Eclipse AAA Research Articles

Knowledge Based Planning for Lung SBRT: Model Transferability between Treatment Systems and Calculation Algorithms

Knowledge based planning (KBP) is known to be effective and clinically feasible in achieving high quality treatment plans with less variability between planners and reduced planning time. A KBP model is highly dependent on characteristics of the training data. Our institution developed and validated a lung SBRT KBP model from plans treated with a C-arm linear accelerator (Linac). This study aims to evaluate the quality of lung SBRT plans with a KBP model when this model is used for an O-ring Linac with different MLC design, beam quality, and calculation algorithm. Two academic hospitals in the same healthcare network were included in this study. One hospital trained a KBP model for five-fraction lung SBRT using 43 plans calculated using the Eclipse AcurosXB v15.6 algorithm for a C-arm Linac. The model was developed to prioritize dose gradient outside the PTV while satisfying institutional critical organ limits and validated using an additional set of 12 plans. A cohort of 10 patients (11 plans) previously treated with lung SBRT using the O-ring Linac at the second hospital was selected. These plans were calculated using the Eclipse AAA v15.6 algorithm with prescription doses of 30-50 Gy in five fractions. Each plan was re-optimized using the KBP model, without manual adjustment of the optimization objectives, using the same beam geometry as in the clinical plans. KBP plans were normalized to achieve the same PTV coverage as the clinical plans. Clinical and KBP plans were evaluated for ITV D98%, PTV conformity index (CI), R50 and maximum dose at 2 cm from the PTV (D2cm). All clinical and KBP plans met critical organ dose constraints. The range of CI was 1-1.13 vs. 0.96-1.05 for clinical plans and KBP plans, respectively. R50 and D2cm were lower with KBP plans (p = 0.007 and p = 0.05, respectively). ITV D98% range was 105-120% vs. 117-122% of the PTV prescription dose for clinical plans and KBP plans, respectively, and higher with KBP plans (p<0.001). For plans where the PTV overlaps the chest wall (n = 7), the maximum chest wall doses (D0.03cc) from KBP plans were higher than the clinical plans (p = 0.16), with an absolute chest wall D0.03cc difference of 9 Gy (54 Gy vs 63 Gy). The KBP model includes an ITV SIB of 120% of the PTV prescription, whereas the clinical plans reduced the ITV SIB when the PTV overlapped the chest wall, highlighting a difference in clinical practice between the two hospitals in this study. This study supports that a lung SBRT KBP model trained using plans for one Linac system and the AcurosXB algorithm can be used to generate clinically acceptable plans without manual planner intervention for a different Linac system and the AAA calculation algorithm, indicating the potential of a KBP model use for centralized planning process across different Linac systems and calculation algorithms.

Monte Carlo Dose Calculation - A QA Method for SRT and SBRT Plans in Treating Multiple and Small Metastatic Lesions.

To provide accurate and fast 3-D dose verification for hypofractionated stereotactic radiotherapy (SRT/SBRT) of small and multi targets calculated with a Varian Eclipse treatment planning system (TPS) delivered on a Varian accelerator. Ten brain and lung hypofractionated SRT/SBRT linac-based and CyberKnife plans were generated by the Eclipse system for delivery on the accelerator with the Millenium-120 leaf multileaf collimator (MLC) and Multiplan for the CyberKnife machine. These clinical SRT/SBRT plans required accurate quality assurance measurements to obtain absolute point dose and 3-D dose distributions due to the low number of fractions and high fraction doses. For small-field and multi-target plans, the EGS4/MCSIM code was used to calculate the dose distribution. A 0.125 cc ion chamber, a 0.016 cc pin-point chamber and Kodak EDR2 film were used for the measurements and the results were compared with Monte Carlo (MC) calculations. The dosimetry for small-field and multi-target treatment plans is challenging due to the comparable range of secondary electrons and the field sizes defined by SRT/SBRT MLC segments. Our MC simulations can accurately reproduce the linac dose distributions (within 1%/1 mm) three dimensionally. For the clinical SRT/SBRT plans investigated in this work, the MC doses agreed within 3% with ion chamber measurements and within 2%/2 mm with film measurements. The doses calculated by the Eclipse AAA algorithm and Multiplan differed by no more than 5% from MC calculations for small (4–40 cc) Planning Target Volumes (PTVs). MC dose calculation provides accurate and fast 3-D dose verification for hypofractionated SRT for small and multi-target treatment plans generated by a Varian Eclipse TPS on a Varian accelerator and Multiplan treatment planning on the CyberKnife System.

Photon beam modeling variations predict errors in IMRT dosimetry audits

Background & purposeTo evaluate treatment planning system (TPS) beam modeling parameters as contributing factors to IMRT audit performance. Materials & methodsWe retrospectively analyzed IROC Houston phantom audit performance and concurrent beam modeling survey responses from 337 irradiations performed between August 2017 and November 2019. Irradiation results were grouped based on the reporting of typical or atypical beam modeling parameter survey responses (<10th or >90th percentile values), and compared for passing versus failing (>7% error) or “poor” (>5% error) irradiation status. Additionally, we assessed the impact on the planned dose distribution from variations in modeling parameter value. Finally, we estimated the overall impact of beam modeling parameter variance on dose calculations, based on reported community variations. ResultsUse of atypical modeling parameters were more frequently seen with failing phantom audit results (p = 0.01). Most pronounced was for Eclipse AAA users, where phantom irradiations with atypical values of dosimetric leaf gap (DLG) showed a greater incidence of both poor-performing (p = 0.048) and failing phantom audits (p = 0.014); and in general, DLG value was correlated with dose calculation accuracy (r = 0.397, p < 0.001). Manipulating TPS parameters induced systematic changes in planned dose distributions which were consistent with prior observations of how failures manifest. Dose change estimations based on these dose calculations agreed well with true dosimetric errors identified. ConclusionAtypical TPS beam modeling parameters are associated with failing phantom audits. This is identified as an important factor contributing to the observed failing phantom results, and highlights the need for accurate beam modeling.

Assessment of the Sun Nuclear ArcCHECK to detect errors in 6MV FFF VMAT delivery of brain SABR using ROC analysis.

Institutions use a range of different detector systems for patient‐specific quality assurance (QA) measurements conducted to assure that the dose delivered by a patient’s radiotherapy treatment plan matches the calculated dose distribution. However, the ability of different detectors to detect errors from different sources is often unreported. This study contains a systematic evaluation of Sun Nuclear’s ArcCHECK in terms of the detectability of potential machine‐related treatment errors. The five investigated sources of error were multileaf collimator (MLC) leaf positions, gantry angle, collimator angle, jaw positions, and dose output. The study encompassed the clinical treatment plans of 29 brain cancer patients who received stereotactic ablative radiotherapy (SABR). Six error magnitudes were investigated per source of error. In addition, the Eclipse AAA beam model dosimetric leaf gap (DLG) parameter was varied with four error magnitudes. Error detectability was determined based on the area under the receiver operating characteristic (ROC) curve (AUC). Detectability of DLG errors was good or excellent (AUC >0.8) at an error magnitude of at least ±0.4 mm, while MLC leaf position and gantry angle errors reached good or excellent detectability at error magnitudes of at least 1.0 mm and 0.6°, respectively. Ideal thresholds, that is, gamma passing rates, to maximize sensitivity and specificity ranged from 79.1% to 98.7%. The detectability of collimator angle, jaw position, and dose output errors was poor for all investigated error magnitudes, with an AUC between 0.5 and 0.6. The ArcCHECK device’s ability to detect errors from treatment machine‐related sources was evaluated, and ideal gamma passing rate thresholds were determined for each source of error. The ArcCHECK was able to detect errors in DLG value, MLC leaf positions, and gantry angle. The ArcCHECK was unable to detect the studied errors in collimator angle, jaw positions, and dose output.

Effects of gas-filled temporary breast tissue expanders on radiation dose from modulated rotational photon beams

Gas-filled temporary tissue expanders (TTEs), implanted to assist in post mastectomy breast reconstructions, are expected to produce increased dosimetric uncertainty in breast radiotherapy treatments, due to their containing both a substantial metallic component and a comparatively large volume of gas. This study therefore builds on previous investigations of the dosimetric effects of gas-filled TTEs in static photon and electron beams, by examining the effects of these implants on dose distributions from common modulated rotational treatment techniques; volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT). Radiochromic film measurements were used to evaluate the accuracy of VMAT and HT dose calculations, for a humanoid phantom augmented with a sample Aeroform CO2-filled TTE (AirXpanders Inc, San Jose, USA) as well as purpose-designed and 3D printed “breast tissue.” Results showed that the TomoTherapy Hi-Art VoLO convolution-superposition algorithm (Accuray Inc, Sunnyvale, USA) produced comparatively accurate calculations of treatment dose within this complex phantom, including immediately anterior and posterior to the TTE. The Varian Eclipse Acuros (AXB) algorithm generally showed better agreement with the film measurement than the Varian Eclipse AAA algorithm (Varian Medical Systems, Palo Alto, USA), although the film measurements showed regions of 5% to 10% disagreement with both AAA and AXB in the dosimetrically-challenging region on the anterior side of the implant. Although the Aeroform CO2-filled TTE has substantial and obvious effects on the downstream dose from a static photon beam, the results of this study showed how inverse-planning of modulated rotational radiotherapy treatments can produce modulated fluence distributions that compensate for the dramatic density heterogeneities in the implant. Despite some disagreements with the planned dose, all film measurements showed that the use of inverse-planned modulated rotational photon beams resulted in comparatively homogeneous coverage of the radiotherapy target, in the complex patient-like phantom with a gas-filled TTE. Due to the importance of matching each planned fluence distribution to the density distribution within each TTE, careful use of available 3D imaging techniques is advisable, when modulated rotational radiotherapy treatments are delivered to patients with gas-filled TTEs.

120. Electron density calibration curve for Philips Brilliance Big Bore CT for radiotherapy: HU stability and comparison with Varian default curve

PurposeThe calibration of the newly installed Philips Brilliance Big Bore CT scanner, finding the most appropriate curve (HU vs electron densities) for Varian Eclipse AAA algorithm. FOV dimensions, CT voltage, object dimensions and reconstruction filters are the major parameters that can affect the response curve, depending on the particular scanner model. They’ve been studied, and differences respect Varian default curve were evaluated. Methods and materialsThe CIRS62M HEAD and BODY phantoms, equipped with low and high densities inserts, of known electron densities were used. Scans were made varying the position of inserts from the centre to the border of the test object, then varying separately CT voltage, FOV, test-object dimensions with respect to FOV, CT protocol (head vs body protocols), with particular attention to reconstruction filters. Test acquisitions regarded both allowed and not-allowed filters (Philips gives general indications about filters to be avoided for planning, cause their effect on HU is not negligible). ResultsCT voltage and FOV dimensions have shown not to influence the response curve. Variability among allowed reconstruction filters is acceptable even for high HU (<2% at 1000 HU), while the not-allowed filters move the HU values till 3% and 12% (at 1000 HU, body and head respectively). With allowed filters, the factor mainly influencing the HU curve has proved to be the object dimension: the head phantom scanned with a body protocol, independently of FOV, (e.g head-and-neck or arm/leg scans) gives differences till 9% at 1000 HU: the head curve should be used. ConclusionMeasures analysis suggests to create two different curves for planning purposes, with the average data of allowed filters: (1) head and (2) body curves. Varian default curve is much similar to the head one, while differences are not negligible with the body curve extrapolating at very high densities: titanium HU is evaluated 5200 against Varian 5600.▪

Impact of dose calculation algorithms on the dosimetric and radiobiological indices for lung stereotactic body radiotherapy (SBRT) plans calculated using LQ–L model

AbstractPurposeTo investigate discrepancies in dose calculation algorithms used for lung stereotactic body radiotherapy (SBRT) plans.Methods and materialsIn total, 30 patients lung SBRT treatment plans, initially generated using BrainLab Pencil Beam (BL_PB) algorithm for 10 Gy×5 Fractions to the planning target volume (PTV) were included in the study. These plans were recalculated using BrainLab Monte Carlo (BL_MC), Eclipse AAA (EC_AAA), Eclipse Acuros XB (EC_AXB) and ADAC Pinnacle CCC (AP_CCC) algorithms. Dose volume histograms of PTV were used to calculate dosimetric and radiobiological quality indices, and equivalent dose to 2 Gy per fraction using linear-quadratic-linear model. The BL_MC algorithm is considered gold standard tool to compare PTV parameters and quality indices to investigate dose calculation discrepancies of abovementioned plans.ResultsBL_PB overestimates doses that may be due to inability of the algorithm to properly account for electron scattering and transport in inhomogeneous medium. Compared with BL_MCNO plans, the EC_AAA and EC_AXB yield lower homogeneity indices and overestimate the dose in the penumbra region, whereas AP_CCC plans were comparable for small PTV (≈8 cc) and had significant difference for large PTV.ConclusionBL_PB algorithm overestimates PTV doses than BL_MC calculated doses. The EC_AAA, EC_AXB and AP_CCC algorithms calculate doses within acceptable limits of radiotherapy dose delivery recommendations.

RP-HPLC method for simultaneous estimation of vigabatrin, gamma-aminobutyric acid and taurine in biological samples

Vigabatrin is used as first line drug in treatment of infantile spasms for its potential benefit overweighing risk of causing permanent peripheral visual field defects and retinal damage. Chronic administration of vigabatrin in rats has demonstrated these ocular events are result of GABA accumulation and depletion of taurine levels in retinal tissues. In vigabatrin clinical studies taurine plasma level is considered as biomarker for studying structure and function of retina. The analytical method is essential to monitor taurine levels along with vigabatrin and GABA. A RP-HPLC method has been developed and validated for simultaneous estimation of vigabatrin, GABA and taurine using surrogate matrix. Analytes were extracted from human plasma, rat plasma, retina and brain by simple protein precipitation method and derivatized by naphthalene 2, 3‑dicarboxaldehyde to produce stable fluorescent active isoindole derivatives. The chromatographic analysis was performed on Zorbax Eclipse AAA column using gradient elution profile and eluent was monitored using fluorescence detector. A linear plot of calibration curve was observed in concentration range of 64.6 to 6458, 51.5 to 5150 and 62.5 to 6258 ng/mL for vigabatrin, GABA and taurine, respectively with r2 ≥ 0.997 for all analytes. The method was successfully applied for estimating levels of vigabatrin and its modulator effect on GABA and taurine levels in rat plasma, brain and retinal tissue. This RP-HPLC method can be applied in clinical and preclinical studies to explore the effect of taurine deficiency and to investigate novel approaches for alleviating vigabatrin induced ocular toxicity.

Rapid and sensitive method for determining free amino acids in plant tissue by high-performance liquid chromatography with fluorescence detection

This paper describes a reliable and rapid method for the complete separation and quantitation of twenty-five amino acids typically found in plants, based on reversed phase high-performance liquid chromatography–linked fluorescence detector using a 150 × 4.6 mm Zorbax Eclipse AAA column. Plant tissue free amino acids (FAA) were extracted by ultrasonication with 5% (v/v) aqueous trifluoroacetic acid followed by ultrafiltration of extracts. The following analysis of amino acids was performed through programmed precolumn derivatization with ortho-phthalaldehyde and 9-fluorenylmethyl chloroformate reagents and efficient elution of derivatives within 26 min using binary gradient scheme. The method was validated over a concentration range of 4.5–450 μmol L−1 (μM). Separation analysis showed good selectivity (resolution > 1.5) for most amino acids. The average repeatability (RSD%, relative standard deviation) of the analysis at seven calibration concentrations was below 4% and ranged from 1.13% to 12.04%. The intra-day mean coefficient of variation at two concentrations (22.5 and 90 μM) was within 2%, and the intermediate precision was less than 4%. The limits of detection were between 0.012 and 6.68 μM. The coefficients of determination (R2) of the linear calibration curves were from 0.9989 to 0.9999. When the method was applied to plant samples, the FAA recoveries at two spiked levels (25 and 100 μM) ranged from 67.0% to 108.9% with an average of 94.4%, and the precision was 0.26%–12.31% RSD. A specific application combining this method with optimized extraction and interference removal procedures was successfully used to determine the FAA pools in different plant tissues. Finally, a PLS-DA multivariate statistics model was validated for the classification of three plant species according to their FAA profiles.

Abstract ID: 101 Comparison of dose calculation between AAA algorithm and Monte Carlo calculation for prostate cancer

In radiotherapy, the accuracy of dose calculation is very important for a treatment success. Analytical algorithms implemented in commercial Treatment Planning Systems (TPS) achieve this calculation. In order to reduce time calculation; these algorithms do some assumptions, which cause important errors, especially in the cases with high heterogeneities. The aim of our study is to evaluate the AAA analytical algorithm of dose calculation using a Monte Carlo method, which is the most accurate, and considered as a reference in dose calculation. To accomplish this purpose, we started by the modeling of the treatment machine (Varian 2100C LINear ACcelerator) for the photons mode and the energy of 18MV, using EGSnrc Monte Carlo code. After that, we used this model to calculate dose distribution in patients treated for prostate cancer. Finally, the results are compared to those of Eclipse AAA TPS.

39. Dose computation for lung stereotactic radiotherapy during deep inspiration breath-hold: A comparison between AAA and Geant4

Introduction Limits of Radiotherapy Treatment Planning System (TPS) when computing dose in moderate density media are known. Previous studies were published using Monte Carlo methods [1] . When treating lung cancer during Deep Inspiration Breath Hold (DIBH), lung density can decrease up to 0.12 g cm−3. The goal of this study is to assess performances of a TPS (Eclipse AAA) and of a MC model under these particular conditions: small fields and very low densities. Films measurements in heterogeneous phantoms are compared to both computation systems. Material and methods MC computation and model of the TrueBeam Linac head (Varian, Palo Alto, CA) are achieved using GATE/Geant4. Geometrical dimensions as well as the space phase files are furnished by Varian [2] . Jaws, mylar and baseplate are modeled and validated for the following energies: X6, X10, X6FFF and X10FFF. For this validation, MC computation are compared with Semiflex (PTW) ion chamber measurements in a water tank for a large range of field sizes (3 × 3 to 20 × 20 cm2). Percentage depth dose (PDD) and lateral dose profiles at different depths obtained by measurement and computation are compared (Gamma Index method). Two different qualities of cork (densities of 0.24 and 0.12 g cm3 are used to build two heterogeneous water/lung phantoms. These densities correspond to the lung densities during free breathing and DIBH respectively. Phantoms are composed of 20 × 20 cm2 slabs: PMMA (3 cm) + cork (7 cm) + PMMA (15 cm). See Fig. 1. Dose is measured at several depths using EBT3 gafchromic films (Ashland). Field sizes were 3 × 3 and 8 × 8 cm2 for X6FFF and X10FFF photon fields. These phantoms are implemented both in Eclipse (AAA) and MC. Download : Download high-res image (121KB) Download : Download full-size image Results TrueBeam MC model and measurements are in good agreement for all field sizes in both water and heterogeneous phantoms. More than 99% of PDD and lateral profiles passed the gamma index test (2% 2 mm). In lung heterogeneity, differences are observed between AAA and measurement results. These differences were higher for small field and very low density. As an example, for a 3 × 3 cm2 field at density 0.12 g cm−3, dose at a depth of 7 cm was 56.61%, 56.25%, and 67.54% for measure, MC and AAA respectively. Conclusion For very low densities, AAA shows its limits when MC is in good agreement with measurements. This MC model will allow us to assess our TPS results in complex cases like lung patients treated by Stereotactic Radiotherapy during DIBH.

An HPLC Procedure for the Quantification of Aloin in Latex and Gel from Aloe barbadensis Leaves.

Aloin is an anthraquinone-C-glycoside present in Aloe vera. This compound is extremely variable among different species and highly depends on the growing conditions of the plants. The quantification of aloin in different extraction preparations has been a frequent problem due to the high instability of the compound. The aim of the present study is to develop and validated an analytical method for aloin detection in fresh and dry samples of Aloe barbadensis gel and latex using high performance liquid chromatography coupled to a diode array detector (HPLC-DAD). Phosphate buffered saline (pH 3) was selected as the extraction solvent. The aloin was separated using a Zorbax Eclipse AAA column (4.6 × 150 mm) at 35°C, and water and acetonitrile were used as the mobile phase at a flow rate of 0.9 mL/min. The linearity was satisfactory with a correlation coefficient greater than 0.999. Under these conditions, the method precision (relative standard deviation) was 3.71% for FL, 4.41% for dry latex, 0.81% for fresh gel and 4.42% for dry gel samples. Aloe latex was determined to have a greater amount of aloin than aloe gel. The method validation was satisfactory and exhibited adequate linearity, repeatability and accuracy.

English

This study provides data on distribution of phthalate esters (PAEs) in the water and sediments in the Ori stream, Iwo, Nigeria. The liquid-liquid extraction method was employed, followed by cleaned up in an alumina column using hexane. Chromatographic separation and quantification were done using Zorbax Eclipse AAA C18 analytical column. The concentrations of DMP, DBP, DEP, and DPP in water (µg/L) ranged from 1.29±1.3 in DBP to 938±780 in DMP while in sediment (µg/kg) ranged from 12.7±15 in DEP to 152±140 µg/kg in DBP. These concentrations were above the USEPA limits of 3 µg/L as recommended for phthalates in water. Key words: Phthalates, liquid-liquid extraction, high performance liquid chromatography (HPLC), endocrine disruptors, stream.

Prostate SBRT: The use of ERB and MOSFET in vivo dosimetry feasibility

Introduction Hypofractionated treatment regimens are indicated for some stages of prostate cancer. In order to provide the reproducibility of the relative position between rectum and prostate and to allow that only a small volume of the rectal wall remains close to the prostate, an endorectal balloon (ERB) may be used. Purpose This work aims to assess the dose on the rectal wall in the presence of the ERB filled with water or air and evaluate the deviation between the measured and calculated doses using two different algorithms (Eclipse AAA and iPlan PencilBeam) Materials and methods Two CT scans were obtained for a modified Rando phantom where an ERB (filled with water or air) with three MOSFET detectors was inserted. A simple 4 field in box plan and an IMRT (6MV beam) clinical plan were calculated in both CT sets, using two different algorithms. The treatment plans were delivered to the phantom using a Varian Novalis linac. Four sets of measurements were obtained and the results were compared with the calculated values. Results The difference between the calculated and measured doses around the ERB is lower when it is filled with water for both algorithms. The maximum relative differences when the ERB is filled with air are 1.8% for Eclipse and 5.5% for iPlan. Conclusion Both algorithms show a better a performance in the presence of water. Water filled ERB seems to be a suitable option. MOSFET dosimetry in association with ERBs for real-time in vivo during hypofractionated treatment of the prostate is a practical and reliable procedure. Disclosure The authors have no relevant financial or non-financial relationships to disclose.

Which impact of tumor density variations on absorbed dose in external radiotherapy

Introduction Dosimetry in external radiotherapy is based on tomodensitometry images that are required to be previously calibrated and assume that the electron density is comparable to the mass density. In Magnetic Resonance Imaging, and in particular diffusion sequence, significant heterogeneities have been shown. These heterogeneities result in density variations within the tumor, e.g. glioblastomas. Purpose The aim of this study is to evaluate slight influence of mass density variations within the tumor via a dosimetric study. This can then be used to quantify the impact of those variations on modifications of dose 3D distribution. Materials and methods This study is devoted to dosimetric calculations realization using TPS Eclipse AAA used in clinical. Treatments were carried out with 5 X-ray beams of 6 MeV, in optimized dynamic Intensity-Modulated-Radiotherapy. Mass density changes of GTV for the same treatment plans have been tested from 1.04 (default tumor density) to 1.8 g.cm−3 in steps of 0.1. Results Dose-volume-histograms and profiles studies show a slight dose increase in surface GTV for the highest densities with significant changes isodose curves. Conversely, upon leaving the GTV, an under dosage of the tumor has been observed from a density variation from 1.04 to 1.2 g cm−3. It was noted that density differences of 0.5 and 0.8 g cm−3 caused failure to respect the prescribed dose limits of ± 3% in brain volumes of 1.5 and 25 cm3, respectively. Conclusion Dose distributions variations were recorded by densities changes. From a change of 0.5 g/cm3 within the tumor, dose prescriptions limits are not any more guarantees.

Monte-Carlo dose computation in radiotherapy for lung at very low density

Introduction In radiotherapy, some limits of clinical algorithms have been reported in low-density media. When patients are treated during Deep Inspiration Breath-Hold (DIBH), lung density can fall to 0.12 g cm -3 . Dose calculation with this density must be validated, in particular for Stereotactic (SBRT) treatment. Purpose We propose to evaluate the performance of Eclipse AAA algorithm (Varian) for SBRT lung treatment (including DIBH) with a Monte-Carlo model based on Geant4 (GATE). GATE, AAA, and measurements are compared for several phantoms including lung heterogeneity. Materials and methods A model of the Varian TrueBeam head was modelled using GATE. Validation was achieved by comparison with measurements in a homogeneous water tank for several static fields (3 × 3 to 20 × 20 cm 2 ) and four beam modes. Subsequently, two phantoms containing layers of water-equivalent material and cork, corresponding to lung during DIBH and free-breathing (0.12 and 0.24 g cm -3 ) were implemented. Percentage depth dose (PDD) measured by Gafchromic films were compared to AAA and GATE. Results GATE model showed a good agreement with experimental results in homogeneous and heterogeneous phantoms. In lung heterogeneity, differences were observed between AAA and measurements. These differences are higher for small fields and lower densities. As an example, for 3 × 3 field size inside a 0.12 g cm -3 heterogeneity, relative dose was 56,6%, 56,2%, and 67,5% for measurement, GATE, and AAA respectively. Conclusions In very low densities, AAA quickly showed its limits while GATE was validated. This will allow us to perform calculations for non measurable cases, for example patients treated by SBRT during DIBH.

SU‐F‐P‐39: End‐To‐End Validation of a 6 MV High Dose Rate Photon Beam, Configured for Eclipse AAA Algorithm Using Golden Beam Data, for SBRT Treatments Using RapidArc

Purpose:To use end‐to‐end testing to validate a 6 MV high dose rate photon beam, configured for Eclipse AAA algorithm using Golden Beam Data (GBD), for SBRT treatments using RapidArc.Methods:Beam data was configured for Varian Eclipse AAA algorithm using the GBD provided by the vendor. Transverse and diagonals dose profiles, PDDs and output factors down to a field size of 2×2 cm2 were measured on a Varian Trilogy Linac and compared with GBD library using 2% 2mm 1D gamma analysis. The MLC transmission factor and dosimetric leaf gap were determined to characterize the MLC in Eclipse. Mechanical and dosimetric tests were performed combining different gantry rotation speeds, dose rates and leaf speeds to evaluate the delivery system performance according to VMAT accuracy requirements. An end‐to‐end test was implemented planning several SBRT RapidArc treatments on a CIRS 002LFC IMRT Thorax Phantom. The CT scanner calibration curve was acquired and loaded in Eclipse. PTW 31013 ionization chamber was used with Keithley 35617EBS electrometer for absolute point dose measurements in water and lung equivalent inserts. TPS calculated planar dose distributions were compared to those measured using EPID and MapCheck, as an independent verification method. Results were evaluated with gamma criteria of 2% dose difference and 2mm DTA for 95% of points.Results:GBD set vs. measured data passed 2% 2mm 1D gamma analysis even for small fields. Machine performance tests show results are independent of machine delivery configuration, as expected. Absolute point dosimetry comparison resulted within 4% for the worst case scenario in lung. Over 97% of the points evaluated in dose distributions passed gamma index analysis.Conclusion:Eclipse AAA algorithm configuration of the 6 MV high dose rate photon beam using GBD proved efficient. End‐to‐end test dose calculation results indicate it can be used clinically for SBRT using RapidArc.

SU‐F‐T‐311: Comparison of Measurements From Two Diode Array QA Devices to Deep Point Dose Measurements with Two Treatment Planning Model Settings for Brain VMAT SRT Patient‐Specific QA

Purpose:To investigate the sensitivity of traditional gamma‐index‐based analysis using diode arrays to detect significant dosimetric errors in patientspecific measurements for VMAT brain SRT plans.Methods:Quality assurance measurements were performed using an ion chamber and two different diode array devices for four brain SRT treatment plans. All cases were planned with Eclipse AAA RapidArc using 6MV VMAT for a TrueBeam STx. Ion chambers measurements were performed with a PinPoint 3D micro‐ion chamber in both the Standard Imaging Stereotactic Dose Verification (SI) Phantom and in the ArcCheck (AC) cavity plug. Point dose measurements were taken at isocenter within the target. Array measurements were acquired with the ArcCheck and gantry‐mounted MapCheck (MC) and assessed using the SNC patient software with a 3%/3mm absolute dose gamma‐index analysis with global normalization and a 10% dose threshold. Calculations and their corresponding measurements were performed and compared before and after a dynamic leaf gap (DLG) adjustment.Results:The measured doses at isocenter dropped from 6.3%–8.7% in the SI phantom and 4.0%–9.3% for AC down to −0.1%–1.1% in the SI phantom and 0.8%–2.9% in the AC after adjusting the DLG. However, the cumulative dose differences to MC's central diode ranged −2.0%–1.9% before the DLG adjustment and were −7.7%–0.2% afterwards. Similarly, the gamma‐index analysis results for MC were found to be 94.1%–100% before the DLG adjustment and 94.4%–100% afterwards. The gamma‐index results from AC were 72.9%–98.7% before and 95.9%–99.7% after.Conclusion:Our data suggest that traditional passing criteria using gantry‐mounted MapCheck may not have the sensitivity to catch significant dose errors introduced by DLG settings. ArcCheck measurements did flag some dose differences, however there appears to be a possibility that dose discrepancies may not always be observed for VMAT SRT treatments, and point dose measurements are recommended for verification.

On the accuracy of dose prediction near metal fixation devices for spine SBRT.

The metallic fixations used in surgical procedures to support the spine mechanically usually consist of high‐density materials. Radiation therapy to palliate spinal cord compression can include prophylactic inclusion of potential tumor around the site of such fixation devices. Determination of the correct density and shape of the spine fixation device has a direct effect on the dose calculation of the radiation field. Even with the application of modern computed tomography (CT), under‐ or overestimation of dose, both immediately next to the device and in the surrounding tissues, can occur due to inaccuracies in the dose prediction algorithm. In this study, two commercially available dose prediction algorithms (Eclipse AAA and ACUROS), EGSnrc Monte Carlo, and GAFchromic film measurements were compared for a clinical spine SBRT case to determine their accuracy. An open six‐field plan and a clinical nine‐field IMRT plan were applied to a phantom containing a metal spine fixation device. Dose difference and gamma analysis were performed in and around the tumor region adjacent to the fixation device. Dose calculation inconsistency was observed in the open field plan. However, in the IMRT plan, the dose perturbation effect was not observed beyond 5 mm. Our results suggest that the dose effect of the metal fixation device to the spinal cord and the tumor volume is not observable, and all dose calculation algorithms evaluated can provide clinically acceptable accuracy in the case of spinal SBRT, with the tolerance of 95% for gamma criteria of 3%/3 mm.PACS number(s): 87.53.bn, 87.53.Ly, 87.55.kd

Going the distance: validation of Acuros and AAA at an extended SSD of 400 cm

Accurate dose calculation and treatment delivery is essential for total body irradiation (TBI). In an effort to verify the accuracy of TBI dose calculation at our institution, we evaluated both the Varian Eclipse AAA and Acuros algorithms to predict dose distributions at an extended source‐to‐surface distance (SSD) of 400 cm. Measurements were compared to calculated values for a 6 MV beam in physical and virtual phantoms at 400 cm SSD using open beams for both 5×5 and 40×40 cm2 field sizes. Inline and crossline profiles were acquired at equivalent depths of 5 cm, 10 cm, and 20 cm. Depth‐dose curves were acquired using EBT2 film and an ion chamber for both field sizes. Finally, a RANDO phantom was used to simulate an actual TBI treatment. At this extended SSD, care must be taken using the planning system as there is good relative agreement between measured and calculated profiles for both algorithms, but there are deviations in terms of the absolute dose. Acuros has better agreement than AAA in the penumbra region.PACS number(s): 87.55.kd