matched Varian iX linacs (Varian Medical Systems, Palo Alto, USA). This study used nominal sizes from 0.6×0.6 to 10×10cm(2), for jaw-collimated fields, and from 1×1 to 10×10cm(2) for MLC-collimated fields, delivered from a zero (head up, beam directed vertically downward) gantry angle. Differences between the field sizes measured for the eight linacs exceeded the uncertainty of the film measurements and the repositioning uncertainty of the jaws and MLCs on one linac. The dimensions of fields defined by MLC leaves were more consistent between linacs, while also differing more from their nominal values than fields defined by orthogonal jaws. The field output factors measured for the different linacs generally increased with increasing measured field size for the nominal 0.6×0.6 to 1×1cm(2) fields, and became consistent between linacs for nominal field sizes of 2×2cm(2) and larger. The inclusion in radiotherapy treatment planning system beam data of small field output factors acquired in fields collimated by jaws (rather than the more-reproducible MLCs), associated with either the nominal or the measured field sizes, should be viewed with caution. The size and reproducibility of the fields (especially the small fields) used to acquire treatment planning data should be investigated thoroughly as part of the linac or planning system commissioning process. Further investigation of these issues, using different linac models, collimation systems and beam orientations, is recommended.

A geometrical model for the Monte Carlo simulation of the TrueBeam linac

Monte Carlo simulation of linear accelerators (linacs) depends on the accurate geometrical description of the linac head. The geometry of the Varian TrueBeam linac is not available to researchers. Instead, the company distributes phase-space files of the flattening-filter-free (FFF) beams tallied at a plane located just upstream of the jaws. Yet, Monte Carlo simulations based on third-party tallied phase spaces are subject to limitations. In this work, an experimentally based geometry developed for the simulation of the FFF beams of the Varian TrueBeam linac is presented. The Monte Carlo geometrical model of the TrueBeam linac uses information provided by Varian that reveals large similarities between the TrueBeam machine and the Clinac 2100 downstream of the jaws. Thus, the upper part of the TrueBeam linac was modeled by introducing modifications to the Varian Clinac 2100 linac geometry. The most important of these modifications is the replacement of the standard flattening filters by ad hoc thin filters. These filters were modeled by comparing dose measurements and simulations. The experimental dose profiles for the 6 MV and 10 MV FFF beams were obtained from the Varian Golden Data Set and from in-house measurements performed with a diode detector for radiation fields ranging from 3 × 3 to 40 × 40 cm2 at depths of maximum dose of 5 and 10 cm. Indicators of agreement between the experimental data and the simulation results obtained with the proposed geometrical model were the dose differences, the root-mean-square error and the gamma index. The same comparisons were performed for dose profiles obtained from Monte Carlo simulations using the phase-space files distributed by Varian for the TrueBeam linac as the sources of particles. Results of comparisons show a good agreement of the dose for the ansatz geometry similar to that obtained for the simulations with the TrueBeam phase-space files for all fields and depths considered, except for the 40 × 40 cm2 field where the ansatz geometry was able to reproduce the measured dose more accurately. Our approach overcomes some of the limitations of using the Varian phase-space files. It makes it possible to: (i) adapt the initial beam parameters to match measured dose profiles; (ii) reduce the statistical uncertainty to arbitrarily low values; and (iii) assess systematic uncertainties (type B) by using different Monte Carlo codes. One limitation of using phase-space files that is retained in our model is the impossibility of performing accurate absolute dosimetry simulations because the geometrical description of the TrueBeam ionization chamber remains unknown.

A Monte Carlo simulation framework for electron beam dose calculations using Varian phase space files for TrueBeam Linacs.

To develop a framework for accurate electron Monte Carlo dose calculation. In this study, comprehensive validations of vendor provided electron beam phase space files for Varian TrueBeam Linacs against measurement data are presented. In this framework, the Monte Carlo generated phase space files were provided by the vendor and used as input to the downstream plan-specific simulations including jaws, electron applicators, and water phantom computed in the EGSnrc environment. The phase space files were generated based on open field commissioning data. A subset of electron energies of 6, 9, 12, 16, and 20 MeV and open and collimated field sizes 3 × 3, 4 × 4, 5 × 5, 6 × 6, 10 × 10, 15 × 15, 20 × 20, and 25 × 25 cm(2) were evaluated. Measurements acquired with a CC13 cylindrical ionization chamber and electron diode detector and simulations from this framework were compared for a water phantom geometry. The evaluation metrics include percent depth dose, orthogonal and diagonal profiles at depths R100, R50, Rp, and Rp+ for standard and extended source-to-surface distances (SSD), as well as cone and cut-out output factors. Agreement for the percent depth dose and orthogonal profiles between measurement and Monte Carlo was generally within 2% or 1 mm. The largest discrepancies were observed within depths of 5 mm from phantom surface. Differences in field size, penumbra, and flatness for the orthogonal profiles at depths R100, R50, and Rp were within 1 mm, 1 mm, and 2%, respectively. Orthogonal profiles at SSDs of 100 and 120 cm showed the same level of agreement. Cone and cut-out output factors agreed well with maximum differences within 2.5% for 6 MeV and 1% for all other energies. Cone output factors at extended SSDs of 105, 110, 115, and 120 cm exhibited similar levels of agreement. We have presented a Monte Carlo simulation framework for electron beam dose calculations for Varian TrueBeam Linacs. Electron beam energies of 6 to 20 MeV for open and collimated field sizes from 3 × 3 to 25 × 25 cm(2) were studied and results were compared to the measurement data with excellent agreement. Application of this framework can thus be used as the platform for treatment planning of dynamic electron arc radiotherapy and other advanced dynamic techniques with electron beams.

Investigating in-field and out-of-field neutron contamination in high-energy medical linear accelerators based on the treatment factors of field size, depth, beam modifiers, and beam type

PurposeWe analysed the effects of field size, depth, beam modifier and beam type on the amount of in-field and out-of-field neutron contamination for medical linear accelerators (linacs). MethodsMeasurements were carried out for three high-energy medical linacs of Elekta Synergy Platform, Varian Clinac DHX High Performance and Philips SL25 using bubble detectors. The photo-neutron measurements were taken in the first two linacs with 18 MV nominal energy, whereas the electro-neutrons were measured in the three linacs with 9 MeV, 10 MeV, 15 MeV and 18 MeV. ResultsThe central neutron doses increased with larger field sizes as a dramatic drop off was observed in peripheral areas. Comparing with the jaws-shaped open-field of 10 × 10 cm, the motorised and physical wedges contributed to neutron contamination at central axis by 60% and 18%, respectively. The similar dose increment was observed in MLC-shaped fields. The contributions of MLCs were in the range of 55–59% and 19–22% in Elekta and Varian linacs comparing with 10 × 10 and 20 × 20 cm open fields shaped by the jaws, respectively. The neutron doses at shallow depths were found to be higher than the doses found at deeper regions. The electro-neutron dose at the 18 MeV energy was higher than the doses at the electron energies of 15 MeV and 9 MeV by a factor of 3 and 50, respectively. ConclusionThe photo- and electro-neutron dose should be taken into consideration in the radiation treatment with high photon and electron energies.

Output calculation of electron therapy at extended SSD using an improved LBR method.

To calculate the output factor (OPF) of any irregularly shaped electron beam at extended SSD. Circular cutouts were prepared from 2.0 cm diameter to the maximum possible size for 15 × 15 applicator cone. In addition, two irregular cutouts were prepared. For each cutout, percentage depth dose (PDD) at the standard SSD and doses at different SSD values were measured using 6, 9, 12, and 16 MeV electron beam energies on a Varian 2100C LINAC and the distance at which the central axis electron fluence becomes independent of cutout size was determined. The measurements were repeated with an ELEKTA Synergy LINAC using 14 × 14 applicator cone and electron beam energies of 6, 9, 12, and 15 MeV. The PDD measurements were performed using a scanning system and two diodes-one for the signal and the other a stationary reference outside the tank. The doses of the circular cutouts at different SSDs were measured using PTW 0.125 cm(3) Semiflex ion-chamber and EDR2 films. The electron fluence was measured using EDR2 films. For each circular cutout, the lateral buildup ratio (LBR) was calculated from the measured PDD curve using the open applicator cone as the reference field. The effective SSD (SSDeff) of each circular cutout was calculated from the measured doses at different SSD values. Using the LBR value and the radius of the circular cutout, the corresponding lateral spread parameter [σR(z)] was calculated. Taking the cutout size dependence of σR(z) into account, the PDD curves of the irregularly shaped cutouts at the standard SSD were calculated. Using the calculated PDD curve of the irregularly shaped cutout along with the LBR and SSDeff values of the circular cutouts, the output factor of the irregularly shaped cutout at extended SSD was calculated. Finally, both the calculated PDD curves and output factor values were compared with the measured values. The improved LBR method has been generalized to calculate the output factor of electron therapy at extended SSD. The percentage difference between the calculated and the measured output factors of irregularly shaped cutouts in a clinical useful SSD region was within 2%. Similar results were obtained for all available electron energies of both Varian 2100C and ELEKTA Synergy machines.

Optimization of heart block in the left-sided whole breast radiation treatments.

Purpose: Blocks have been used to protect heart from potential radiation damage in left-sided breast treatments. Since cardiac motion pattern may not be fully captured on conventional 3DCT or 4DCT simulation scans, this study was intended to investigate the optimization of the heart block design taking the cardiac motion into consideration.Materials and Methods: Whole breast treatment plans using two opposed tangential fields were designed based on 4DCT simulation images for 10 left-sided breast cancer patients. Using an OBI system equipped to a Varian Linac, beam-eye viewed fluoroscopy images were acquired for each of the treatment beams after patient treatment setup, and the MLC heart blocks were overlaid onto the fluoroscopy images with an in-house software package. A non-rigid image registration and tracking algorithm was utilized to track the cardiac motion on the fluoroscopy images with minimal manual delineation for initialization, and the tracked cardiac motion information was used to optimize the heart block design to minimize the radiation damage to heart while avoiding the over-shielding that may lead to underdosing certain breast tissues.Results: Twenty-three sets of fluoroscopy images were acquired on 23 different days of treatment for the 10 patients. As expected, heart moved under the influences of both respiratory and cardiac motion. It was observed that for 16 out of the 23 treatments, heart moved beyond the planed heart block into treatment fields and MLC had to be adjusted to fully block heart. The adjustment was made for all but one patient. The number of the adjusted MLC leaves ranged from 1 to 16 (mean = 10), and the MLC leaf position adjustment ranged from 2 to 10 mm (mean = 6 mm). The added heart block areas ranged from 3 to 1230 mm2 (mean = 331 mm2).Conclusion: In left-sided whole breast radiation treatments, simulation CT (and 4DCT) based heart block design may not provide adequate heart protection for all the treatments. A fluoroscopy-based method has been developed to adaptively optimize the heart MLC block to achieve optimal heart protection.