Anisotropic Analytical Algorithm Algorithm Research Articles

SU-D-BRB-07: Lipiodol Impact On Dose Distribution in Liver SBRT After TACE

Purpose: Stereotactic body radiotherapy (SBRT) combining transarterial chemoembolization (TACE) with Lipiodol is expected to improve local control. This study aims to evaluate the impact of Lipiodol on dose distribution by comparing the dosimetric performance of the Acuros XB (AXB) algorithm, anisotropic analytical algorithm (AAA), and Monte Carlo (MC) method using a virtual heterogeneous phantom and a treatment plan for liver SBRT after TACE. Methods: The dose distributions calculated using AAA and AXB algorithm, both in Eclipse (ver. 11; Varian Medical Systems, Palo Alto, CA), and EGSnrc-MC were compared. First, the inhomogeneity correction accuracy of the AXB algorithm and AAA was evaluated by comparing the percent depth dose (PDD) obtained from the algorithms with that from the MC calculations using a virtual inhomogeneity phantom, which included water and Lipiodol. Second, the dose distribution of a liver SBRT patient treatment plan was compared between the calculation algorithms. Results In the virtual phantom, compared with the MC calculations, AAA underestimated the doses just before and in the Lipiodol region by 5.1% and 9.5%, respectively, and overestimated the doses behind the region by 6.0%. Furthermore, compared with the MC calculations, the AXB algorithm underestimated the doses just before and in the Lipiodol region by 4.5% and 10.5%, respectively, and overestimated the doses behind the region by 4.2%. In the SBRT plan, the AAA and AXB algorithm underestimated the maximum doses in the Lipiodol region by 9.0% in comparison with the MC calculations. In clinical cases, the dose enhancement in the Lipiodol region can approximately 10% increases in tumor dose without increase of dose to normal tissue. Conclusion: The MC method demonstrated a larger increase in the dose in the Lipiodol region than the AAA and AXB algorithm. Notably, dose enhancement were observed in the tumor area; this may lead to a clinical benefit.

SU‐E‐T‐317: Dosimetric Evaluation of Acuros XB Advanced Dose Calculation Algorithm in Head and Neck Patients

Purpose:The Acuros XB photon dose calculation algorithm is a newly implemented calculation technique within the Eclipse treatment planning system using deterministic solutions to the linear Boltzmann transport equations. The goal of this study is to assess the clinical impact of dose differences arising from a retrospective comparison of calculations performed using the Analytical Anisotropic Algorithm (AAA) and Acuros XB on patients.Methods:Ten head and neck patients receiving intensity modulated radiation therapy were selected as a pilot study. Initial evaluation was based on the percentage of the planning target volume (PTV) covered by the prescription dose, minimum dose within the PTV, and dose differences in critical structures. For patients receiving boost plans, dosimetric evaluations were performed on the plan sum of the primary and boost plans.Results:Among the ten patients there were a total of 21 PTVs corresponding to primary and boost volumes. Using the same normalization within Eclipse, the average percentage of the PTVs receiving the prescription dose were 95.6% for AAA and Acuros XB. The average minimum doses within the PTVs, expressed as a percentage of the prescription to the volume, were 82.3% and 83.6% for AAA and Acuros XB respectively. Neither comparison showed differences with statistical significance when subjected to a paired t‐test. Statistical significance was found in the average difference of the maximum dose for the mandible (242.5cGy, p=0.0005) and cord with a 5mm radial expansion (105.0cGy, p=0.0005) and in the median dose for the left parotid (25.0cGy, p=0.0423) and oral cavity (36.3cGy, p=0.002).Conclusion:The Acuros XB dose calculation algorithm did not exhibit significant differences in PTV coverage when compared to the AAA algorithm. Significant differences in critical structures are likely attributed to the structures proximity to high atomic number materials or air cavities, regions of known difficulty for the AAA algorithm.

A phantom study on the behavior of Acuros XB algorithm in flattening filter free photon beams

To study the behavior of Acuros XB algorithm for flattening filter free (FFF) photon beams in comparison with the anisotropic analytical algorithm (AAA) when applied to homogeneous and heterogeneous phantoms in conventional and RapidArc techniques. Acuros XB (Eclipse version 10.0, Varian Medical Systems, CA, USA) and AAA algorithms were used to calculate dose distributions for both 6X FFF and 10X FFF energies. RapidArc plans were created on Catphan phantom 504 and conventional plans on virtual homogeneous water phantom 30 × 30 × 30 cm3, virtual heterogeneous phantom with various inserts and on solid water phantom with air cavity. Dose at various inserts with different densities were measured in both AAA and Acuros algorithms. The maximum % variation in dose was observed in (−944 HU) air insert and minimum in (85 HU) acrylic insert in both 6X FFF and 10X FFF photons. Less than 1% variation observed between −149 HU and 282 HU for both energies. At −40 HU and 765 HU Acuros behaved quite contrarily with 10X FFF. Maximum % variation in dose was observed in less HU values and minimum variation in higher HU values for both FFF energies. Global maximum dose observed at higher depths for Acuros for both energies compared with AAA. Increase in dose was observed with Acuros algorithm in almost all densities and decrease at few densities ranging from 282 to 643 HU values. Field size, depth, beam energy, and material density influenced the dose difference between two algorithms.

Photon beam dosimetry with EBT3 film in heterogeneous regions: Application to the evaluation of dose-calculation algorithms

For a better understanding of the accuracy of state-of-the-art-radiation therapies, 2-dimensional dosimetry in a patient-like environment will be helpful. Therefore, the dosimetry of EBT3 films in non-water-equivalent tissues was investigated, and the accuracy of commercially-used dose-calculation algorithms was evaluated with EBT3 measurement. Dose distributions were measured with EBT3 films for an in-house-designed phantom that contained a lung or a bone substitute, i.e., an air cavity (3 × 3 × 3 cm3) or teflon (2 × 2 × 2 cm3 or 3 × 3 × 3 cm3), respectively. The phantom was irradiated with 6-MV X-rays with field sizes of 2 × 2, 3 × 3, and 5 × 5 cm2. The accuracy of EBT3 dosimetry was evaluated by comparing the measured dose with the dose obtained from Monte Carlo (MC) simulations. A dose-to-bone-equivalent material was obtained by multiplying the EBT3 measurements by the stopping power ratio (SPR). The EBT3 measurements were then compared with the predictions from four algorithms: Monte Carlo (MC) in iPlan, acuros XB (AXB), analytical anisotropic algorithm (AAA) in Eclipse, and superposition-convolution (SC) in Pinnacle. For the air cavity, the EBT3 measurements agreed with the MC calculation to within 2% on average. For teflon, the EBT3 measurements differed by 9.297% (±0.9229%) on average from the Monte Carlo calculation before dose conversion, and by 0.717% (±0.6546%) after applying the SPR. The doses calculated by using the MC, AXB, AAA, and SC algorithms for the air cavity differed from the EBT3 measurements on average by 2.174, 2.863, 18.01, and 8.391%, respectively; for teflon, the average differences were 3.447, 4.113, 7.589, and 5.102%. The EBT3 measurements corrected with the SPR agreed with 2% on average both within and beyond the heterogeneities with MC results, thereby indicating that EBT3 dosimetry can be used in heterogeneous media. The MC and the AXB dose calculation algorithms exhibited clinically-acceptable accuracy (<5%) in heterogeneities.

The accuracy of the out-of-field dose calculations using a model based algorithm in a commercial treatment planning system

The out-of-field dose can be clinically important as it relates to the dose of the organ-at-risk, although the accuracy of its calculation in commercial radiotherapy treatment planning systems (TPSs) receives less attention. This study evaluates the uncertainties of out-of-field dose calculated with a model based dose calculation algorithm, anisotropic analytical algorithm (AAA), implemented in a commercial radiotherapy TPS, Varian Eclipse V10, by using Monte Carlo (MC) simulations, in which the entire accelerator head is modeled including the multi-leaf collimators. The MC calculated out-of-field doses were validated by experimental measurements. The dose calculations were performed in a water phantom as well as CT based patient geometries and both static and highly modulated intensity-modulated radiation therapy (IMRT) fields were evaluated. We compared the calculated out-of-field doses, defined as lower than 5% of the prescription dose, in four H&N cancer patients and two lung cancer patients treated with volumetric modulated arc therapy (VMAT) and IMRT techniques. The results show that the discrepancy of calculated out-of-field dose profiles between AAA and the MC depends on the depth and is generally less than 1% for in water phantom comparisons and in CT based patient dose calculations for static field and IMRT. In cases of VMAT plans, the difference between AAA and MC is <0.5%. The clinical impact resulting from the error on the calculated organ doses were analyzed by using dose-volume histograms. Although the AAA algorithm significantly underestimated the out-of-field doses, the clinical impact on the calculated organ doses in out-of-field regions may not be significant in practice due to very low out-of-field doses relative to the target dose.

SU‐E‐T‐481: Dosimetric Comparison of Acuros XB and Anisotropic Analytic Algorithm with Commercial Monte Carlo Based Dose Calculation Algorithm for Stereotactic Body Radiation Therapy of Lung Cancer

Purpose:To evaluate performance of three commercially available treatment planning systems for stereotactic body radiation therapy (SBRT) of lung cancer using the following algorithms: Boltzmann transport equation based algorithm (AcurosXB AXB), convolution based algorithm Anisotropic Analytic Algorithm (AAA); and Monte Carlo based algorithm (XVMC).Methods:A total of 10 patients with early stage non‐small cell peripheral lung cancer were included. The initial clinical plans were generated using the XVMC based treatment planning system with a prescription of 54Gy in 3 fractions following RTOG0613 protocol. The plans were recalculated with the same beam parameters and monitor units using AAA and AXB algorithms. A calculation grid size of 2mm was used for all algorithms. The dose distribution, conformity, and dosimetric parameters for the targets and organs at risk (OAR) are compared between the algorithms.Results:The average PTV volume was 19.6mL (range 4.2–47.2mL). The volume of PTV covered by the prescribed dose (PTV‐V100) were 93.97±2.00%, 95.07±2.07% and 95.10±2.97% for XVMC, AXB and AAA algorithms, respectively. There was no significant difference in high dose conformity index; however, XVMC predicted slightly higher values (p=0.04) for the ratio of 50% prescription isodose volume to PTV (R50%). The percentage volume of total lungs receiving dose &gt;20Gy (LungV20Gy) were 4.03±2.26%, 3.86±2.22% and 3.85±2.21% for XVMC, AXB and AAA algorithms. Examination of dose volume histograms (DVH) revealed small differences in targets and OARs for most patients. However, the AAA algorithm was found to predict considerable higher PTV coverage compared with AXB and XVMC algorithms in two cases. The dose difference was found to be primarily located at the periphery region of the target.Conclusion:For clinical SBRT lung treatment planning, the dosimetric differences between three commercially available algorithms are generally small except at target periphery. XVMC and AXB algorithms are recommended for accurate dose estimation at tissue boundaries.

SU‐E‐T‐313: The Accuracy of the Acuros XB Advanced Dose Calculation Algorithm for IMRT Dose Distributions in Head and Neck

Purpose:To investigate the accuracy of the Acuros XB version 11 (AXB11) advanced dose calculation algorithm by comparing with Monte Caro (MC) calculations. The comparisons were performed with dose distributions for a virtual inhomogeneity phantom and intensity‐modulated radiotherapy (IMRT) in head and neck.Methods:Recently, AXB based on Linear Boltzmann Transport Equation has been installed in the Eclipse treatment planning system (Varian Medical Oncology System, USA). The dose calculation accuracy of AXB11 was tested by the EGSnrc‐MC calculations. In additions, AXB version 10 (AXB10) and Analytical Anisotropic Algorithm (AAA) were also used. First the accuracy of an inhomogeneity correction for AXB and AAA algorithms was evaluated by comparing with MC‐calculated dose distributions for a virtual inhomogeneity phantom that includes water, bone, air, adipose, muscle, and aluminum. Next the IMRT dose distributions for head and neck were compared with the AXB and AAA algorithms and MC by means of dose volume histograms and three dimensional gamma analysis for each structure (CTV, OAR, etc.).Results:For dose distributions with the virtual inhomogeneity phantom, AXB was in good agreement with those of MC, except the dose in air region. The dose in air region decreased in order of MC&lt;AXB11&lt;AXB10. This may be caused by the difference of the electron cut‐off energy for algorithms, ie: 0.700 MeV for MC, 0.711 MeV for AXB11, and 1.011 MeV for AXB 10. Since the AAA algorithm is based on the dose kernel of water, the doses in regions for air, bone, and aluminum considerably became higher than those of AXB and MC. The pass rates of the gamma analysis for IMRT dose distributions in head and neck were similar to those of MC in order of AXB11&lt;AXB10&lt;AAA.Conclusion:The dose calculation accuracy of AXB11 was almost equivalent to the MC dose calculation.

SU-E-T-137: Dosimetric Validation for Pinnacle, Acuros, AAA, and Brainlab Algorithms with Induced Inhomogenieties

Purpose: To compare the dosimetric accuracy of the Eclipse 11.0 Acuros XB and Anisotropic Analytical Algorithm (AAA), Pinnacle-3 9.2 Collapsed Cone Convolution, and the iPlan 4.1 Monte Carlo (MC) and Pencil Beam (PB) algorithms using measurement as the gold standard. Methods: Ion chamber and diode measurements were taken for 6, 10, and 18 MV beams in a phantom made up of slab densities corresponding to solid water, lung, and bone. The phantom was setup at source-to-surface distance of 100 cm, and the field sizes were 3.0 × 3.0, 5.0 × 5.0, and 10.0 × 10.0 cm2. Data from the planning systems were computed along the central axis of the beam. The measurements were taken using a pinpoint chamber and edge diode for interface regions. Results: The best agreement between data from the algorithms and our measurements occurs away from the slab interfaces. For the 6 MV beam, iPlan 4.1 MC software performs the best with 1.7% absolute average percent difference from measurement. For the 10 MV beam, iPlan 4.1 PB performs the best with 2.7% absolute average percent difference from measurement. For the 18 MV beam, Acuros performs the best with 2.0% absolute average percent difference from measurement. It is interesting to note that the steepest drop in dose occurred the at lung heterogeneity-solid water interface of the18 MV, 3.0 × 3.0 cm2 field size setup. In this situation, Acuros and AAA performed best with an average percent difference within −1.1% of measurement, followed by iPlan 4.1 MC, which was within 4.9%. Conclusion: This study shows that all of the algorithms perform reasonably well in computing dose in a heterogeneous slab phantom. Moreover, Acuros and AAA perform particularly well at the lung-solid water interfaces for higher energy beams and small field sizes.

SU-E-T-219: Investigation of IMRT Out-Of-Field Dose Calculation Accuracy for a Commercial Treatment Planning System

Medical PhysicsVolume 41, Issue 6Part14 p. 273-273 Fifty-sixth annual meeting of the American association of physicists in medicine SU-E-T-219: Investigation of IMRT Out-Of-Field Dose Calculation Accuracy for a Commercial Treatment Planning System First published: 29 May 2014 https://doi.org/10.1118/1.4888549 Vanderbilt University, Nashville, TN About ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Purpose: Inaccuracies in out-of-field calculations could lead to underestimation of dose to organs-at-risk. This study evaluates the dose calculation accuracy of a model-based calculation algorithm at points outside the primary treatment field for an intensity modulated radiation therapy (IMRT) plan using experimental measurements. Methods: The treatment planning system investigated is Varian Eclipse V.10 with Analytical Anisotropic Algorithm (AAA). The IMRT fields investigated are from real patient treatment plans. The doses from a dynamic (DMLC) IMRT brain plan were calculated and compared with measured doses at locations outside the primary treatment fields. Measurements were performed with a MatriXX system (2-D chamber array) placed in solid water. All fields were set vertically incident on the phantom and were 9 cm × 6 cm or smaller. The dose was normalized to the central axis for points up to 15 cm off isocenter. The comparisons were performed at depths of 2, 10, 15, and 20 cm Results: The measurements have shown that AAA calculations underestimate doses at points outside the primary treatment field. The underestimation occurs at 2 cm depth and decreases down to a factor of 2 as depth increases to 20 cm. In low dose (<2% of target dose) regions outside the primary fields the local dose underestimations can be >200% compared to measured doses. Relative to the plan target dose, the measured doses to points outside the field were less than 1% at shallow depths and less than 2% at greater depths. Conclusion: Compared to measurements, the AAA algorithm underestimated the dose at points outside the treatment field with the greatest differences observed at shallow depths. Despite large local dose uncertainties predicted by the treatment planning system, the impact of these uncertainties is expected to be insignificant as doses at these points were less than 1-2% of the prescribed treatment dose. Volume41, Issue6Part14June 2014Pages 273-273 RelatedInformation

Irradiation of lung and esophagus tumors: A comparison of dose distributions calculated by anisotropic analytical algorithm and pencil beam convolution algorithm, a retrospective dosimetric study

Purpose: The aim of this study was to evaluate dosimetric differences between pencil beam convolution (PBC) algorithm and anisotropic analytical algorithm (AAA) calculations in patients with lung and esophageal cancers. Methods : The existing plans calculated with PBC for 60 patients treated in 2012 were recalculated with AAA maintaining the same beam geometry and dose monitor units. For these plans, dose prescription ranges were 41.4 Gy to 56.0 Gy for esophageal cancers and 50.0 Gy to 64.0 Gy for lung cancers. Dosimetric variables were the 95% PTV coverage, mean PTV dose, maximum spinal cord dose, lung V 5Gy , and lung V 20Gy . Results: The 95% PTV coverage’s for both lung and esophageal tumors were reduced when recalculated with AAA. Maximum spinal cord doses for lung cancer patients were reduced by 0.7 Gy and by 0.3 Gy for esophageal cancer patients on AAA. On the other hand, lung V 5Gy had 3.5% increase for both lung and esophageal cancer patients on AAA, whereas lung V 20Gy increased by 1.5% also on AAA for esophageal cancer patients. Conclusion : These clinical results confirm the differences between AAA and PBC algorithms as observed in phantom dosimetric studies, and give an indication of the clinical implications of changing from one calculation algorithm to another. --------------------------------------- Cite this article as: Amankwaa-Frempong E, Vernimmen F, Blay S, Ezhilalan R. Irradiation of lung and esophagus tumors: A comparison of dose distributions calculated by anisotropic analytical algorithm and pencil beam convolution algorithm, a retrospective dosimetric study. Int J Cancer Ther Oncol 2014; 2(2):020210. DOI: 10.14319/ijcto.0202.10

Development of New 4D Phantom Model in Respiratory Gated Volumetric Modulated Arc Therapy for Lung SBRT

†In stereotactic body radiotherapy (SBRT), the accurate location of treatment sites should be guaranteed from the respiratory motions of patients. Lots of studies on this topic have been conducted. In this letter, a new verification method simulating the real respiratory motion of heterogenous treatment regions was proposed to investigate the accuracy of lung SBRT for Volumetric Modulated Arc Therapy. Based on the CT images of lung cancer patients, lung phantoms were fabricated to equip in QUAS ARTM respiratory moving phantom using 3D printer. The phantom was bisected in order to measure 2D dose distributions by the insertion of EBT3 film. To ensure the dose calculation accuracy in heterogeneous condition, The homogeneous plastic phantom were also utilized. Two dose algorithms; Analytical Anisotropic Algorithm (AAA) and AcurosXB (AXB) were applied in plan dose calculation processes. In order to evaluate the accuracy of treatments under respiratory motion, we analyzed the gamma index between the plan dose and film dose measured under various moving conditions; static and moving target with or without gating. The CT number of GTV region was 78 HU for real patient and 92 HU for the homemade lung phantom. The gamma pass rates with 3%/3 mm cr iteria between the plan dose calculated by AAA algorithm and the film doses measured in heterogeneous l ung phantom under gated and no gated beam delivery with respiratory motion were 88% and 78%. In static case, 95% of gamma pass rate was presented. In the all cases of homogeneous phantom, the gamma pass rates were more than 99%. Applied AcurosXB algorithm, for heterogeneous phantom, more than 98% and for hom ogeneous phantom, more than 99% of gamma pass rates were achieved. Since the respiratory amplitude was relatively small and the breath pattern had the longer exhale phase than inhale, the gamma pass rates i n 3%/3 mm criteria didn’t make any significant difference for various motion conditions. In this study, the new phantom model of 4D dose distribution verification using patient-specific lung phantoms moving in real breathing patterns was successfully implemented. It was also evaluated that the model provides the capability to verify dose distributions delivered in the more realistic condition and also the accuracy of dose calculation.

Effect of Acuros XB algorithm on monitor units for stereotactic body radiotherapy planning of lung cancer

Stereotactic body radiotherapy (SBRT) is a curative regimen that uses hypofractionated radiation-absorbed dose to achieve a high degree of local control in early stage non–small cell lung cancer (NSCLC). In the presence of heterogeneities, the dose calculation for the lungs becomes challenging. We have evaluated the dosimetric effect of the recently introduced advanced dose-calculation algorithm, Acuros XB (AXB), for SBRT of NSCLC. A total of 97 patients with early-stage lung cancer who underwent SBRT at our cancer center during last 4 years were included. Initial clinical plans were created in Aria Eclipse version 8.9 or prior, using 6 to 10 fields with 6-MV beams, and dose was calculated using the anisotropic analytic algorithm (AAA) as implemented in Eclipse treatment planning system. The clinical plans were recalculated in Aria Eclipse 11.0.21 using both AAA and AXB algorithms. Both sets of plans were normalized to the same prescription point at the center of mass of the target. A secondary monitor unit (MU) calculation was performed using commercial program RadCalc for all of the fields. For the planning target volumes ranging from 19 to 375cm3, a comparison of MUs was performed for both set of algorithms on field and plan basis. In total, variation of MUs for 677 treatment fields was investigated in terms of equivalent depth and the equivalent square of the field. Overall, MUs required by AXB to deliver the prescribed dose are on an average 2% higher than AAA. Using a 2-tailed paired t-test, the MUs from the 2 algorithms were found to be significantly different (p < 0.001). The secondary independent MU calculator RadCalc underestimates the required MUs (on an average by 4% to 5%) in the lung relative to either of the 2 dose algorithms.

Clinical impact of using the deterministic patient dose calculation algorithm Acuros XB for lung stereotactic body radiation therapy

Purpose. To evaluate the clinical impact of using the deterministic dose calculation algorithm, Acuros XB, for early stage lung cancer patients undergoing stereotactic body radiotherapy (SBRT). Material and methods. Seventy-seven stage I non-small cell lung cancer patients who underwent lung SBRT from 2008 to 2012 at our center were included in this study. All treatment plans originally calculated by the anisotropic analytical algorithm (AAA) were recalculated using the AAA and Acuros XB algorithms with identical monitor units and beam arrangements. The dose, dose distribution, conformality number (CN) and heterogeneity index (HI) of the target were determined for each plan. A paired matched t-test was used to evaluate the difference between the mean dose, the dose distribution, and the CN and HI for the target. The importance of tumor (volume, location), patient (pulmonary functional, body mass index) and treatment (number of SBRT beams) on the dose distributions obtained from the two algorithms was statistically determined using linear regression analyses. Results. The mean target dose was same for both algorithms. Compared to AAA, a small and significant difference in dose distribution in the target was found for the Acuros XB algorithm, resulting in lower conformity (−2.1%, p < 0.0001) and higher heterogeneity (p < 0.0001) of dose. Single logistic regression identified pulmonary function, number of beams and target location as being correlated with the difference of CN between the two calculations. Multivariate analysis indicated that the patient's pulmonary function (p = 0.0296) was the only predictor for the difference in conformality between the two dose calculation algorithms. Conclusions. In lung SBRT, the patient's pulmonary function is responsible for the difference in target dose distribution between the Acuros XB and AAA algorithms. The Acuros XB algorithm could be used to advantage in patients with compromised pulmonary function based on its accurate modeling of lung tissue in comparison to AAA.

Critical appraisal of the accuracy of Acuros-XB and Anisotropic Analytical Algorithm compared to measurement and calculations with the compass system in the delivery of RapidArc clinical plans

BackgroundThe accuracy of the two dose calculation engines available for RapidArc planning (both released for clinical use) is investigated in comparison to the COMPASS data.MethodsTwo dose calculation algorithms (Acuros-XB and Anisotropic Analytic Algorithm (AAA)) were used to calculate RA plans and compared to calculations with the Collapsed Cone Convolution algorithm (CC) from the COMPASS system (IBA Dosimetry). CC calculations, performed on patient data, are based on experimental fluence measurements with a 2D array of ion chambers mounted on the linac head. The study was conducted on clinical cases treated with RA. Five cases for each of the following groups were included: Brain, Head and Neck, Thorax, Pelvis and stereotactic body radiation therapy for hypo-fractionated treatments with small fields. COMPASS measurements were performed with the iMatrixx-2D array. RapidArc plans were optimized for delivery using 6MV photons from a Clinac-iX (Varian, Palo Alto, USA).Accuracy of the RA calculation was appraised by means of: 1) comparison of Dose Volume histograms (DVH) metrics; 2) analysis of differential dose distributions and determination of mean dose differences per organ; 3) 3D gamma analysis with distance-to-agreement and dose difference thresholds set to 3%/3 mm or 2%/2 mm for targets, organs at risks and for the volumes encompassed by the 50 and 10% isodoses.ResultsFor almost all parameters, the better agreement was between Acuros-XB and COMPASS independently from the anatomical site and fractionation. The same result was obtained from the mean dose difference per organ with Acuros-CC average differences below 0.5% while for AAA-CC data, average deviations exceeded 0.5% and in the case of the pelvis 1%. Relevance of observed differences determined with the 3D gamma analysis resulted in a pass rate exceeding 99.5% for Acuros-CC and exceeding 97.5% for AAA-CC.ConclusionsThis study demonstrated that i) a good agreement exists between COMPASS-CC calculations based on measured fluences with respect to dose distributions obtained with both Acuros-XB and AAA algorithms; ii) 3D dose distributions reconstructed from actual delivery coincide very precisely with the planned data; iii) a slight preference in favor of Acuros-XB was observed suggesting the preference for this algorithm in clinical applications.

A comparison between anisotropic analytical and multigrid superposition dose calculation algorithms in radiotherapy treatment planning

Monte Carlo (MC) simulation is currently the most accurate dose calculation algorithm in radiotherapy planning but requires relatively long processing time. Faster model-based algorithms such as the anisotropic analytical algorithm (AAA) by the Eclipse treatment planning system and multigrid superposition (MGS) by the XiO treatment planning system are 2 commonly used algorithms. This study compared AAA and MGS against MC, as the gold standard, on brain, nasopharynx, lung, and prostate cancer patients. Computed tomography of 6 patients of each cancer type was used. The same hypothetical treatment plan using the same machine and treatment prescription was computed for each case by each planning system using their respective dose calculation algorithm. The doses at reference points including (1) soft tissues only, (2) bones only, (3) air cavities only, (4) soft tissue-bone boundary (Soft/Bone), (5) soft tissue-air boundary (Soft/Air), and (6) bone-air boundary (Bone/Air), were measured and compared using the mean absolute percentage error (MAPE), which was a function of the percentage dose deviations from MC. Besides, the computation time of each treatment plan was recorded and compared. The MAPEs of MGS were significantly lower than AAA in all types of cancers (p<0.001). With regards to body density combinations, the MAPE of AAA ranged from 1.8% (soft tissue) to 4.9% (Bone/Air), whereas that of MGS from 1.6% (air cavities) to 2.9% (Soft/Bone). The MAPEs of MGS (2.6%±2.1) were significantly lower than that of AAA (3.7%±2.5) in all tissue density combinations (p<0.001). The mean computation time of AAA for all treatment plans was significantly lower than that of the MGS (p<0.001). Both AAA and MGS algorithms demonstrated dose deviations of less than 4.0% in most clinical cases and their performance was better in homogeneous tissues than at tissue boundaries. In general, MGS demonstrated relatively smaller dose deviations than AAA but required longer computation time.

Algorithms Used in Heterogeneous Dose Calculations Show Systematic Differences as Measured With the Radiological Physics Center's Anthropomorphic Thorax Phantom Used for RTOG Credentialing

To determine the impact of treatment planning algorithm on the accuracy of heterogeneous dose calculations in the Radiological Physics Center (RPC) thorax phantom. We retrospectively analyzed the results of 304 irradiations of the RPC thorax phantom at 221 different institutions as part of credentialing for Radiation Therapy Oncology Group clinical trials; the irradiations were all done using 6-MV beams. Treatment plans included those for intensity-modulated radiation therapy (IMRT) as well as 3-dimensional conformal therapy (3D-CRT). Heterogeneous plans were developed using Monte Carlo (MC), convolution/superposition (CS), and the anisotropic analytic algorithm (AAA), as well as pencil beam (PB) algorithms. For each plan and delivery, the absolute dose measured in the center of a lung target was compared to the calculated dose, as was the planar dose in 3 orthogonal planes. The difference between measured and calculated dose was examined as a function of planning algorithm as well as use of IMRT. PB algorithms overestimated the dose delivered to the center of the target by 4.9% on average. Surprisingly, CS algorithms and AAA also showed a systematic overestimation of the dose to the center of the target, by 3.7% on average. In contrast, the MC algorithm dose calculations agreed with measurement within 0.6% on average. There was no difference observed between IMRT and 3D CRT calculation accuracy. Unexpectedly, advanced treatment planning systems (those using CS and AAA algorithms) overestimated the dose that was delivered to the lung target. This issue requires attention in terms of heterogeneity calculations and potentially in terms of clinical practice.

Algorithms Used in Heterogeneous Dose Calculations Show Systematic Error as Measured With the Radiological Physics Center's Anthropomorphic Thorax Phantom Used for RTOG Credentialing

To determine the impact of treatment planning algorithm on the accuracy of heterogeneous dose calculations in the Radiological Physics Center (RPC) thorax phantom. We retrospectively analyzed the results of 304 irradiations of the RPC thorax phantom at 221 different institutions as part of credentialing for RTOG clinical trials; the irradiations were all done using 6-MV beams. Heterogeneous treatment plans were developed using Monte Carlo algorithms (MC), convolution/superposition (CS) algorithms and the anisotropic analytic algorithm (AAA), as well as pencil beam (PB) algorithms. For each plan and delivery, the absolute dose measured in the center of a lung target, as well as planar dose measurements throughout the target, were compared to the calculated dose. The difference between measured and calculated dose was examined as a function of planning algorithm, as well as use of IMRT versus 3D conformal therapy, use of a motion platform versus static irradiation, and date of irradiation. Only minor differences were observed in the planar agreement between different algorithms. Absolute dose agreement showed sizeable and significant variation between algorithms. PB algorithms overestimated the dose delivered to the center of the target by 4.9% on average. Surprisingly, CS algorithms and AAA also showed a systematic overestimation of the dose to the center of the target, by 3.7% on average. In contrast, the MC algorithm dose calculations agreed with measurement within 0.6% on average. There was no significant difference observed between IMRT and 3D CRT calculation accuracy, between moving or static irradiations, or as a function of time (i.e., agreement has not improved in recent years). Unexpectedly, advanced treatment planning systems (those using CS and AAA algorithms) overestimated the dose that was delivered to the lung target. This large dosimetric difference requires attention in terms of heterogeneity calculations and potentially in terms of clinical practice.Oral Scientific Abstract 319; TableMean ratio and standard deviation of measured to calculated doseAlgorithmMonte CarloAAAPinnacleXioTomotherapyPBMean0.9940.9590.9680.9550.9710.951Standard Deviation0.0300.0270.0240.0250.0240.027 Open table in a new tab

SU-E-T-538: Evaluation of IMRT Dose Calculation Based on Pencil-Beam and AAA Algorithms.

To evaluate the accuracy of dose calculation for intensity modulated radiation therapy (IMRT) based on Pencil Beam (PB) and Analytical Anisotropic Algorithm (AAA) computation algorithms. IMRT plans of twelve patients with different treatment sites, including head/neck, lung and pelvis, were investigated. For each patient, dose calculation with PB and AAA algorithms using dose grid sizes of 0.5 mm, 0.25 mm, and 0.125 mm, were compared with composite-beam ion chamber and film measurements in patient specific QA. Discrepancies between the calculation and the measurement were evaluated by percentage error for ion chamber dose and γ〉l failure rate in gamma analysis (3%/3mm) for film dosimetry. For 9 patients, ion chamber dose calculated with AAA-algorithms is closer to ion chamber measurement than that calculated with PB algorithm with grid size of 2.5 mm, though all calculated ion chamber doses are within 3% of the measurements. For head/neck patients and other patients with large treatment volumes, γ〉l failure rate is significantly reduced (within 5%) with AAA-based treatment planning compared to generally more than 10% with PB-based treatment planning (grid size=2.5 mm). For lung and brain cancer patients with medium and small treatment volumes, γ〉l failure rates are typically within 5% for both AAA and PB-based treatment planning (grid size=2.5 mm). For both PB and AAA-based treatment planning, improvements of dose calculation accuracy with finer dose grids were observed in film dosimetry of 11 patients and in ion chamber measurements for 3 patients. AAA-based treatment planning provides more accurate dose calculation for head/neck patients and other patients with large treatment volumes. Compared with film dosimetry, a γ〉l failure rate within 5% can be achieved for AAA-based treatment planning.

SU‐E‐T‐353: Dosimetric Verification of Rectal Wall Sparing Effect in Intensity‐Modulated Radiation Therapy for Prostate Cancer Patients with and Without the Use of a Rectal Balloon

Purpose: To evaluate dosimetric effect of a rectal balloon and calculation accuracy due to dose perturbation by air cavities in the rectum, rectal wall doses with and without a rectal balloon were verified using rolled‐up and ‐ out techniques of radiochromic films in prostate cancer cases. Methods: The prescribed dose of 74 Gy in 37 fractions was delivered to the 6 patients with similar sized‐intact prostates using the same plan parameters and constraints in intensity‐modulated radiation therapy. The rectum was contoured from the anus to the rectosigmoid flexure for all divided patient groups according to the rectal filling status. Calculated rectal wall doses were evaluated by the analytical anisotropic algorithm (AAA) and the pencil beam convolution (PBC) algorithm. The acrylic inserts dedicated to measure rectal wall doses were developed for simulating the 70 cc air‐filled and the empty rectum. Rolled‐up Gafchromic EBT2 films are closely placed in the interspaces film layer thick between 2 hollow cylinders and in the innermost inside of the rectal wall 4 mm thick and 4.5 mm in diameter. Results: The AAA algorithm more accurately yielded perturbed doses near the air‐tissue interfaces by describing anterior rectal wall sparing and higher doses at the posterior rectal wall than estimated values in PBC. Delivered doses on rolled‐up films showed more than 93% gamma pass rate. Although the air‐filled rectum achieved rectal wall sparing at the doses very close to the prescribed one, the rectal wall volumes received 70% or adjacent to the prescribed dose were increased. Conclusion: Delivered rectal wall doses to the rolled‐up films suggested that the AAA algorithm reflected more accurate dosimetric effect by the air cavity in the rectum. The rectal balloon can be useful to spare the small fraction of rectal volume where high doses reaching up to the prescribed level were delivered.

SU-E-T-516: Dosimetric Validation of AcurosXB Algorithm in Comparison with AAA & CCC Algorithms for VMAT Technique.

To dosimetrically validate AcurosXB algorithm for Volumetric Modulated Arc Therapy (VMAT) in comparison with standard clinical Anisotropic Analytic Algorithm(AAA) and Collapsed Cone Convolution(CCC) dose calculation algorithms. AcurosXB dose calculation algorithm is available with Varian Eclipse treatment planning system (V10). It uses grid-based Boltzmann equation solver to predict dose precisely in lesser time. This study was made to realize algorithms ability to predict dose accurately as its delivery for which five clinical cases each of Brain, Head&Neck, Thoracic, Pelvic and SBRT were taken. Verification plans were created on multicube phantom with iMatrixx-2D detector array and then dose prediction was done with AcurosXB, AAA & CCC (COMPASS System) algorithm and the same were delivered onto CLINAC-iX treatment machine. Delivered dose was captured in iMatrixx plane for all 25 plans. Measured dose was taken as reference to quantify the agreement between AcurosXB calculation algorithm against previously validated AAA and CCC algorithm. Gamma evaluation was performed with clinical criteria distance-to-agreement 3&2mm and dose difference 3&2% in omnipro-I'MRT software. Plans were evaluated in terms of correlation coefficient, quantitative area gamma and average gamma. Study shows good agreement between mean correlation 0.9979±0.0012, 0.9984±0.0009 & 0.9979±0.0011 for AAA, CCC & Acuros respectively. Mean area gamma for criteria 3mm/3% was found to be 98.80±1.04, 98.14±2.31, 98.08±2.01 and 2mm/2% was found to be 93.94±3.83, 87.17±10.54 & 92.36±5.46 for AAA, CCC & Acuros respectively. Mean average gamma for 3mm/3% was 0.26±0.07, 0.42±0.08, 0.28±0.09 and 2mm/2% was found to be 0.39±0.10, 0.64±0.11, 0.42±0.13 for AAA, CCC & Acuros respectively. This study demonstrated that the AcurosXB algorithm had a good agreement with the AAA & CCC in terms of dose prediction. In conclusion AcurosXB algorithm provides a valid, accurate and speedy alternative to AAA and CCC algorithms in a busy clinical environment.