Elekta Versa HD Linac Research Articles

HDR brachytherapy versus robotic-based and linac-based stereotactic ablative body radiotherapy in the treatment of liver metastases – A dosimetric comparison study of three radioablative techniques

PurposeThe aim of our study was to compare dosimetric aspects of three radioablation modalities – direct high-dose-rate brachytherapy (HDR-BT) and virtually planned stereotactic body radiation therapy performed on CyberKnife (SBRTck) and Elekta Versa HD LINAC (SBRTe) applied in patients with liver metastases. Material and methodsWe selected 30 patients with liver metastases, who received liver interstitial HDR-BT and virtually prepared plans for SBRTck and SBRTe. In all the cases, the prescribed dose was a single fraction of 25 Gy. Treatment delivery time, doses delivered to PTV and organs at risk, as well as conformity indices, were calculated and compared. ResultsThe longest median treatment delivery time was observed in SBRTck in contrast to HDR-BT and SBRTe which were significantly shorter and comparable. HDR-BT plans achieved better coverage of PTV (except for D98%) in contrast to SBRT modalities. Between both SBRT modalities, SBRTck plans resulted in better dose coverage in Dmean, D50%, and D90% values compared to SBRTe without difference in D98%. The SBRTe was the most advantageous considering the PCI and R100%. SBRTck plans achieved the best HI, while R50% value was comparable between SBRTe and SBRTck. The lowest median doses delivered to uninvolved liver volume (V5Gy, V9.1Gy) were achieved with HDR-BT, while the difference between SBRT modalities was insignificant. SBRT plans were better regarding more favourable dose distribution in the duodenum and right kidney, while HDR-BT achieved lower doses in the stomach, heart, great vessels, ribs, skin and spinal cord. There were no significant differences in bowel and biliary tract dose distribution between all selected modalities. ConclusionsHDR-BT resulted in more favourable dose distribution within PTVs and lower doses in organs at risk, which suggests that this treatment modality could be regarded as an alternative to other local ablative therapies in carefully selected patients’ with liver malignancies. Future studies should further address the issue of comparing treatment modalities in different liver locations and clinical scenarios.

Comparison between Volumetric Modulated arc Therapy based Coplanar and Noncoplanar Planning for Stereotactic Body Radiation Therapy of Liver

The study aims to investigate potential dosimetric benefits between non-coplanar and coplanar beam arrangements of Volumetric-Modulated Arc Therapy (VMAT) plans for liver stereotactic body radiotherapy (SBRT). Thirteen patients who had undergone liver SBRT treatment in our department were chosen retrospectively for the study. Two sets of SBRT-VMAT plans namely, non-coplanar (NC-VMAT) and Coplanar (C-VMAT) were generated in Monaco(v5.11) planning system for Elekta Versa HD Linac using unflatten 6MV photon. The NC-VMAT plans were created by two/three non-coplanar partial arcs with couch rotation of ±150 and had an arc span of 1300 to 1600 whereas the C-VMAT plans consisted of a full arc. Both plans were compared by statistically analyzing various dosimetric and technical parameters. There is no statistically significant difference observed between the C-VMAT and NC-VMAT plans for planning target volume (PTV) coverage. However, the spine dose (D1cc) was much less in the NC-VMAT plan compared to the C-VMAT plan, with mean values of 6.127 ± 3.08Gy and 9.058 ± 4.76Gy, respectively (p-value=0.002). The low dose spillage to the healthy tissue was compared by the volume receiving 5Gy (V5Gy) and 10Gy (V10Gy). V5Gy of the NC-VMAT plan was 2399.23±1870.76cc while that of C-VMAT plans was 2835.36±1930.20cc with the p-value <0.001. Moreover, the monitor units(MU) were less with NC-VMAT than with C-VMAT SBRT plans (p=0.015). The plan quality of NC-VMAT plans was favorable compared to C-VMAT plans for liver SBRT especially in reducing spine dose, low dose spillage to healthy tissue, and MU.

Precise dosimetric comparison between GAMOS and the collapsed cone convolution algorithm of 4D DOSE accumulated in lung SBRT treatments

BackgroundIt is widely accepted that Monte Carlo dose calculations offers a higher precision that the commercially available dose calculation algorithms. This advantage may be especially relevant for lung Stereotactic Body Radiation Therapy (SBRT), as this is a precise technique applied to an area of big inhomogeneity. PurposeWe conducted a comparative study to reveal the differences between the doses calculated using the Collapsed Cone Convolution algorithm and the GAMOS/Geant4 Monte Carlo calculation for lung cancer patients treated with Stereotactic Body Radiation Therapy on an Elekta Versa HD linac. MethodsFor this study a set of ten patient treatments carried out at the Clínica Universidad de Navarra was selected. Theanalysis is based on the comparison of several dosimetric quantities for the Gross Tumor Volume (GTV) and several OrgansAt Risk (OARs), and also a gamma index calculation with distance-to-agreement set to 2 mm and dose difference to 3%, as recommended by ICRU to assess clinical impact. In order to guarantee a small uncertainty in the Monte Carlo calculation of the dosimetric quantities, we studied in detail the validity of different methods that may be used to determine this uncertainty.To compensate for lung movements, a 4D-Cone-beam Computed Tomography (CBCT) was acquired before treatment, whichallowed us to identify eight respiratory phases using a temporal binning. Using commercial MIM software®, we performed a deformable image registration between the eight CT respiration phases to construct the 4D doses. The same procedure was applied for the Treatment Planning System (TPS) dose files and for the Monte Carlo dose files. ResultsThe differences between the two algorithms reveal the known weaknesses of the Collapsed Cone Convolution (CCC) algorithm for the calculation of lateral doses and in regions of large density change. The comparison between the two algorithms for individual phase doses shows differences up to 5% of the GTV D95 or 3–4 Gy in some OARs, which may have a clinical impact. Nevertheless these differences are reduced for the 4D dose in most quantities under study. ConclusionsComparing the dose calculated with a Collapsed Cone Convolution algorithm with GAMOS/Geant4 for ten patients and eight respiratory phases, we found some differences that could have a clinical impact. When combining the eight temporal phases into a 4D dose using the MIM Deformable Image Registration software, the differences diminished substantially.Our statistical analysis concludes that dose uncertainty in the voxels with a maximum dose below a given percentage guarantees uncertainty in the dosimetric quantities below that figure.

Effects of reconstruction methods on dose distribution for lung stereotactic body radiotherapy treatment plans.

The aim of the present study was to investigate the effect of tumour motion on various imaging strategies as well as on treatment plan accuracy for lung stereotactic body radiotherapy treatment (SBRT) cases. The ExacTrac gating phantom and paraffin were used to investigate respiratory motion and represent a lung tumour, respectively. Four-dimensional computed tomography (4DCT) imaging was performed, while the phantom was moving sinusoidally with 4s cycling time with three different amplitudes of 8, 16, and 24mm. Reconstructions were done with maximum (MIP) and average intensity projection (AIP) methods. Comparisons of target density and volume were performed using two reconstruction techniques and references values. Volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) were planned based on reconstructed computed tomography (CT) sets, and it was examined how density variations affect the dose-volume histogram (DVH) parameters. 4D cone beam computed tomography (CBCT) was performed with the Elekta Versa HD linac imaging system before irradiation and compared with 3D CBCT. Thus, various combinations of 4DCT reconstruction methods and treatment alignment methods have been investigated. Point measurements as well as 2 and 3D dose measurements were done by optically stimulated luminescence (OSL), gafchromic films, and electronic portal imaging devices (EPIDs), respectively. The mean volume reduction was 7.8% for the AIP and 2.6% for the MIP method. The obtained Hounsfield Unit (HU) values were lower for AIP and higher for MIP when compared with the reference volume density. In DVH analysis, there were no statistical differences for D95%, D98%, and Dmean (p > 0.05). However, D2% was significantly affected by HU changes (p < 0.01). A positional variation was obtained up to 2mm in moving direction when 4D CBCT was applied after 3D CBCT. Dosimetric measurements showed that the main part of the observed dose deviation was due to movement. In lung SBRT treatment plans, D2% doses differ significantly according to the reconstruction method. Additionally, it has been observed that setups based on 3D imaging can cause a positional error of up to 2mm compared to setups based on 4D imaging. It is concluded that MIP has advantages over AIP in defining internal target volume (ITV) in lung SBRT applications. In addition, 4D CBCT and 3D EPID dosimetry are recommended for lung SBRT treatments.

Developing a new Monte Carlo algorithm as an alternative tool to simulate virtual source model on an Elekta Versa HD Linac

The aim of this study is to develop in-house Monte Carlo (MC) algorithm as an alternative tool to simulate all mechanisms of interactions for dose calculation in radiation therapy. Additionally, evaluating the MC-SCAN algorithm with beam modeling of Elekta VersaHD Linac was aimed. Mohan 6 MV photon spectrum was used and the source was modeled as a point source. The absorbed dose is calculated via modeling all interaction mechanisms to gain energy deposition. TPR20/10, PDD, photon profile of different field sizes as well as point dose was calculated after that, a comparison of all the achieved results with experimental measurements went through. The difference between calculated mean energy and theoretical one was obtained <1%. The calculated TPR20/10 value was (0.685 ± 0.03). The difference between experimental PDD and photon profile, and the calculated ones by new model was <3% for all field sizes. A new MC algorithm can be used as an alternative tool to simulate virtual source model on an Elekta VersaHD Linac. All calculated values turned out to present the difference between experimental measurements to be <3% for homogeneous mediums. Nonetheless, further research was needed to be done for the better improvement.

A novel bone suppression algorithm in intensity-based 2D/3D image registration for real-time tumor motion monitoring: Development and phantom-based validation.

BackgroundReal‐time tumor motion monitoring (TMM) is a crucial process for intra‐fractional respiration management in lung cancer radiotherapy. Since the tumor can be partly or fully located behind the ribs, the TMM is challenging.PurposeThe aim of this work was to develop a bone suppression (BS) algorithm designed for real‐time 2D/3D marker‐less TMM to increase the visibility of the tumor when overlapping with bony structures and consequently to improve the accuracy of TMM.MethodA BS method was implemented in the in‐house developed software for ultrafast intensity‐based 2D/3D tumor registration (Fast Image‐based Registration [FIRE]). The method operates on both, digitally reconstructed radiograph (DRR) and intra‐fractional X‐ray images. The bony structures are derived from computed tomography data by thresholding during ray‐casting, and the resulting bone DRR is subtracted from intra‐fractional X‐ray images to obtain a soft‐tissue‐only image for subsequent tumor registration. The accuracy of TMM utilizing BS was evaluated within a retrospective phantom study with nine different 3D‐printed tumor phantoms placed in the in‐house developed Advanced Radiation DOSimetry (ARDOS) breathing phantom. A 24 mm craniocaudal tumor motion, including rib eclipses, was simulated, and X‐ray images were acquired on the Elekta Versa HD Linac in the lateral and posterior–anterior directions. An error assessment for BS images was evaluated with respect to the ground truth tumor position.ResultsA total error (root mean square error) of 0.87 ± 0.23 mm and 1.03 ± 0.26 mm was found for posterior–anterior and lateral imaging; the mean time for BS was 8.03 ± 1.54 ms. Without utilizing BS, TMM failed in all X‐ray images since the registration algorithm focused on the rib position due to the predominant intensity of this tissue within DRR and X‐ray images.ConclusionThe BS algorithm developed and implemented improved the accuracy, robustness, and stability of real‐time TMM in lung cancer in a phantom study, even in the case of rib interlude where normal tumor registration fails.

Beam modeling and commissioning for Monte Carlo photon beam on an Elekta Versa HD LINAC

PurposeThis study aims at analyzing beam data commissioning along with verifying Monte Carlo-based treatment planning system on the basis of the manufacturer guidelines for Elekta Versa HD Linear Accelerator. Moreover, evaluating the beam match process in terms of quality index, photon profile and multi leaf collimator (MLC) offset is aimed as well. Materials and methodsThe process of collecting beam data for Monaco 5.51 Treatment Planning System commissioning was done based on the instructions provided by the manufacturer as well as AAPM TG-106. Monte Carlo analysis was done for output factors in water, percent depth dose and beam profiles. A set of eight static and intensity modulated radiation therapy fields were used to verify the MLC parameters. ResultsThe difference between the measured and modeled penetrative quality (D10) was achieved to be 0.54%. The output factors for 6 MV photon energy were measured and the difference between the measured and Monte Carlo output results was smaller than 1% for all the fields. The average percentage of passing the gamma criteria for commissioning test fields was (95+-4)%, however, the minimum agreement was 87.5% belonging to “7SEGA”. Additionally, the agreement between both LINAC is 96%, however, the second LINAC reveals a positive offset in the point of approximately −4 cm on the x-axis at the crossplane. ConclusionTest commissioning was successfully verified using a homogeneous phantom for point dose measurements, post modelling MLC parameters and patient QA plans. All plan parameters pass the gamma criteria. 6 MV photon beam was successfully commissioned for Elekta VersaHD LINAC and is ready for clinical implementation.

Experimental determination of thermal neutron fluence around Elekta Versa HD linear accelerator for various photon energies

A complex neutron spectrum generated along with a useful photon beam imposes an additional radiation protection risk around medical linear accelerators (linac). The thermal neutron component of this complex neutron spectrum formed during different photon modes of operation of Elekta Versa HD linac has been quantified using Indium foil activation technique. The thermal neutron fluence (Φ th ) at isocenter for 15 MV, 10 MV and 10 MV FFF beams was found to be 2.45 × 105, 4.35 × 104 and 3.2 × 104 neutrons cm−2 Gy−1, respectively. The analysis shows a reduction in the Φ th as the flattening filter is being taken out from the beam path. A negative correlation in Φ th with respect to field size has been observed with an average 18% reduction in Φ th per monitor units as field size changes from 10 cm × 10 cm to 40 cm × 40 cm. For particular field size and photon energy, Φ th was found to be uniform across the patient plane. From the measured gamma ray spectrum inside the treatment room six major isotopes have been identified which were 122Sb, 187W, 82Br, 56Mn, 24Na and 28Al.

Assessment of an Elekta Versa HD linear accelerator for stereotactic radiosurgery with circular cone collimators.

Versa HD linear accelerators (linacs) are used for stereotactic radiosurgery treatment. However, the mechanical accuracy of such systems remains a concern. The purpose of this study was to evaluate the accuracy of an Elekta Versa HD linac. We performed measurements with a ball bearing phantom to calculate the rotational isocenter radii of the linac's gantry, collimator, and table, and determine the relative locations of those isocenters. We evaluated the accuracy of the cone-beam computed tomography (CBCT) guidance with a film-embedding head phantom and circular cone-collimated radiation beams. We also performed dosimetric simulations to study the effects of the linac mechanical uncertainties on non-coplanar cone arc delivery. The mechanical uncertainty of the linac gantry rotation was 0.78 mm in radius, whereas that of the collimator and the table was <0.1 mm and 0.33 mm, respectively. The axes of rotation of the collimator and the table were coinciding with and 0.13 mm away from the gantry isocenter, respectively. Experiments with test plans demonstrated the limited dosimetric consequences on the circular arc delivery given the aforementioned mechanical uncertainties. End-to-end measurements determined that the uncertainty of the CBCT guidance was≤1 mm in each direction with respect to the reference CT image. In arc delivery, the mechanical uncertainties associated with the gantry and the table do not require remarkable increases in geometric margins. If large enough, the residual setup errors following CBCT guidance will dominate the overall dosimetric consequence. Therefore, the Versa HD linac is a valid system for stereotactic radiosurgery using non-coplanar arc delivery.

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams.

PurposeThe goal of this work is to provide a large and consistent set of data for detector‐specific output correction factors, kQclin,Qreffclin,fref, for small static fields for seven solid‐state detectors and to determine field output factors, ΩQclin,Qreffclin,fref, using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS‐483 for small‐field dosimetry were followed throughout the study.Methods ΩQclin,Qreffclin,fref were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm2 to 10 × 10 cm2. Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two‐dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. kQclin,Qreffclin,fref were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector.ResultsField output factors and detector‐specific kQclin,Qreffclin,fref are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small‐field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for kQclin,Qreffclin,fref are found to differ significantly (P < 0.05) from the corresponding data published in TRS‐483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between kQclin,Qreffclin,fref values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm2 and 0.8 × 0.8 cm2. Significant differences in kQclin,Qreffclin,fref between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in kQclin,Qreffclin,fref are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant.ConclusionsSelection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of ΩQclin,Qreffclin,fref for small static fields in megavoltage photon beams. Large set of kQclin,Qreffclin,fref data for seven solid‐state detectors and four beam energies determined on two linacs by a single group of researchers can be considered a valuable supplement to the literature and the TRS‐483 dataset.

Adaptation of NCS report 24 for flattening filter free VMAT quality assurance

Volumetric Arc radiotherapy (VMAT) was introduced nearly 10 years ago and is an advancement on the technology of Intensity-Modulated Radiotherapy (IMRT). The main difference between VMAT & IMRT are the increased number of degrees of freedom during patient treatment including gantry rotation, dose rate variation, multi-leaf collimator (MLC) movement and in some cases collimator movement1. Because of the aforementioned differences between IMRT and VMAT, the established IMRT Quality Assurance (QA) procedures alone are not sufficient for VMAT QA. To address this shortfall, in 2015 the Nederlandse Commissie voor Stralingsdosimetrie (NCS) published report 24 which is the first fully comprehensive guide of its kind aimed explicitly at VMAT QA and therefore has the potential to be utilised to improve the quality assurance of VMAT. The aim of this work was to integrate machine QA aspects of NCS report 24 as part of Galway Clinic’s VMAT QA process & furthermore investigate if this can be adapted to flattening filter free (FFF) beams. QA tests outlined by NCS 24 were performed on an Elekta Versa HD linac. The methods, equipment required to conduct the test set out by NCS 24 was investigated. Analysis of the test results was performed to the standards set by NCS 24 and compared to the standards of TG 142, which is the standard currently employed by the Galway Clinic. The tests set out by NCS 24 were also translated to FFF beam energies using additional sources of information.

Dosimetric characterization of Elekta stereotactic cones

PurposeDosimetry of small fields defined by stereotactic cones remains a challenging task. In this work, we report the results of commissioning measurements for the new Elekta stereotactic conical collimator system attached to the Elekta VersaHD linac and present the comparison between the measured and Monte Carlo (MC) calculated data for the 6 MV FFF beam. In addition, relative output factor (ROF) dependence on the stereotactic cone aperture variation was studied and penumbra comparison for small MLC‐based and cone‐based fields was performed.MethodsCones with nominal diameters of 15 mm, 12.5 mm, 10 mm, 7.5 mm, and 5 mm were employed in our study. Percentage depth dose (PDD), off‐axis ratios (OAR), and ROF were measured using a stereotactic field diode (SFD). BEAMnrc code was used for MC simulations.ResultsMC calculated and measured PDDs for all cones agreed within 1%/0.5 mm, and OAR profiles agreed within 1%/0.5 mm. ROF obtained from the measurements and MC calculations agreed within 2% for all cone sizes. Small‐field correction factors for the SFD detector Kfield,3 × 3(SFD) were derived using MC calculations as a baseline and were found to be 0.982, 0.992, 0.997, 1.015, and 1.017 for the 5, 7.5, 10, 12.5, and 15‐mm cones respectively. The difference in ROF was about 10%, 6%, 3.5%, 3%, 2.5%, and 2% for ±0.3 mm variations in 5, 7.5, 10, 12.5, and 15‐mm cone aperture respectively. In case of single static field, cone‐based collimation produced a sharper penumbra compared to the MLC‐based.ConclusionsAccurate MC simulation can be an effective tool for verification of dosimetric measurements of small fields. Due to the very high sensitivity of output factors on the cone diameter, manufacture‐related variations in cone size may lead to considerable variations in dosimetric characteristics of stereotactic cones.

Abstract ID: 77 Monte Carlo simulation of the Elekta VersaHD linac with Elekta’s Stereotactic Conical Collimation System

Purpose Small field dosimetry remains a challenging task. The difficulties associated with small fields measurements become more pronounced for field sizes less than 1 cm. To overcome these difficulties it has been suggested to employ Monte Carlo (MC) techniques. The purpose of this work was to report the results of commissioning and MC validation of the new Elekta stereotactic conical collimator system for 6 MV flattening filter free (FFF) beams on Elekta VersaHD linac. Methods Commissioning measurements for stereotactic cones with diameters of 5 mm, 7.5 mm, 10 mm, 12.5 mm and 15 mm included relative output factor (ROF), percentage depth dose (PDD) and off-axis ratio (OAR) measurements and were performed with SRS diode. MC simulations were performed using EGSnrc. The incident electron beam parameters were adjusted to match the measured data (PDD and OAR) for set of MLC-based open fields. For each cone, aperture size was adjusted to match MC calculated and measured OAR profiles. Penumbra width (defined as 20%–80% distance) comparison of MLC-based and cone-based fields for a static angle case and for a full arc was performed. Angular distribution of photons collimated with MLC and with the cones was calculated. Leakage for the MLC and for the cones was determined. Results For MLC-based fields, agreement between calculated and measured values was within 1%/0.5 mm for PDDs, within 1%/1 mm for OARs, and within 3% for ROFs. For stereotactic cones, the agreement in PDD and OAR profiles was within 1%/0.5 mm and the ROFs agreed within 3%. The measured ROFs were 0.564, 0.645, 0.706, 0740 and 0.770 for the 5 mm, 7.5 mm, 10 mm, 12.5 mm and 15 mm cones, respectively. For one static MLC-based 1 × 1 cm field, penumbra was 5.6 mm in the in-plane direction and 8.4 mm in the cross-plane direction, compared to 4 mm in both directions for the 10 mm cone. The cone leakage was found to be 0.5% for both energies. Conclusions Results of our MC calculations were found to be in good agreement with the measurements thus confirming accuracy of our MC model for the Agility MLC.

Accounting for the ion recombination factor in relative dosimetry of flattening filter free photon radiation

Flattening filter free (FFF) linear accelerators (linacs) are increasingly being used to deliver external beam radiotherapy. Advantages of FFF include lower out-of-field dose to the patient due to reduced head scatter, and shorter treatment times due to increased dose rate. The ion recombination factor (kion) is known to increase linearly with dose per pulse in pulsed photon beams. This variation is neglected when performing relative dosimetry in flattened beams, however could be significant in the higher dose rates encountered with FFF. kion has been derived from theory and measured using Jaffe plots and two voltage analysis for three ionisation chambers at a range of depths and points off-axis in 6 and 10 MV FFF beams produced by an Elekta Versa HD linac. The chambers under investigation were the PTW 30013 ‘Farmer’ (0.60 cc), the Exradin A1SL (0.053 cc) and the IBA CC04 (0.04 cc). The effect of neglecting ion recombination corrections in commissioning data was quantified using a 1D gamma analysis (1%/2 mm) on depth dose curves and beam profiles. The largest variation in kion was for a Farmer chamber in a 10 MV FFF beam, where kion varied by up to 1.2% ± 0.15% (95% CL) between different points in a water tank. The Farmer variation with depth was comparable to that measured by Kry et al (2012 J. Appl. Clin. Med. Phys. 13 318–325). Other chambers exhibited negligible changes in kion. It was concluded that neglecting ion recombination in relative measurements at 6 MV FFF would have a negligible effect on dosimetric accuracy. Furthermore kion could be safely neglected when acquiring relative data with small volume chambers for commissioning 10 MV FFF, although may need to be accounted for if relative data are acquired with a Farmer chamber and when routine energy checks are performed.

Dose-to-medium vs. dose-to-water: Dosimetric evaluation of head and neck VMAT cases using Monaco treatment planning system

Purpose: In this paper, we evaluate the dosimetric differences between absorbed dose to water and absorbed dose to medium in Monte Carlo (MC)-based calculations used for radiation therapy treatment plans. Methods: Thirty-four treated Head and Neck simultaneously integrated boost cases were analyzed retrospectively. All of them were planned by Monaco treatment planning system (TPS), calculated and reviewed on absorbed dose to medium (D m ) calculations and treated in Elekta Versa HD LINAC. Absorbed dose to medium D m was converted to absorbed dose to water D w in Monaco treatment planning system using the procedure based on stopping power ratios and the Bragg-Gray cavity theory. Dosimetric parameters were then compared and analyzed with respect to absorbed dose to medium (D m ) calculations for multiple planning target volumes (PTVs) and critical organs such as brainstem, spinal cord, left and right lens, left and right parotids, larynx, left and right middle ear and lips. Results: It was found that mean and minimum D w (i.e. D w mean and D w min ) of organs at risk did not differ much (hardly differing by 0.8-2%) with respect to those of the absorbed dose to medium. However maximum D w (i.e. D w max ) in case of lips, left and right middle ear were found to differ more than 4% with respect to D m max . For serial organs brainstem and spinal cord, maximum dose D w max were found to vary around 1% and 2%, respectively, with respect to absorbed dose to medium dose calculation. In case of PTVs, the mean percentages variation of D w min and D w mean were found to be less than 1 %, although the variation of maximum D w was found to be high around 5-7% with respect to that of D m . Conclusion: The comparative analysis of dosimetric parameters in the present study shows that the selection of either D m or D w in Monaco planning system is less likely to produce any significant clinical effect in tumor control and to the damage of organs at risk.

TH-CD-BRA-12: Impact of a Magnetic Field On the Response From a Plastic Scintillation Detector

Purpose: To study the effect of a strong magnetic field on the scintillation and the stem effect from a plastic scintillation detector (PSD) and evaluate its accuracy to measure dose. Methods: A plastic scintillation detector and a bare plastic fiber were placed inside a magnet of adjustable field strength (B=0−1.5T) and irradiated by a 6-MV photon beam (Elekta Versa HD LINAC). The PSD was built in-house using a scintillating fiber (BCF-60, 3-mm long × 1-mm diameter) coupled to an optical fiber similar to the bare fiber (PMMA, 12-m long, 1-mm diameter). Light output spectra were acquired with a spectrometer. Intensity and shape of the output spectra were compared as a function of the magnetic field strength. The bare fiber was used to study the behavior of the stem effect (composed of Cerenkov and fluorescence). The spectrometry setup allowed to perform a previously demonstrated hyperspectral stem-effect removal and calculated dose was studied as a function of the magnetic field strength. Results: Signal intensities were shown to increase with the magnetic field strength by up to 19% and 79% at 1.5T in comparison to the irradiation without a magnetic field, for respectively the PSD and the bare fiber. The light produced by Cerenkov effect in the optical fiber was shown to be the major component affected by the magnetic field. Effect of the magnetic field on the electrons trajectory may explain this behavior. Finally, accounting for the stem effect using the hyperspectral approach led to accuracy in dose measurement within 2.6%. Interestingly, variations in accuracy were negligible for values over 0.3T. Conclusion: Dependence of PSDs to magnetic field is mainly due to the Cerenkov light. When accounting for it, PSDs become a candidate of choice for both quality assurance and in vivo dosimetry of therapy under strong magnetic fields (e.g. for MRI-Linacs).

SU‐F‐J‐50: Study On the Magnetic Field Effect On the Exradin W1 Plastic Scintillation Detector

Purpose:To study the response of the Exradin W1 plastic scintillator detector to a 6 MV photon field in the presence of a strong magnetic field (B).Methods:An Exradin W1 scintillator detector coupled to a SuperMax two‐channel electrometer, both manufactured by Standard Imaging, Inc., was first calibrated in a Co‐60 beam. The Cerenkov Light Ratio (CLR) was obtained following the procedure recommended by the manufacturer. Subtracting signal in channel 2 multiplied by CLR from the signal in channel 1 should lead to a Cerenkov‐free signal. The W1 scintillator was placed in a plastic phantom inside a dipole electromagnet (GMW Associates) that could produce a strong B field, and irradiated using a 6 MV beam from an Elekta Versa HD LINAC. Signals from both channels of the W1 scintillator were acquired as a function of B (0 ‐ 1.5 T).Results:The signals from both channels increased as a function of the B field strength. At 1.5 T, channel 1 increased by 11% compared to the baseline (B=0 T), while channel 2 increased by 22%. Applying the recommended Cerenkov correction led to a 2% difference between dose measurement with and without a magnetic field. The values between B=0.3 T and B=1.5 T remained constant.Conclusion:Signals from the Exradin W1 plastic scintillation detector increased as the B field increased. This increase mainly comes from a change in the amount of Cerenkov light coupled within the optical fiber. Removing the Cerenkov component following the procedure recommended by the manufacturer showed to be an effective way to measure dose accurately in strong magnetic fields. The cause for the residual 2% difference is currently under investigation.We acknowledge research support from Elekta AB.

TH-CD-BRA-04: Effect of a Strong Magnetic Field On TLDs, OSLDs, and Gafchromic Films Using An Electromagnet

Purpose: To study the effect of strong magnetic field on three types of dosimeters using an electromagnet inside a Linac vault. Materials and Methods: Three types of dosimeters, thermoluminescent Dosimeters (TLDs), optically stimulated luminescent Dosimeters (OSLDs), and EBT3 Film were used to measure radiation dose response inside an electromagnet that could produce a strong magnetic field (B>1.5 T). The dosimeters were placed inside a plastic phantom between the two poles of the magnet, at approximately 3 meters from the iso-center of an Elekta Versa HD Linac. The B field was calibrated with a Gauss meter (Model: GM-2, AlphaLab Inc). The dosimeters received ∼2 Gy with and without the presence of the 1.5 T magnetic field. The EBT3 films were scanned 24 hours before and 24 hours after irradiation. The TLD dosimeters were read 1 week after irradiation. The OSLDs were read two weeks after irradiation. The ratios of signals of dosimeters irradiated with the B field to the signals without the B field were calculated. Two experiments have been conducted so far. Results: The ratios (averaged over two experiments) of dosimeter signals with vs without B field were 0.994 for films, 0.994 for OSLDs, and 1.002 for TLDs. The statistical uncertainty was ∼3%. Conclusions: The three types of dosimeters (film, TLD, OSLD) seem not affected by the presence of a magnetic field (B=1.5 T) with the uncertainty of ∼3%. They may be suitable for QA purposes in a strong B field up to 1.5 T. More measurements will be conducted for reproducibility testing. We acknowledge research support from Elekta AB.

Dosimetric properties of equivalent-quality flattening filter-free (FFF) and flattened photon beams of Versa HD linear accelerator.

This study presents the basic dosimetric properties of photon beams of a Versa HD linear accelerator (linac), which is capable of delivering flattening filter‐free (FFF) beams with a beam quality equivalent to the corresponding flattened beams based on comprehensive beam data measurement. The analyzed data included the PDDs, profiles, penumbra, out‐of‐field doses, surface doses, output factors, head and phantom scatter factors, and MLC transmissions for both FFF and flattened beams of 6 MV and 10 MV energy from an Elekta Versa HD linac. The 6MVFFF and 10MVFFF beams had an equivalent mean energy to the flattened beams and showed less PDD variations with the field sizes. Compared with their corresponding flattened beams, Dmax was deeper for FFF beams for all field sizes; the ionization ratio variations with the field size were lower for FFF beams; the out‐of‐field doses were lower and the penumbras were sharper for the FFF beams; the off‐axis profile variations with the depths were lesser for the FFF beams. Further, the 6MVFFF and 10MVFFF beams had 35.7% and 40.9% less variations in output factor with the field size, respectively. The collimator exchange effect was reduced in the FFF mode. The head scatter factor showed 59.1% and 73.6% less variations, on average, for the 6MVFFF and 10MVFFF beams, respectively; the variations in the phantom scatter factor were also smaller. The surface doses for all beams increased linearly with the field size. The 6MVFFF and 10MVFFF beams had higher surface doses than the corresponding flattened beams for field sizes of up to 10×10cm2 but had lower surface doses for larger fields. Both FFF beams had lower average MLC transmissions than the flattened beams. The finding that the FFF beams were of equivalent quality to the corresponding flattened beams indicates a significant difference from the data on unmatched FFF beams.PACS number(s): 87.56.bd, 87.55.Qr

Commissioning an Elekta Versa HD linear accelerator.

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