Anisotropic Analytical Algorithm Research Articles

Quality Assurance of Multiple Brain Metastases with a Single-Isocenter Plan using Portal Dosimetry and Independent Dose Calculation

Recently, a multiple metastases element (MME) for a linear accelerator (LINAC)-based radiation system was released commercially. It covers multiple brain metastases with a single-isocenter. The purpose of this study is to evaluate a quality assurance (QA) program for multiple targets with a single-isocenter plan. We verify this plan by portal dosimetry using an electronic portal imaging device (EPID) and independent dose calculation. We created 10 QA plans with a median of 3.5 targets (range, 2.0–9.0) using noncoplanar dynamic conformal arcs by MME. The median number of arcs was 6.5 (range, 4.0-9.0). The median planning target volume (PTV) was 0.76 cm3 (range, 0.45-2.76). These plans were delivered from a medical linear accelerator with a 6-MV X-ray beam. First, dosimetric comparison was conducted between MME with pencil beam (PB) and a treatment planning system with analytical anisotropic algorithm (AAA). The plan was transferred to the treatment planning system from the MME. The dose was then recalculated using the AAA with the same monitor unit (MU) and beam arrangement as the plans created using MME. Thereafter, the point dose of the PTV using the different dose-calculation algorithms were compared. Second, a portal dosimetry plan of the imported MME plan was created by an oncology information system. For the gamma analysis, a calculated portal image was acquired through a portal dose calculation algorithm in the treatment planning system, and a measured portal image was obtained using an EPID. Thereafter, the gamma analysis was performed using the portal dosimetry software. The resulting dose distributions were then compared with the corresponding planned dose distributions using the 2D gamma index with 3%/2 mm and 3%/1 mm passing criteria. In addition, LINAC delivery log files were acquired after the delivery of each arc plan. An in-house software was used to analyze the root mean square of the multi leaf collimator (MLC) error which was defined as the difference between the planned position and the actual position. The point dose for AAA was higher than the dose for PB by 0.5 ± 1.4 %. However, the differences were not statistically significant for AAA and PB (p-value=0.051). Only minor differences were observed in dose when comparing the two-treatment planning software related to the same plan. The average pass rates using the EPID images were 99.6±0.6% (3%/2 mm) and 97.3 ± 1.9% (3%1 mm). The methods showed a good agreement between calculation and measurement results in MUs and field shape. The MLC positioning between plan and delivered values were 0.019 ± 0.004 mm. There was no correlation between MLC errors and the pass rate. Our results indicate that the use of the recalculated dose and EPID are viable methods for the QA of multiple targets with single-isocenter plans. This QA method is useful as a rapid and convenient verification tool.

Local Control Analyses of Pulmonary Oligometastases Treated By Stereotactic Body Radiation Therapy (SBRT) from a Multi-institutional Survey in Japan

This study aimed to investigate local control (LC) rate and to identify affecting factors for LC of pulmonary oligometastases treated by SBRT using large data sets from a multi-institutional nationwide survey in Japan. A total of 1374 patients with 1545 targets from 68 institutions were collected. Eligibility was as follows: SBRT for pulmonary oligometastases was performed between 2004 January and 2015 June, biological effective dose (BED10) was 75 Gy or more. Local recurrence of thoracic primary tumor was excluded and all extrathoracic lesions were controlled at the time of SBRT. Cumulative LC rate was calculated using Kaplan-Meier method and the 95% confidence interval (95% CI) was calculated using Greenwood's formula. Cox proportional hazards model was applied for LC analyses and variables with a p-value of < 0.20 in univariate analyses were put in multivariate analysis (MVA). The median targeted tumor diameter was 1.5 cm (range, 0.3-6.5), and the median overall treatment time (OTT) of SBRT was 7 days (range, 3-81). Dose calculation algorithm was divided into type B (51.7%) which was equivalent of Analytical Anisotropic Algorithm, type C (9.3%) which was equivalent of Monte Carlo Algorithm and type A (34.8%) which was older generation algorithm than both. The median heterogeneity correction considered BED10 around isocenter was 126.9 Gy (range, 76.8-352.7). Primary sites were separated into lung (29.1%), colorectum (25.3%), genitourinary (13.5%), head and neck (8.6%), esophagus (8.5%) and others. Median follow-up period was 24.2 months (range, 0.1-143.7). Local recurrence occurred in 222 tumors and median time to local recurrence was 12.4 months (range, 2.9-98.7). Estimated 1-year, 3-year and 5-year LC were 92.0% (95% CI, 90.4-93.4%), 81.3% (95% CI, 76.7-83.6%) and 78.5% (95% CI, 75.6-81.2%), respectively. The result of MVA (n=785) showed that maximum diameter of targeted tumor (per 1 cm increase; Hazard ratio [HR], 1.320; p=0.023), dose calculation algorithm (type B vs. type A; HR, 0.541; p=0.012), OTT (per 10 days prolonged; HR, 0.564; p=0.022) and primary site of metastases were significantly associated with LC. In regard to primary sites, colorectum showed significantly lower LC rate compared to lung (HR, 2.837; p<0.001), genitourinary (HR, 6.473; p<0.001), head and neck (HR, 4.879; p<0.001) and others (HR, 2.478; p=0.005), genitourinary showed significantly better LC compared to colorectum (HR, 0.154; p<0.001) and esophagus (HR, 0.291; p=0.020) but the others showed no significant association. BED10 showed the tendency (per 10 Gy increase; HR, 0.924; p=0.102). To procure excellent LC rate, the use of type A algorithm should be avoided, the strategy of tumor diameter and primary sites should be considered and higher BED with longer OTT might help.

Comparison of measured and calculated doses in a Rando phantom with a realistic lung radiotherapy treatment plan including heterogeneities

In this work, dose measurements were performed to evaluate an external radiotherapy treatment plan and, particularly, to validate dose calculations for a lung lesion case. Doses were calculated by the Varian Eclipse treatment planning system using the AAA anisotropic analytical algorithm. The measurements were performed using a Rando anthropomorphic phantom and TLD700 thermoluminescent dosimeters. The comparison between doses calculated and doses measured by means of thermoluminescence (TL) shows compatibility except for a few points, due to the limitations in the heterogeneity correction used for the case studied here. The deviation between the calculated and measured doses is about 6.5% for low (< 0.5Gy) doses and about 1% for higher doses (> 0.5Gy).The deviation between AAA-calculated and TL-measured doses was also found to be higher in proximity to heterogeneous tissue interfaces.

Technical and dosimetric implications of respiratory induced density variations in a heterogeneous lung phantom

BackgroundStereotactic Body Radiotherapy (SBRT) is an ablative dose delivery technique which requires the highest levels of precision and accuracy. Modeling dose to a lung treatment volume has remained a complex and challenging endeavor due to target motion and the low density of the surrounding media. When coupled together, these factors give rise to pulmonary induced tissue heterogeneities which can lead to inaccuracies in dose computation. This investigation aims to determine which combination of imaging techniques and computational algorithms best compensates for time dependent lung target displacements.MethodsA Quasar phantom was employed to simulate respiratory motion for target ranges up to 3 cm. 4DCT imaging was used to generate Average Intensity Projection (AIP), Free Breathing (FB), and Maximum Intensity Projection (MIP) image sets. In addition, we introduce and compare a fourth dataset for dose computation based on a novel phase weighted density (PWD) technique. All plans were created using Eclipse version 13.6 treatment planning system and calculated using the Analytical Anisotropic Algorithm and Acuros XB. Dose delivery was performed using Truebeam STx linear accelerator where radiochromic film measurements were accessed using gamma analysis to compare planned versus delivered dose.ResultsIn the most extreme case scenario, the mean CT difference between FB and MIP datasets was found to be greater than 200 HU. The near maximum dose discrepancies between AAA and AXB algorithms were determined to be marginal (< 2.2%), with a greater variability occurring within the near minimum dose regime (< 7%). Radiochromatic film verification demonstrated all AIP and FB based computations exceeded 98% passing rates under conventional radiotherapy tolerances (gamma 3%, 3 mm). Under more stringent SBRT tolerances (gamma 3%, 1 mm), the AIP and FB based treatment plans exhibited higher pass rates (> 85%) when compared to MIP and PWD (< 85%) for AAA computations. For AXB, however, the delivery accuracy for all datasets were greater than 85% (gamma 3%,1 mm), with a corresponding reduction in overall lung irradiation.ConclusionsDespite the substantial density variations between computational datasets over an extensive range of target movement, the dose difference between CT datasets is small and could not be quantified with ion chamber. Radiochromatic film analysis suggests the optimal CT dataset is dependent on the dose algorithm used for evaluation. With AAA, AIP and FB resulted in the best conformance between measured versus calculated dose for target motion ranging up to 3 cm under both conventional and SBRT tolerance criteria. With AXB, pass rates improved for all datasets with the PWD technique demonstrating slightly better conformity over AIP and FB based computations (gamma 3%, 1 mm). As verified in previous studies, our results confirm a clear advantage in delivery accuracy along with a relative decrease in calculated dose to the lung when using Acuros XB over AAA.

A depth dose study between AAA and AXB algorithm against Monte Carlo simulation using AIP CT of a 4D dataset from a moving phantom

To identifying depth dose differences between the two versions of the algorithms using AIP CT of a 4D dataset. Motion due to respiration may challenge dose prediction of dose calculation algorithms during treatment planning. The two versions of depth dose calculation algorithms, namely, Anisotropic Analytical Algorithm (AAA) version 10.0 (AAAv10.0), AAA version 13.6 (AAAv13.6) and Acuros XB dose calculation (AXB) algorithm version 10.0 (AXBv10.0), AXB version 13.6 (AXBv13.6), were compared against a full MC simulated 6X photon beam using QUASAR respiratory motion phantom with a moving chest wall. To simulate the moving chest wall, a 4cm thick wax mould was attached to the lung insert of the phantom. Depth doses along the central axis were compared in the anterior and lateral beam direction for field sizes 2×2cm2, 4×4cm2 and 10×10cm2. For the lateral beam direction, the moving chest wall highlighted differences of up to 105% for AAAv10.0 and 40% for AXBv10.0 from MC calculations in the surface and buildup doses. AAAv13.6 and AXBv13.6 agrees with MC predictions to within 10% at similar depth. For anterior beam doses, dose differences predicted for both versions of AAA and AXB algorithm were within 7% and results were consistent with static heterogeneous studies. The presence of the moving chest wall was capable of identifying depth dose differences between the two versions of the algorithms. These differences could not be identified in the static chest wall as shown in the anterior beam depth dose calculations.

Comparative study between Acuros XB algorithm and Anisotropic Analytical Algorithm in the case of heterogeneity for the treatment of lung cancer

Abstract The aim of this study was to investigate the impact of heterogeneity on the dose calculation for two algorithms implemented in the TPS “Analytical Anisotropic Algorithm (AAA) and Acuros XB” and validated the use of Acuros XB algorithm in clinical routine. First, we compare the dose calculated by these algorithms and the dose measured at the given point P, which is found after heterogeneity insert. Second, we extend our work on clinical cases that present a complex heterogeneity. By evaluating the impact of the choice of the algorithm on the dose coverage of the tumor, and the dose received by the organs at risk for 20 patients affected by lung cancer. The result of our phantom study showed a good agreement with several studies that showed the superiority of the Acuros XB over the AAA in predicting dose when it concerns heterogeneous media. The treatment plans for 20 lung cancers were calculated by two algorithms AAA and Acuros XB, the results show a statistical significant difference between algorithms for Homogeneity Index and the maximum dose of planning target volume (HI: 0.11±0.01 vs 0.05±0.01 p = 0.04; Dmax: 69.30±3.12 vs 68.51±2.64 p = 0.02). Instead, no statistically significant difference was observed for conformity index CI and mean dose (CI: 0.98±0.18 vs 0.99±0.14 p = 0.33; Dmean: 66.3±0.65 vs 66.10 ±0.61 p = 0.54). For organs at risk, the maximum dose for spinal cord, mean dose and D37 % of lung minus GTV (dose receiving 37% of lung volume) were found to be lower for AAA plans than Acuros XB and the differences were statistically significant (p&lt;0.05). For the heart D33% and D67% were found to be higher for AAA plans than Acuros XB and the differences were statistically significant (p&lt;0.05), but No difference was observed for D100% of the heart. The use of the AXB algorithm is suitable in the case of presence of heterogeneity, because it allows to have a better accuracy close to the Monte Carlo calculation.

Comparison of absorbed dose distribution 10 MV photon beam on water phantom using Monte Carlo method and Analytical Anisotropic Algorithm

Treatment planning is a process of radiation therapy performed by medical physicists in the hospital. This process can be understood as a simulation to determine parameters such as photon energy, field size, and the absorbable dose to be received by the patient. Simulation treatment planning is usually done using TPS ECLIPSE with AAA or Acuros XB method that has been integrated into it. In this research, the comparison of the distribution of absorbent dose of 10 MV photon at water phantom using Monte Carlo-EGSnrc and AAA-ECLIPSE method was used. Monte Carlo simulations were performed with 6x6 cm2, 10x10 cm2, and 20x20 cm2 field size variations and initial electron energy variations 10.1 MeV, 10.2 MeV, 10.3 MeV and 10.4 MeV. The design and simulation of Linac heads are done using BEAMnrc and the calculation of absorbent doses in this water phantom using DOSXYZnrc. The BEAMnrc simulated particle information was analyzed using BEAMDP. To obtain the depth dose distribution or PDD and PROFILE the dose is done using statdose code. Based on the data processing, the result shows that the mean deviation of PDD and Dmax from the three field sizes is 7.09% for 10.1 MeV, 2.71% for 10.2 MeV, 2.86% for 10.3 MeV and 7.39% for 10.4 MeV. These results indicate that the initial electron energy has the least deviation of 10.2 MeV. As for the dose profile in the field size 10x10 cm2 obtained deviation 6.65% to 10.2 MeV

Phantom Verification of AAA and Acuros Dose Calculations for Lung Cancer: Do Tumor Size and Regression Matter?

PurposeThis study aimed to evaluate dose calculation accuracy for the Eclipse Analytical Anisotropic Algorithm (AAA) and Acuros XB algorithm for various lung tumor sizes and to investigate dosimetric changes associated with treatment of regressing tumors. Methods and materialsA water phantom with cylindrical cork inserts (lung surrogates) was fabricated. Large (202 cm3), medium (54 cm3), and small (3 cm3) solid water tumors were implanted within cork inserts. A plain cork insert was used to simulate a lung without a tumor. The cork inserts and tumors were cut along the long axis, and Gafchromic film was placed between the sections to measure dose distributions. Three-dimensional conformal plans were created using 6 MV and 10 MV beams, and volumetric modulated arc therapy plans were created using 6 MV beams for each tumor size. Doses were calculated using Eclipse AAA and Acuros XB. The measured and calculated dose distributions were compared for each tumor size and treatment algorithm. To simulate a regressing tumor, the original plans created for the large tumor were separately delivered to the phantom that contained a small, medium, or no tumor. The dosimetric effects were evaluated using gamma passing rates with a 2%/2 mm criterion and dose profile comparisons. ResultsAgreement between the measurements and AAA calculations decreased as tumor size decreased, but Acuros XB showed better agreement for all tumor sizes. The largest difference was observed for a 6 MV volumetric modulated arc therapy plan created to treat the smallest tumor. The gamma passing rate was 89.7% but that of Acuros was 99.5%. For the tumor regression evaluation, the gamma passing rates ranged from 53% to 99% for AAA. For Acuros XB, the gamma passing rates were >98% for all scenarios. ConclusionBoth AAA and Acuros XB calculated the dose accurately for the largest lung tumor. For the smallest and regressing tumors, Acuros XB more accurately modelled the dose distribution compared with AAA.

A comparison of Monte Carlo, anisotropic analytical algorithm (AAA) and Acuros XB algorithms in assessing dosimetric perturbations during enhanced dynamic wedged radiotherapy deliveries in heterogeneous media

AbstractBackgroundA comparison of anisotropic analytical algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms with Electron Gamma Shower (EGSnrc) Monte Carlo (MC) for modelling lung and bone heterogeneities encountered during enhanced dynamic wedged (EDWs) radiotherapy dose deliveries was carried out.Materials and methodsIn three heterogenous slab phantoms: water–bone, lung–bone and bone–lung, wedged percentage depth doses with EGSnrc, AAA and AXB algorithms for 6 MV photons for various field sizes (5×5, 10×10 and 20×20 cm2) and EDW angles (15°, 30°, 45° and 60°) have been scored.ResultsFor all the scenarios, AAA and AXB results were within ±1% of the MC in the pre-inhomogeneity region. For water–bone AAA and AXB deviated by 6 and 1%, respectively. For lung–bone an underestimation in lung (AAA: 5%, AXB: 2%) and overestimation in bone was observed (AAA: 13%, AXB: 4%). For bone–lung phantom overestimation in bone (AAA: 7%, AXB: 1%), a lung underdosage (AAA: 8%, AXB: 5%) was found. Post bone up to 12% difference in the AAA and MC results was observed as opposed to 6% in case of AXB.ConclusionThis study demonstrated the limitation of the AAA (in certain scenarios) and accuracy of AXB for dose estimation inside and around lung and bone inhomogeneities. The dose perturbation effects were found to be slightly dependent on the field size with no obvious EDW dependence.

P160] From anisotropical analytical algorithm to acuros XB: Clinical implications in vmat technique for lung, head&neck and prostate treatments

Purpose To determine the dose differences and clinical implications of using a high accuracy algorithm, AcurosXB (AXB), instead of the Anisotropic Analytical Algorithm (AAA), for three different treatment sites with VMAT technique, in order to use AXB as routine algorithm. Methods We selected fifteen prostate and head&neck cancer patients, and seventeen lung cancer patients. All of them had been optimized with the Photon Optimizer algorithm and calculated with AAA. These plans were recalculated with AXB to quantify dose differences in significant points of the DVH: coverage and maximum dose criteria for the PTV, and several dose-volume constraints for the involved Organs at Risk (OAR). All data were statistically treated. For the PTV, we analyzed clinical consequences in terms of coverage and homogeneity. For the OAR, it is well known that classic dose tolerances have been stated based on AAA or even simpler algorithms. In our study, we identified those dose-volume points in which AXB provides lower calculated doses than AAA. In these cases, it wouldn’t be safe to plan with AXB according to classic dose tolerances, since it might imply that these criteria weren’t met with AAA. To prevent this, an additional statistically derived restriction to tolerance constrains is proposed when using AXB. Results PTV: For lung and head&neck patients, due to the high heterogeneity involved, we observe a worse coverage and homogeneity in plans recalculated with AXB. Consequently, if we use AXB in our practice, a more demanding optimization is needed to meet objectives, implying a better local control disease. OAR: for most of the dose-volume points studied the differences found are statistically significant. Some of them might lead to clinical consequences for serial OAR. In these cases, we encourage to restrict Dmax tolerance when AXB is used, as discussed above. Conclusions The results obtained with AXB differ from those obtained with AAA. Since AXB is more accurate, we propose using it in clinical practice. Nevertheless, we strongly recommend being cautious about OAR tolerances, setting an additional restriction to the tolerance criteria when required, until these are reviewed and an international consensus is adopted taking into account the new algorithms.

Validation of the 6X Acuros algorithm version 13.7 in the presence of inhomogeneities for VMAT treatment planning

Purpose the aim of this study was to validate the Acuros®XB (AXB) dose calculation algorithm for a Varian TrueBeam Linac against absolute dose measurements in heterogeneous phantoms. Subsequently, the results were benchmarked against the clinically used Anisotropic Analytic Algorithm (AAA). Methods and materials the algorithm was validated in a homogenous water phantom using MapCheck diode array measurements for a range of field sizes and SSDs. To examine AXB in heterogeneous media, two slab phantoms, representing head/neck and abdominal regions, have been created and absolute dose measurements were performed with GafChromic®EBT2 films. The phantoms simulate water, muscle, bone and free air. The setup was modelled in Eclipse and the dose calculated with AXB and AAA. Results The homogenous phantom results met the gamma criteria of 3%, 2 mm. The planar dose differences between AXB and films after an air gap, for a 6 MV beam, are less than 1% in 3 × 3 cm2 field and 2% in 10 × 10 cm2 field. In the case of a 6MV beam, 5 × 5 cm2 field, interacting with a phantom of different densities and an air gap, AXB underestimates the dose after the free air gap, while AAA overestimates the dose. Measurements at 5 mm from the air/water interface show a difference in dose of 3% for AXB and 7% for AAA. Conclusion AXB was validated in a homogeneous phantom. AXB has advanced modelling of lateral electron transport that enables a more accurate dose calculation in heterogeneous regions. As a result, AXB can replace AAA for dose calculations for head/neck and abdominal VMAT plans.

P251] Dosimetric impact of intermediate dose calculation on heterogeneous region radiotherapy planning

Purpose In radiotherapy planning of heterogeneous regions, differences which negatively affect the quality of the plan in terms of homogeneity and conformity may occur between the optimal dose volume histogram (DVH) and the final calculated DVH. The intermediate dose calculation (IDC) module, which is included in the Analytical Anisotropic Algorithm (AAA), is used even though the planning period is slightly increased in radiotherapy plans of lung tumors with low tissue density to reduce these differences. The purpose of this study is to examine the impact of IDC on radiotherapy planning for the heterogeneous maxillary sinus region. Methods In this study, the homogeneity index (HI) and the conformity index (CI) of the intensity modulated radiotherapy plans prepared using Eclipse-TPS and AAA v8.9 of 12 patients with maxillary sinus cancer were calculated. The original plans were re-optimized with the same optimization criteria by using AAA v15.1 with and without IDC. HI and CI values of the new plans were calculated after re-optimization. The HI and CI values, maximum doses and the critical organ doses were compared between the original and new plans. Results AAA v15.1 with IDC increased the plan quality in terms of HI, CI and maximum dose values. The HI values (ideal value=0) were found to be 0.093, 0.090 and 0.067; the CI values (ideal value=1) were found to be 1.149, 1.142 and 1.055; the maximum doses were found to be 108.1%, 107.8% and 105.3% for AAA v8.9, AAA v15.1 without IDC and AAA v15.1 with IDC, respectively. The differences between AAA v15.1 with IDC and the others are statistically significant for HI values, CI values and maximum doses but there is no significant difference for the doses received by critical organs. Conclusions The use of the IDC increases the quality of intensity modulated radiotherapy plans in heterogeneous regions such as head and neck. This work was supported by Scientific Research Projects Coordination Unit of Istanbul University (Project Number: I.U.BAP-23057)

P163] Dosimetric effects from high density markers for prostate cancer treatments

Purpose Implanted high density gold markers are used in radiotherapy of prostate cancer for a more accurate delivery of planned target dose. However, the presence of high density objects inside the prostate causes artefacts in the computed tomography (CT) images from which the dose calculations are based on. The interpretation of the tissues made by the different dose calculation algorithms becomes uncertain and this uncertainty may propagate to the final dose distribution. The aim of this work was to quantify the dosimetric effects from high density markers on dose distributions for prostate cancer treatments using three different algorithms. Methods About 200 patients treated with volumetric modulated arc therapy (VMAT) for prostate cancer were included. Dose calculations (dose to water) were executed using the treatment planning system (TPS) algorithms, anisotropic analytical algorithm (AAA), Acuros XB (AXB) and by an automated Monte Carlo (MC) system based on the EGSnrc code package with modifications. In order to quantify the effect of high density markers, dose distributions for prostate plans (post-operative and with gold markers) were compared. Dose volume histogram (DVH) estimates such as the mean dose to the clinical target volume (CTV), the planning target volume (PTV), D 98 % PTV and D 2 % PTV (dose to xx% of the PTV) were evaluated. Further analysis was performed by modifying the artefact areas in the CT images by setting the material in the PTV to water. Results The mean difference between AAA, AXB and MC estimations of the mean dose to the CTV and PTV was within 0.5% for all cases. Also, mean dose deviations up to 2% were observed for individual plans, especially for plans with gold markers. Dose deviations up to 5%, related to the shape of the DVH, were detected for D 98 % PTV and D 2 % PTV. The size of the CTV showed no impact on dose deviations for prostate plans with gold markers. Conclusions The presence of gold markers increases the variation between CTV and PTV dosimetry parameters obtained by different algorithms. However, no clinically relevant dosimetric effects arise from artefacts caused by high density markers.

OA037] Advanced dose calculation algorithms in lung cancer radiotherapy: Implications when treating in deep inspiration breath hold

Purpose Modern dose calculation algorithms model absence of lateral charged particle equilibrium to a limited extent. The resulting dose calculation uncertainties are most noticeable in strongly heterogeneous regions, like the thorax, and will increase in deep inspiration breath hold (DIBH) due to decreased lung tissue density. Methods For 17 stage I and 17 stage III lung cancer patients, a plan in free breathing (FB, based on midventilation) and in DIBH were generated with Anisotropic Analytical Algorithm (AAA). Stage I disease was treated with 3D-conformal stereotactic radiotherapy (SBRT), 45 Gy in 3 fractions, prescribed to 95% isodose covering 95% of PTV and aiming for 140% dose centrally in the tumour. Stage III disease was treated with volumetric modulated arc therapy (VMAT), 66 Gy in 33 fractions, prescribed to mean PTV dose. Calculation grid size was 1 mm for stage I and 2.5 mm for stage III. All plans were recalculated with AcurosXB with same MU as in AAA, for comparison on target coverage and dose to risk organs. Results Lung volume increase in DIBH resulted in 6% decreased lung density for stage I (from median −757 HU to −811 HU) and 12% for stage III (from median −723 HU to −822 HU). In stage I, AAA overestimated all PTV parameters (p-values Conclusions Choice of calculation algorithm impacts the calculated dose distribution in the target. AAA overestimated target coverage compared to AcurosXB, especially in DIBH for stage I lung cancer treated stereotactically.

P184] Proposed solution to improve the protection of rectum in VMAT prostate treatments

Purpose Volumetric modulated arc therapy (VMAT) is a technique widely used in prostate cancer nowadays, which decreases the delivery time and the dose in the organs at risk. The goal of this work is to achieve a higher dose gradient between the PTV and rectum, meanwhile, decrease the optimization time in our treatment planning system (TPS). Methods Our TPS is Eclipse (Varian Medical System, Palo Alto, California). The VMAT optimizer is Progressive Resolution Optimizer (PRO v13.06.26) and the calculation algorithm is Anisotropic Analytical Algorithm (AAA v13.06.26). Instead the initial state of the multi-leaf collimator (MLC) given by the optimizer, we try to find one more adapted to the organs delimited by the physician. To obtain this initial state, an arc therapy treatment was generated fitting the MLC to the PTV in the beam’s eye view (BEV) with a 5 mm of margin and protecting the rectum without margin, as well. We propose to use this arc therapy plan as an initial plan when optimization starts and only perform the last stages of VMAT optimization, assigning priority only to the objectives of the PTV and the bladder. Results Our group has used this method for more than 20 concomitant and single volume treatments. We have compared the results of our method with those obtained by the usual method of optimization. In both cases, we delivered the dose with a single arch. The coverage of the PTV is similar in both solutions, but the protection of the rectum in low and medium doses is notably better with our technique. The MLC transitions between adjacent Gantry angles become smoother using our solution. The differences between the dose to the bladder and to the femoral heads in both methods are not remarkable. The time used to perform an optimization with the new technique is significantly lower. Conclusions The proposed method increases the protection of the rectum in prostate treatments, decreases the optimization time and simplifies the table of restrictions in the optimizer.

OA036] Clinical implementation and evaluation of the Mobius3D system for independent dose calculation

Purpose The complexity of dose planning and treatment delivery systems has increased and necessitates a more comprehensive physics quality assurance (QA). These QA checks can, however, be quite time consuming. Therefore, there is a need for automation of QA which usually rely on empirically established tolerance levels. Mobius3D is a commercial fully automated QA system that uses a collapsed cone convolution/superposition algorithm for verification of treatment plans. The purpose of this study was to investigate the level of deviation between this system and the clinical commissioned treatment planning system (Eclipse v.13.5) using the Anisotropic Analytical Algorithm when the MLC modeling was optimized. Methods The Mobius3D system comes with golden beam data and calculates the dose on the patient CT data set. For MLC optimization, 8 IMRT/VMAT plans from various anatomical regions were included and recalculated in a cylindrical plastic phantom containing two planar diode arrays (Delta4 phantom, ScandiDos). The plans were then verified experimentally at the accelerator and all passed a local 3%/2 mm gamma criteria (passing rate > 90%). The dosimetric leaf gap (DLG) used to model the MLC in Mobius3D was optimized to keep the PTV mean dose of the 8 plans within 2% in the Delta4 geometry resulting in a +0.6 mm adjustment of the DLG from the default value. To establish the level of dose deviation and establish acceptance criteria, 34 treatment plans in different geometries, such us lung (10), head and neck (HN, 10), and pelvis (14) were evaluated for both IMRT and VMAT. Results The absolute mean and max PTV dose deviation for (lung/HN/pelvis) were: 0.8/2.2/2.1% and 1.4/2.9/2.8%. All max deviations were positive, i.e. a higher dose estimate by Mobius3D. 5 HN IMRT plans before DLG adjustment all showed negative deviations of −3.12/−3.8% (PTV mean/max). Conclusions A random sample of treatment plans were within 2.5% in absolute mean dose of the PTV between Mobius3D and the TPS. MLC parameter optimization plays an important role in establishing an acceptance criterion.

P298] The influence of variations of the beam quality for high energy photon beams on tissue inhomogeneity correction factors for radiotherapy treatment plans

Purpose The purpose of this study was to assess the effect of variations of the beam quality (TPR20,10) for photon beams on tissue inhomogeneity correction factors (ICFs). Methods A total of 90 three-dimensional conformal radiotherapy (3DCRT) treatment plans for three locations (3DCRT lung, gynaecological, prostate) and 15 SRS lung plans were considered for this study. For 3DCRT plans, the ICFs were calculated for a range of beam qualities. For 6 MV, the range of TPR20,10 was 0.670 ± 3%, and for 15 MV, the range of TPR20,10 was 0.760 ± 3%. The stereotactic plans were prepared with two 6 MV beams from a Varian TrueBeam accelerator - one generated with a flattening filter (X6, TPR20,10 = 0.688) and one flattening filter free (X6FFF, TPR20,10 = 0.632). Calculations were performed using Eclipse treatment planning system (TPS) with the anisotropic analytical algorithm (AAA). ICFs were also measured in a CIRS Tissue Simulation Phantom (thorax with lungs) for two fields 5 × 5 and 10 × 10 for X6 and X6FFF beams. PTW Farmer and semiflex ionization chambers were used for measurements. Results Calculations for 3DCRT plans showed that the 6% variations in energy (TPR20,10) lea to average changes of ICFs of 2.0% (6 MV) and 2.6% (15 MV) for lung cases. For gynaecological and prostate patients, the mean differences of ICFs were less than 1%. The difference of 5.6% of TPR20,10 values between X6 and X6 FFF for lung SRS treatment plans led to a difference of ICFs of less than 2%. Measurements with the CIRS phantom also demonstrated differences of ICFs consistently less than 5% between X6 and X6 FFF beams. Conclusions The influence of energy on inhomogeneity correction factors is rather small. Correction factors measured on one accelerator may be used for another accelerator with similar energy provided quality control tests are performed.

P224] Analytical anisotropic algorithm vs Acuros algorithm comparison

Purpose The aim of this work is to compare Acuros and Analytical Anisotropic Algorithm (AAA), focussing on tissue interfaces. Methods 20 head and neck pathology treatment plans were calculated with both algorithms. Density heterogeneities and air tissue interfaces in the lungs and oral cavity have been analysed. Radiotherapy Treatment Planning System (TPS) Eclipse version 13.7 was used. The grid size set was 0.25 cm. Dose matrix were exported from the RTP and compared with gamma 3D evaluation method with different distance and dose parameters using the software VeriSoft version 5.1. For every treatment plan gamma 3D analysis was performed in two dose planes (head and lung) to check dose differences. Values under 20% of the maximum dose are excluded of the comparison. Γ(3/1) and Γ(1/3) were also performed to evaluate dependence on dose and distance. Results The Γ(2/2) in 80% of cases between both dose distribution was higher than 95%. Γ(3/3) was higher than 100% for all the cases tested in the case of the head. For the case of the lung, 90% of cases Γ(2/2) between both dose distribution was higher than 95%. Γ(3/3) was higher than 100% for all the cases tested. Γ(3/1) vs. Γ(1/3) averages were 91.3% vs. 77% for head and 91.8% vs. 56.4% for lung. Conclusions Dose difference is much more dependent on the distance of the points under the Γ criteria than on the dose. Main differences are obtained at the skin, dose calculated with the Acuros algorithm is higher than the values with AAA algorithm, nevertheless maximum dose values are higher when the AAA algorithm is used. No significant dose difference has been observed in lungs.

Comparison of normal tissue dose calculation methods for epidemiological studies of radiotherapy patients

Radiation dosimetry is an essential input for epidemiological studies of radiotherapy patients aimed at quantifying the dose–response relationship of late-term morbidity and mortality. Individualised organ dose must be estimated for all tissues of interest located in-field, near-field, or out-of-field. Whereas conventional measurement approaches are limited to points in water or anthropomorphic phantoms, computational approaches using patient images or human phantoms offer greater flexibility and can provide more detailed three-dimensional dose information. In the current study, we systematically compared four different dose calculation algorithms so that dosimetrists and epidemiologists can better understand the advantages and limitations of the various approaches at their disposal. The four dose calculations algorithms considered were as follows: the (1) Analytical Anisotropic Algorithm (AAA) and (2) Acuros XB algorithm (Acuros XB), as implemented in the Eclipse treatment planning system (TPS); (3) a Monte Carlo radiation transport code, EGSnrc; and (4) an accelerated Monte Carlo code, the x-ray Voxel Monte Carlo (XVMC). The four algorithms were compared in terms of their accuracy and appropriateness in the context of dose reconstruction for epidemiological investigations. Accuracy in peripheral dose was evaluated first by benchmarking the calculated dose profiles against measurements in a homogeneous water phantom. Additional simulations in a heterogeneous cylinder phantom evaluated the performance of the algorithms in the presence of tissue heterogeneity. In general, we found that the algorithms contained within the commercial TPS (AAA and Acuros XB) were fast and accurate in-field or near-field, but not acceptable out-of-field. Therefore, the TPS is best suited for epidemiological studies involving large cohorts and where the organs of interest are located in-field or partially in-field. The EGSnrc and XVMC codes showed excellent agreement with measurements both in-field and out-of-field. The EGSnrc code was the most accurate dosimetry approach, but was too slow to be used for large-scale epidemiological cohorts. The XVMC code showed similar accuracy to EGSnrc, but was significantly faster, and thus epidemiological applications seem feasible, especially when the organs of interest reside far away from the field edge.

A fast jaw-tracking model for VMAT and IMRT Monte Carlo simulations.

Modern radiotherapy techniques involve routine use of volumetric arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) with jaw‐tracking – dynamic motion of the secondary collimators (jaws) in tandem with multi‐leaf collimators (MLCs). These modalities require accurate dose calculations for the purposes of treatment planning and dose verification. Monte Carlo (MC) methods for radiotherapy dose calculation are widely accepted as capable of achieving high accuracy. This paper presents an efficiency‐enhancement method for secondary collimator modeling, presented in the context of a tool for MC‐based dose second checks. The model constitutes an accuracy trade‐off in the source model for the sake of efficiency enhancement, but maintains the advantages of MC transport in patient heterogeneities. The secondary collimator model is called Flat‐Absorbing‐Jaw‐Tracking (FAJT). Transmission through and scatter from the secondary collimators is neglected, and jaws are modeled as perfectly absorbing planes. To couple the motion of secondary collimators with MLCs for jaw‐tracking, the FAJT model was built into the VCU‐MLC model. Gamma‐index analysis of the dose distributions from FAJT against the full BEAMnrc MC simulations showed over 99% pass rate for a range of open fields, two clinical IMRT, and one VMAT treatment plan, for 2%/2 mm criteria above 10%. Using FAJT, the simulation speed of the secondary collimators for open fields increased by a factor of 237, 1489, and 1395 for 4 × 4, 10 × 10, and 30 × 30 cm2, respectively. In general, clinically oriented simulation times are reduced from “hours” to “minutes” on identical hardware. Results for nine representative clinical cases (seven with jaw‐tracking) are presented. The average 2%/2 mm γ‐test success rate above the 80% isodose was 96.8% when tested against the EPIDose electronic portal image‐based dose reconstruction method and 97.3% against the Eclipse analytical anisotropic algorithm.