Elekta Synergy Linac Research Articles

Validating TG119 VMAT Commissioning on a Volumetric Phantom

<h3>Purpose/Objective(s)</h3> The present work adapts AAPM TG119 on a volumetric detector array and evaluates its feasibility for VMAT commissioning for two different Treatment Planning Systems (TPS). <h3>Materials/Methods</h3> TG119 describes a group of test plans for standard structure sets following prescription and dose constraints objectives. These plans were calculated and evaluated using both Elekta Monaco V5.0 (Montecarlo) and Varian Eclipse v15.6 (AAA algorithm) TPS. TG119 structure sets were inserted on virtual CT images for volumetric Quality Assurance (QA) phantom. A set of plans calculated on both TPS were delivered on a 6MV photon beam on an Elekta Synergy Linac. A set of plans was generated only on Eclipse for a Varian Halcyon Linac with 6MV Flattening Filter Free beam. Generated plans were delivered, dose distribution was concurrently measured with Ion Chamber (IC) point doses using a cavity plug holder inserted on the QA phantom. Dose constraints and goals comparison between TPS was executed. Gamma analysis was performed for 3%3mm and 3%2mm global gamma index and Confidence Limits (CL) according to the TG119 definition of Normal quantile were calculated. Measured and planned point doses on relevant positions were compared. <h3>Results</h3> TG119 goals were mostly satisfied except for some critical situations: PTV D99 goal on head & neck plan calculated on Eclipse TPS and Superior Target D10 goal on Multitarget Plan for Monaco TPS calculation. Gamma Analysis for 3%3mm range of passing rate was 96.8% to 100% while CLs range between 4.1% and 4.7% for a Normal quantile for Monaco TPS. For Eclipse TPS range of passing rate achieved was among 97.2% and 99.9%, CLs range was between 3.2% and 3.1%. For Eclipse plans delivered on Halcyon, the passing rate range was among 98.6% and 99.8%, CLs between 1.6% and 1.7%. 3%2mm global gamma index analysis shows a range of passing rate of 93.5% and 100% for Monaco TPS while CLs range between 4.8% and 8.3%. For Elekta plans on Eclipse TPS range of passing rate was 95.6% and 98.8%, CLs range was between 5.1% and 4.5%. The gamma passing rate range for Halcyon plans was between 94.5% and 99.3 and CLs between 3.4% and 6.0%. IC measurements for Synergy Linac showed an average dose difference of 0.84% between measured and calculated plans on Monaco, range among -3.15% and 7.01%, the greatest differences were found on C-Shape plan for high gradient regions measurements. For Eclipse TPS, the average dose difference was 1.28%, range between -1.04% and 10.26%, the highest difference was found in high gradient region C-Shape plan. For Halcyon plans, IC measurements showed an average dose difference of 0.74%, ranging between -0.27% and 3.38%. <h3>Conclusion</h3> CLs agreed with TG119 reference values even when stricter requirements were used. New criteria are more appropriate for modern treatments, but TG119 methodology remains a valid procedure for VMAT. The methodology described for commissioning VMAT technique on a volumetric QA phantom shows to be robust and consistent for various TPS and lilacs.

Performance of 3D diamond detectors in small field dosimetry: The impact of pixel size

Purpose:Small photon beams used in radiotherapy techniques have inherent characteristics of charge particle disequilibrium and high-dose gradient making accurate dosimetry for such fields very challenging. By means of a 3D manufacturing technique, it is possible to create arrays of pixels with a very small sensitive volume for radiotherapy dosimetry. We investigate the impact of 3D pixels size on absorbed dose sensitivity, linearity of response with dose rate, reproducibility and beam profile measurements. Methods:Diamond detectors with different pixel sizes have been produced in the 3DOSE experiment framework. To investigate the pixels size impact, they were tested using an Elekta Synergy LINAC. Dose rate dependence, absorbed dose sensitivity, reproducibility and beam profile measurement accuracy have been investigated and compared with PTW 60019 and IBA SFD reference dosimeters. Results:All of the 3D pixels had a linear and reproducible response to the dose rate. The sensitivity of a pixel decreases with its size, although even the smallest pixel has a high absorbed dose sensitivity (15 nC/Gy). The penumbra width measured with the smallest pixel size was consistent with the PTW microDiamond and differed by 0.2 mm from the IBA SFD diode. Conclusions:The study demonstrates that variation in pixel size do not affect the linearity of response with dose rate and the reproducibility of response. Due to the 3D geometry, the absorbed dose sensitivity of the detector remains high even for the smallest pixel, furthermore the pixel size was demonstrated to be of fundamental importance in the measurement of beam profiles.

Electron ultra-high dose rate FLASH irradiation study using a clinical linac: Linac modification, dosimetry, and radiobiological outcome.

Ultra-high dose rate FLASH irradiation (FLASH-IR) has been shown to cause less normal tissue damage compared with conventional irradiation (CONV-IR), this is known as the "FLASH effect." It has attracted immense research interest because its underlying mechanism is scarcely known. The purpose of this study was to determine whether FLASH-IR and CONV-IR induce differential inflammatory cytokine expression using a modified clinical linac. An Elekta Synergy linac was used to deliver 6 MeV CONV-IR and modified to deliver FLASH-IR. Female FvB mice were randomly assigned to three different groups: a non-irradiated control, CONV-IR, or FLASH-IR. The FLASH-IR beam was produced by single pulses repeated manually with a 20-s interval (Strategy 1), or single-trigger multiple pulses with a 10 ms interval (Strategy 2). Mice were immobilized in the prone position in a custom-designed applicator with Gafchromic films positioned under the body. The prescribed doses for the mice were 6 to 18 Gy and verified using Gafchromic films. Cytokine expression of three pro-inflammatory cytokines (tumor necrosis factor-α [TNF-α], interferon-γ [IFN-γ], interleukin-6 [IL-6]) and one anti-inflammatory cytokine (IL-10) in serum samples and skin tissue were examined within 1 month post-IR. The modified linac delivered radiation at an intra-pulse dose rate of around 1 × 106 Gy/s and a dose per pulse over 2 Gy at a source-to-surface distance (SSD) of 13 to 15 cm. The achieved dose coverage was 90%-105% of the maximum dose within -20 to 20 mm in the X direction and 95% within -30 to 30 mm in the Y direction. The absolute deviations between the prescribed dose and the actual dose were 2.21%, 6.04%, 2.09%, and 2.73% for 6, 9, 12, and 15 Gy as measured by EBT3 films, respectively; and 4.00%, 4.49%, and 2.30% for 10, 14, and 18 Gy as measured by the EBT XD films, respectively. The reductions in the CONV-IR versus the FLASH-IR group were 4.89%, 10.28%, -7.8%, and -22.17% for TNF-α, IFN-γ, IL-6, and IL-10 in the serum on D6, respectively; 37.26%, 67.16%, 56.68%, and -18.95% in the serum on D31, respectively; and 62.67%, 35.65%, 37.75%, and -12.20% for TNF-α, IFN-γ, IL-6, and IL-10 in the skin tissue, respectively. Ultra-high dose rate electron FLASH caused lower pro-inflammatory cytokine levels in serum and skin tissue which might mediate differential tissue damage between FLASH-IR and CONV-IR.

The Influence of the Technical Parameters of the SRS Plans on the Dosimetric and Radiobiologic Evaluation

Background: The purpose of this study was to evaluate influence of technical parameters on Stereotactic Radiosurgery (SRS) plans on dosimetric evaluation. Materials and methods: There were considered and selected two patients who had brain tumors and had been treating in Tomsk Regional Oncology Center. All treatment plans were simulated using the Monaco treatment planning system v 5.11 (Elekta Instrument AB, Stockholm) on the Elekta Synergy linac with photon beam nominal energy 6 MV. For study there were created several plans with different technical parameters (number of arcs, increment, gantry angle, collimator angle, couch angle, minimum segment width). Each plan was evaluated by metrics such as Conformal, Homogeneity and Gradient indexes. Results: If we use a large increment value it may create few sectors and which can produce low quality plans, and increase treatment time, otherwise if we use a too small increment value it will give us more sectors and they can increase the quality of the plan. The dose which was measured with higher MSW showed to us better agreement with the calculated dose. Conclusion: It is valuable to carefully select technical parameters. The use of rigid increment value, segment width or increasing number of arcs can make plans have lower quality and to rise time of treatment.

A Monte Carlo model of an agility head for a 10-MV photon beam

Objective: Modelling linear accelerator (linac) photon beams using the Monte Carlo (MC) technique is presently the most precise technique with which to calculate radiotherapy beam data. Complicated treatment plans can be achieved by using the Agility multileaf collimator. The focus of this study is to model a linac, integrated with an Agility head, using EGSnrc MC code and to investigate its accuracy compared with actual measurements. Methodology: A 10-MV photon beam from an Agility head installed in an Elekta Synergy linac was simulated using BEAMnrc MC code. Various incident electron energies were used to generate percentage depth dose curves for the MC model and were compared with measurements using gamma analysis (γ). Consequently, the focal spot with different full width half-maximums (FWHMs) was used to tune dose profiles. In addition, any differences in dose profiles were further minimized using different angular divergences. Results: The electron energy of 9.6 MeV was found to represent the best match with the measurements, where the γ pass rate was 100%. For dose profile comparisons, a circular focal spot with an FWHM of 0.45 cm and an angular divergence of 0.04 produced the best match with the measurements. Conclusion: The presented MC linac model integrated with an Agility head for a 10-MV photon beam can be reliably used for dose calculations.

Dosimetric Characterization of Small Radiotherapy Electron Beams Collimated by Circular Applicators with the New Microsilicon Detector

High-energy small electron beams, generated by linear accelerators, are used for radiotherapy of localized superficial tumours. The aim of the present study is to assess the dosimetric performance under small radiation therapy electron beams of the novel PTW microSilicon detector compared to other available dosimeters. Relative dose measurements of circular fields with 20, 30, 40, and 50 mm aperture diameters were performed for electron beams generated by an Elekta Synergy linac, with energy between 4 and 12 MeV. Percentage depth dose, transverse profiles, and output factors, normalized to the 10 × 10 cm2 reference field, were measured. All dosimetric data were collected in a PTW MP3 motorized water phantom, at SSD of 100 cm, by using the novel PTW microSilicon detector. The PTW diode E and the PTW microDiamond were also used in all beam apertures for benchmarking. Data for the biggest field size were also measured by the PTW Advanced Markus ionization chamber. Measurements performed by the microSilicon are in good agreement with the reference values for all the tubular applicators and beam energies within the stated uncertainties. This confirms the reliability of the microSilicon detector for relative dosimetry of small radiation therapy electron beams collimated by circular applicators.

Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo.

Accuracy of ionization chamber (IC) to measure the scatter output factor (Scp) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the Scp for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)20,10) with the IC measurement. The validated linac model was simulated to calculate the Scp consisting of collimator scatter factor (Sc) and phantom scatter factor (Sp). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR20,10 value was 0.675 for field size 10 cm × 10 cm. The Scp, Sc, and Sp simulated values were close to the IC measurement within ±2% difference. The simulation for Sc and Sp in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the Scp calculation including small-field size for linac.

Comparison of treatment plans for hypofractionated high-dose prostate cancer radiotherapy using the Varian Halcyon and the Elekta Synergy platforms.

PurposeTo compare radiotherapy plans between an O‐ring and a conventional C‐arm linac for hypofractionated high‐dose prostate radiotherapy in terms of plan quality, dose distribution, and quality assurance in a multi‐vendor environment.MethodsTwenty prostate cancer treatment plans were irradiated on the O‐ring Varian Halcyon linac and were re‐optimized for the C‐arm Elekta Synergy Agility linac. Dose‐volume histogram metrics for target coverage and organ at risk dose, quality assurance, and monitor units were retrospectively compared. Patient‐specific quality assurance with ion chamber measurements, gamma index analysis, and portal dosimetry was performed using the Varian Portal Dosimetry system and the ArcCHECK® phantom (Sun Nuclear Corporation). Prostate‐only radiotherapy was delivered with simultaneous integrated boost (SIB) volumetric modulated arc therapy (VMAT) in 20 fractions of 2.5/3.0 Gy each.ResultsFor both linacs, target coverage was excellent and plan quality comparable. Homogeneity in PTVBoost was high for Synergy as well as Halcyon with a mean homogeneity index of 0.07 ± 0.01 and 0.05 ± 0.01, respectively. Mean dose for the organs at risk rectum and bladder differed not significantly between the linacs but were higher for the femoral heads and penile bulb for Halcyon. Quality assurance showed no significant differences in terms of ArcCHECK gamma pass rates. Median pass rate for 3%/2 mm was 99.3% (96.7 to 99.8%) for Synergy and 99.8% (95.6 to 100%) for Halcyon. Agreement between calculated and measured dose was high with a median deviation of −0.6% (−1.7 to 0.8%) for Synergy and 0.2% (−0.6 to 2.3%) for Halcyon. Monitor units were higher for the Halcyon by approximately 20% (p < 0.001).ConclusionHypofractionated high‐dose prostate cancer SIB VMAT on the Halcyon system is feasible with comparable plan quality in reference to a standard C‐arm Elekta Synergy linac.

High-Resolution MLC Leaf Position Measurements With a Large Area MAPS

Multileaf collimators (MLCs) are an essential component in modern radiotherapy that shape the X-ray treatment beam. Currently, MLC leaf position accuracy is verified to ±1 mm every month. However, leaf position accuracy can drift between verification dates and treatment verification only occurs pretreatment. To prevent serious errors, it would be highly beneficial to use a real-time verification system. We are developing a system based on monolithic active pixel sensors (MAPSs). MAPS are radiation hard under photon and electron irradiation, have high readout rates, low attenuation and are suitable for high-resolution applications making them an ideal upstream radiation detector. Here, we report results using the Lassena MAPS, which measures 12×14 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and is three-side buttable, allowing full treatment fields to be monitored. The Lassena detector was placed in the treatment field of an Elekta Synergy LINAC which has an MLC leaf width of 0.5 cm. An MLC leaf was extended to ten different positions within the field. Sobel-based methods were used to reconstruct the leaf edge position. Correspondence between reconstructed and set leaf position was excellent and resolutions ranged between 60.6± 8 and 109±12 μm for a central leaf with leaf extensions ranging from 1 to 35 mm using ~0.3 s of treatment beam time while the sensor was placed at an SSD of 85 cm.

Analysis of characteristics and validation of 6 MV photon beam produced by Elekta Synergy linear accelerator using EGSnrc Monte Carlo code

The most important step of the Monte Carlo (MC) dose calculation is the correct modelling of the radiation device. This study was aimed to determine the initial electron beam characteristics of the BEAMnrc model of 6 MV photon beam of Elekta Synergy IGRT System linear accelerator and to validate the simulation. For this purpose, EGSnrc/BEAMnrc/DOSXYZnrc Monte Carlo code were used for an accurate model of a 6 MV Elekta Synergy linac head. To analyze MC model, the dose distributions were calculated by EGSnrc/DOSXYZnrc simulation in a homogeneous water phantom. Then the results were compared with the measurements performed in the water phantom. In order to determine the mean energy and Full Width at Half Maximum (FWHM) suitable for the device in MC model, the Percentage of Depth Dose (PDD) and profile curves were compared for 10 × 10 cm2 field size. All data were analyzed using MATLAB. As a result, the mean energy of the device with the MC model was found to be compatible with the 6.2 MeV spectrum and 1 mm FWHM. For validation of MC model, PDD and profile curves for 5 × 5, 10 × 10, 15 × 15, 20 × 20 and 30 × 30 cm2 field sizes with 6.2 MeV spectrum and 1 mm FWHM were compared with water phantom measurement results.

Validation of a 10 MV photon beam Elekta Synergy linear accelerator using the BEAMnrc MC code

Objective: The Monte Carlo (MC) technique can accurately model any linear accelerator where accurate details about the treatment head and the incident electron beam have been provided. However, manufacturers generally do not provide such details as the energy, radial intensity (spot size), or the angular spread of the incident electron beam. The aim of this study is to predict these details and validate a MC Linac model with the measurements.Methodology: A 10 MV photon beam from an Elekta Synergy Linac was modelled using the BEAMnrc code. The percentage depth dose of the MC model was generated using different electron energies and compared with the measurements using a Gamma index (γ) with two different criteria sets. The dose profiles of the fine-tuned electron energy were generated using different spot sizes and compared with appropriate measurements. In addition, the fine-tuned electron energy and spot size dose profiles were generated using different angular spreads to minimise any differences. Finally, the output factors for different field sizes, with and without a wedge, and also the quality index, were compared.Results: The fine-tuned electron energy of the MC model was found to be 9.8 MeV, where 94.12% of the calculation points pass the γ test. For the spot size, a circular radial intensity of 0.35 cm best described the 10 MV photon beam. Furthermore, a mean angular spread of 0.05° minimised the cross-field profile differences between calculation and measurement. The largest differences between the output factors from the MC model and measurements were -0.8% and 4.7% for an open and wedged field, respectively. Ultimately, a difference of 0.82% in the quality index was achieved.Conclusion: A reliable MC model of a 10 MV photon beam Elekta Synergy Linac can be achieved as presented in this study using the BEAMnrc code. This model can be reliably used to calculate dose distributions.

Pre-treatment and real-time image guidance for a fixed-beam radiotherapy system

Purpose. A radiotherapy system with a fixed treatment beam and a rotating patient positioning system could be smaller, more robust and more cost effective compared to conventional rotating gantry systems. However, patient rotation could cause anatomical deformation and compromise treatment delivery. In this work, we demonstrate an image-guided treatment workflow with a fixed beam prototype system that accounts for deformation during rotation to maintain dosimetric accuracy. Methods. The prototype system consists of an Elekta Synergy linac with the therapy beam orientated downward and a custom-built patient rotation system (PRS). A phantom that deforms with rotation was constructed and rotated within the PRS to quantify the performance of two image guidance techniques: motion compensated cone-beam CT (CBCT) for pre-treatment volumetric imaging and kilovoltage infraction monitoring (KIM) for real-time image guidance. The phantom was irradiated with a 3D conformal beam to evaluate the dosimetric accuracy of the workflow. Results. The motion compensated CBCT was used to verify pre-treatment position and the average calculated position was within −0.3 ± 1.1 mm of the phantom’s ground truth position at 0°. KIM tracked the position of the target in real-time as the phantom was rotated and the average calculated position was within −0.2 ± 0.8 mm of the phantom’s ground truth position. A 3D conformal treatment delivered on the prototype system with image guidance had a 3%/2 mm gamma pass rate of 96.3% compared to 98.6% delivered using a conventional rotating gantry linac. Conclusions. In this work, we have shown that image guidance can be used with fixed-beam treatment systems to measure and account for changes in target position in order to maintain dosimetric coverage during horizontal rotation. This treatment modality could provide a viable treatment option when there insufficient space for a conventional linear accelerator or where the cost is prohibitive.

Electron Beam Profile Assessment of Linear Accelerator Using Startrack Quality Assurance Device

Electron beam therapy using linear accelerator done for patients with superficial cancerous tumors. Daily quality assurance is preferable to assess a good treatment for the patients. This study focuses on penumbra, flatness, and symmetry determination for four types of an electron beam using StarTrack 2D array for quality assurance. Eelectron beam with energies: 6MeV, 9MeV, 12 MeV, and 15MeV for both in-plane (x-axis) and cross-plane (y-axis) using Elekta synergy linac exposed to StarTrack 2D array readings during 16 weeks for testing the performance and stability of StarTrack. The testing protocol used is IEC.The beam profile estimation of variation, when compared with standard values of penumbra, flatness, and symmetry of electron beam energy at the time of commissioning, reveals that all had a variation but these variations are within the limits. It is concluded that StarTrack 2D array detector is favourable in the QA process. Their implementation is not only easy, but gives information about the beam’s temporal stability and is particularly suitable for beam steering mounting.

Feasibility of using a single transmission factor for the Integral Quality Monitor ® on dynamic 15 MV photon beams

PurposeTo investigate the feasibility of using a single transmission factor (TF) to account for the attenuation of the iRT Systems Integral Quality Monitor (IQM) on dynamic 15 MV photon beams supported by Gafchromic EBT3 film measurements in a realistic pelvic prosthesis phantom. Materials and methodsIQM TFs for 2 × 2 to 20 × 20 cm2 15 MV photon beams produced by an Elekta Synergy linac were measured with a 0.6 cm3 cylindrical chamber inside RW3 phantom slabs. A Monaco treatment planning system (v. 5.00.00) that uses the X-ray voxel Monte Carlo algorithm was used to generate intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans in a computed tomography scanned-based model of the pelvic phantom. The phantom was irradiated according to the plans with and without the IQM mounted on the linac. In each case, the absorbed dose in the phantom was measured with EBT3 Gafchromic film. In a second step each plan was re-scaled to new monitor units (MUs) taking into account the IQM TF. The phantom was irradiated according to the plans that now have the re-scaled MUs. Again, film dosimetry was performed with the IQM mounted to the linac. Monaco dose values were compared with film measurements using percentage differences and gamma index criteria of 3%/3 mm. ResultsThe measured IQM TFs varied within 0.1% over the range of fields evaluated. For the treatment plans, inside the target area, dose differences ≥4% were found between Monaco and EBT3 film for plans with unscaled MUs with the IQM mounted. Plans delivered with unscaled MUs without the IQM as well as plans delivered with scaled MUs with the IQM showed dose differences within 2% between Monaco and film. In these cases, the corresponding gamma pass rate between Monaco and film data was ≥90%. ConclusionA single TF is sufficient to scale MUs of IMRT/VMAT plans so that the attenuation in IQM is taken into account for dynamic 15 MV photon beams. Dose differences between corrected plans and film measurements are then within 2%, as opposed to ≥4% when not taken into account.

Evaluation of the RUBY modular QA phantom for planar and non-coplanar VMAT and stereotactic radiations.

PurposeThis study evaluates the clinical use of the RUBY modular QA phantom for linac QA to validate the integrity of IGRT workflows including the congruence of machine isocenter, imaging isocenter, and room lasers. The results have been benchmarked against those obtained with widely used systems. Additionally, the RUBY phantom has been implemented to perform system QA (End‐to‐End testing) from imaging to radiation for IGRT‐based VMAT and stereotactic radiations at an Elekta Synergy linac.Material and MethodsThe daily check of IGRT workflow was performed using the RUBY phantom, the Penta‐Guide, and the STEEV phantom. Furthermore, Winston–Lutz tests was carried out with the RUBY phantom and a ball‐bearing phantom to determine the offsets and the diameters of the isospheres of gantry, collimator, and couch rotations, with respect to the room lasers and kV‐imaging isocenter. System QA was performed with the RUBY phantom and STEEV phantom for eight VMAT treatment plans. Additionally, the visibility of the embedded objects within these phantoms in the images and the results of CT and MR image fusions were evaluated.ResultsAll systems used for daily QA of IGRT workflows show comparable results. Calculated shifts based on CBCT imaging agree within 1 mm to the expected values. The results of the Winston–Lutz test based on kV imaging (2D planar and CBCT) or room lasers are consistent regardless of the system tested. The point dose values in the RUBY phantom agree to the expected values calculated using algorithms in Masterplan and Monte Carlo engine in Monaco within 3% of the clinical acceptance criteria.ConclusionAll the systems evaluated in this study yielded comparable results in terms of linac QA and system QA procedures. A system QA protocol has been derived using the RUBY phantom to check the IGRT‐based VMAT and stereotactic radiations workflow at an Elekta Synergy linac.

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets-Quantification and possible corrections.

PurposeTo assess the impact of isocenter shifts due to linac gantry and table rotation during cranial stereotactic radiosurgery on D98, target volume coverage (TVC), conformity (CI), and gradient index (GI).MethodsWinston‐Lutz (WL) checks were performed on two Elekta Synergy linacs. A stereotactic quality assurance (QA) plan was applied to the ArcCHECK phantom to assess the impact of isocenter shift corrections on Gamma pass rates. These corrections included gantry sag, distance of collimator and couch axes to the gantry axis, and distance between cone‐beam computed tomography (CBCT) isocenter and treatment beam (MV) isocenter. We applied the shifts via script to the treatment plan in Pinnacle 16.2. In a planning study, isocenter and mechanical rotation axis shifts of 0.25 to 2 mm were applied to stereotactic plans of spherical planning target volumes (PTVs) of various volumes. The shifts determined via WL measurements were applied to 16 patient plans with PTV sizes between 0.22 and 10.4 cm3.ResultsArcCHECK measurements of a stereotactic treatment showed significant increases in Gamma pass rate for all three measurements (up to 3.8 percentage points) after correction of measured isocenter deviations. For spherical targets of 1 cm3, CI was most severely affected by increasing the distance of the CBCT isocenter (1.22 to 1.62). Gradient index increased with an isocenter‐collimator axis distance of 1.5 mm (3.84 vs 4.62). D98 (normalized to reference) dropped to 0.85 (CBCT), 0.92 (table axis), 0.95 (collimator axis), and 0.98 (gantry sag), with similar but smaller changes for larger targets. Applying measured shifts to patient plans lead to relevant drops in D98 and TVC (7%) for targets below 2 cm3 treated on linac 1.ConclusionMechanical deviations during gantry, collimator, and table rotation may adversely affect the treatment of small stereotactic lesions. Adjustments of beam isocenters in the treatment planning system (TPS) can be used to both quantify their impact and for prospective correction of treatment plans.

Dosimetric Comparison between Monaco TPS and EGSnrc Monte Carlo simulation on Titanium Rod in 12bit and 16bit Image Format

This study purpose is to investigate the effects of using different bit depth DICOM images for the 6 MV photon beam on a titanium rod. An EGSnrc Monte Carlo simulation on the Elekta Synergy LINAC e...

Evaluation of the IAEA-TRS483 protocol for the dosimetry of small fields (square and stereotactic cones) using multiple detectors.

The IAEA TRS 483 protocol1 for the dosimetry of small static fields in radiotherapy was used to calculate output factors for the Elekta Synergy linac at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Small field output factors for both square and circular fields were measured using nine different detectors. The “corrected” output factors (ratio of detector readings multiplied by the appropriate correction factor from the protocol) showed better consistency compared to the “uncorrected” output factors (ratio of detector readings only), with the relative standard deviation decreasing by approximately 1% after the application of the relevant correction factors. Comparisons relative to an arbitrarily chosen PTW 60019 microDiamond detector showed a reduction of maximal variation for the corrected values of approximately 3%. A full uncertainty budget was prepared to analyze the consistency of the output factors. Agreement within uncertainties between all detectors and field sizes was found, except for the 15 mm circular field. The results of this study show that the application of IAEA TRS 4831 when measuring small fields will improve the consistency of small field measurements when using multiple detectors contained within the protocol.

Assessing the dependency of the uncertainties in the Elekta Agility MLC calibration procedure on the focal spot position.

The effectiveness of radiotherapy treatments depends on the accuracy of the dose delivery process. The majority of radiotherapy courses are delivered on linear accelerators with a Multi Leaf Collimator (MLC) in 3D conformal Radiation Therapy, Intensity Modulated Radiation Therapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT) modes that require accurate MLC positioning. This study investigates the MLC calibration accuracy, following manufacturer procedures for an Elekta Synergy linac with the Agility head, against the radiation focal spot offset (alignment with the collimator axis of rotation). If the radiation focal spot is not aligned ideally with the collimator axis of rotation then a systematic error can be introduced into the calibration procedure affecting absolute MLC leaf positions. Calibration of diaphrams is equally affected; however they are not investigated here. The results indicate that an estimated 0.15 mm MLC uncertainty in all MLC leaves positions can be introduced due to uncertainty of the radiation focal spot positionof 0.21 mm.

Organ sparing of linac-based targeted marrow irradiation over total body irradiation.

PurposeTargeted marrow irradiation (TMI) is an alternative conditioning regimen to total body irradiation (TBI) before bone marrow transplantation in hematologic malignancies. Intensity‐modulation methods of external beam radiation therapy are intended to permit significant organ sparing while maintaining adequate target coverage, improving the therapeutic ratio. This study directly compares the dose distributions to targets and organs at risk from TMI and TBI, both modalities conducted by general‐use medical linacs at our institution.MethodsTMI treatments were planned for 10 patients using multi‐isocentric feathered volumetric arc therapy (VMAT) plans, delivered by 6 MV photon beams of Elekta Synergy linacs. The computed tomography (CT) datasets used to obtain these plans were also used to generate dose distributions of TBI treatments given in the AP/PA extended‐field method. We compared dose distributions normalized to the same prescription for both plan types. The generalized equivalent uniform dose (gEUD) of Niemierko for organs and target volumes was used to quantify effective whole structure dose and dose savings.ResultsFor the clinical target volume (CTV), no significant differences were found in mean dose or gEUD, although the radical dose homogeneity index (minimum dose divided by maximum dose) was 31.7% lower (P = 0.002) and the standard deviation of dose was 28.0% greater (P = 0.027) in the TMI plans than in the TBI plans. For the TMI plans, gEUD to the lungs, brain, kidneys, and liver was significantly lower (P < 0.001) by 47.8%, 33.3%, 55.4%, and 51.0%, respectively.ConclusionTMI is capable of maintaining CTV coverage as compared to that achieved in TBI, while significantly sparing organs at risk. Improvement on sparing organs at risk permits a higher prescribed dose to the target or the maximum number of times marrow conditioning may be delivered to a patient while maintaining similar typical tissue complication rates.