Photon Dose Calculation Algorithms Research Articles

End-to-End Verification and Dosimetric Comparative Study of Brainlab Monte Carlo and Eclipse Acuros XB Photon Dose Calculation Algorithm for VMAT SRS Planning

We commissioned the Brainlab Monte Carlo Dose Calculation Algorithm for VMAT SRS planning and verified it by measurements and comparisons with two other algorithms: Pencil beam and Eclipse Acuros. The end-to-end verification and dosimetric comparative study results are reported in this abstract. Brainlab Elements Cranial SRS (v3.0) provides single lesion planning using VMAT optimization, thus allowing dose modulation with MLC leaf positioning, dose rate, and gantry speed. We commissioned and performed quality assurance for the available dose calculation algorithms, including Pencil Beam (PB) algorithm with only reference beam data (RBM PB) and PB with full beam data (Elements PB), and Monte Carlo (MC) algorithm XVMC. Calculation between the two were compared to each other and to Acuros XB dose calculation algorithm by Varian. Open field MLC, static cone, dynamic MLC, and conical cone plans were calculated with each algorithm. IBA CC01 ionization chamber was used for each measurement, and small field correction factors as determined in TRT-483 were applied. End-to-end tests were performed using an anthropomorphic stereotactic end-to-end verification (STEEV) phantom from CIRS and a phantom/diode array, StereoPHAN/SRSMapCHECK from SunNuclear. Each phantom was CT scanned with 0.5 mm slice thickness. Four clinical plans were optimized in Brainlab Elements Treatment Planning System (TPS). Each plan was calculated with the following algorithms: Elements PB, RBM PB, MC, and Acuros XB with 1 mm dose grid size. The delivered dose was measured with a SunNuclear SRSMapCHECK diode array detector. The calculated dose was compared with the measured dose by Gamma analysis with DTA 1%/1mm, 2%/1mm, and 3%/1mm. The results of point dose measurements for field size larger than 1 × 1 cm2 among measured dose, dose calculated by pencil beam algorithms, and dose calculated by Monte Carlo algorithm were in excellent agreement (1.45% ± 0.7%). For the small rectangular field sizes (0.4 × 1, 0.2 × 1 cm2), Monte Carlo calculation showed superior accuracy and robustness to pencil beam algorithm when compared with measured dose, with percent dose difference 13% and -2.2%, respectively. Monte Carlo calculation have higher gamma results compared with pencil beam algorithm due more accurate calculations with heterogenous materials. Acuros XB showed comparable accuracy with MC in terms of gamma passing rate. The SRS MapCHECK measurement agrees dosimetrically with the plans calculated by Pencil Beam algorithm and the Monte Carlo algorithm XVMC by Brainlab, as well as the Acuros XB dose calculation algorithm by Varian. Point dose calculation for small field dosimetry and gamma analysis using SRS MapCHECK both showed that Monte Carlo algorithm is more robust and is superior in regions of heterogenous materials. Therefore, we recommend that Monte Carlo algorithm for dose calculation be used in clinical SRS and SRT cases.

Technical Report: Efficient constancy evaluation of the Eclipse treatment planning system's photon and electron dose calculation algorithms aided by the third-party ClearCheck software

Our institutional approach to performing periodic constancy checks on dose calculations for the Varian Eclipse treatment planning system is described, including details of how the Radformation ClearCheck software is leveraged for highly efficient results analysis. Photon- and electron-specific synthetic phantoms are created which allow all modality-specific fields to be spatially separated and calculated in a single plan, with field-specific structures encompassing individual irradiated volumes facilitating field-specific dose distribution evaluations. Plan comparison templates in ClearCheck allow for the near-instantaneous evaluation of 116 and 165 individual calculation metrics for photon and electron fields, respectively. The methodology is further applied to the Radformation ClearCalc finite-size pencil beam and electron algorithms for secondary algorithm validation. The total time required for constancy evaluations of 6 primary algorithms and 4 secondary algorithms is less than one hour, with much of that time used for passive dose calculation.

Evaluation of Manually Sculpted Trajectories and a New Trajectory Optimizer Dedicated to a Novel Patient Positioning System on a Truebeam Linac

The new patient positioning system is a robotized couch of Varian associated to a TrueBeam Linac This new robotic couch enables Volumetric Modulated Arc Therapy (VMAT) treatments optimization with new dynamic trajectories, thanks to precise, simultaneous and continuous movements of gantry, couch and collimators. The aim of this study was to evaluate the dosimetric improvement of these VMAT treatments compared with our common ones, by manual design of more general treatment trajectories in the treatment planning system (TPS). This study exposed the first results obtained for the treatment of intracranial localization, particularly for re-irradiation of relapsing glioblastoma, for which the proximity of organs at risk is a critical issue due to a previous irradiation or de novo glioblastoma irradiation. First, trajectories were designed manually in the beam’s eye view of the TPS. A script was used in which each control point of the trajectory contained a combination of gantry angle, collimator angle, couch angle, isocenter position and jaws size chosen to spare at best organs at risk while targeting the tumor. Photon Optimizer and dose calculation algorithms were adapted to take into account the movement of the couch. Second, dosimetric improvement of these new VMAT treatments realized thanks to new dynamic trajectories were evaluated and compared to VMAT or Dynamical Conformal Arc treatments usually performed in our center. Dynamic trajectories were first tested on six cases of glioblastoma and four cases of reirradiation. The comparison was made by estimating the relative differences of dosimetric criteria from the ICRU report 83 for VMAT and the ICRU report 91 for DCA. The first results showed for glioblastoma a reduction of the maximal dose to optical structures (optical nerves, chiasma and lenses) from 5% to 80%. Brainstem was also better protected by reducing the maximal dose from 3% to 40% according to the position of the target. In some cases, the D40% delivered to the contralateral hippocampus was reduced from 10% to 40%. In cases of reirradiation, the mean dose delivered to the brain was decreased by 30%. In all cases, the dose reduction at organs at risk did not impact the planning target volume (PTV) coverage. Trajectories defined with the new patient positioning system are very promising as they show a PTV coverage equivalent to the common treatments achieved in our center with a better sparing of the organs at risk. New studies are currently ongoing using a new Trajectory Optimizer (TO) developed at Varian, which provides patient-specific trajectories in an automated fashion, and progressively extended to other localizations as vertebra, lung and liver.

A preliminary study of a photon dose calculation algorithm using a convolutional neural network

The aim of dose calculation algorithm research is to improve the calculation accuracy while maximizing the calculation efficiency. In this study, the three-dimensional distribution of total energy release per unit mass (TERMA) and the electron density (ED) distribution are considered inputs in a method for calculating the three-dimensional dose distribution based on a convolutional neural network (CNN). Attempts are made to improve the efficiency of the collapsed cone convolution/superposition (CCCS) algorithm while providing an approach to improve the efficiency of other traditional dose calculation algorithms. Twelve sets of computed tomography (CT) images were employed for training. Data sets were generated by the CCCS algorithm with a random beam configuration. For each monoenergetic photon model, 7500 samples were generated for the training set, and 1500 samples were generated for the validation set. Training occurred for 0.5 MeV, 1 MeV, 2 MeV, 3 MeV, 4 MeV, 5 MeV, and 6 MeV monoenergetic photon models. To evaluate the usability under linac conditions, a comparison between CCCS and CNN-Dose was performed for the Mohan 6-MV spectrum for 12 additional new sets of CT images with different anatomies. A total of 1512 test samples were generated. For all anatomies, the mean value, 95% lower confidence limit (LCL) and 95% upper confidence limit (UCL) were 99.56%, 99.51% and 99.61%, respectively, at the 3%/2 mm criteria. The mean value, 95% LCL and 95% UCL were 98.57%, 98.46% and 98.67%, respectively, at the 2%/2 mm criteria. The results meet the relevant clinical requirements. In the proposed methods, the dose distribution of clinical energy can be obtained by TERMA, and the electronic density can be obtained with a CNN. This method can also be used for other traditional dose algorithms and displays potential in treatment planning, adaptive radiation therapy, and in vivo verification.

Dosimetric validation of Acuros XB photon dose calculation algorithm on an indigenously fabricated low-density heterogeneous phantom

The aim of this study was to validate Acuros XB (AXB) algorithm for photon dose calculation on an indigenously fabricated low-density heterogeneous phantom. Phantom was fabricated using poly (methyl methacrylate) (PMMA) and racemosa wood. The measured Hounsfield units, relative electron density, and mass density were 726.5, 0.273, and 0.212 g/cc and 201.8, 1.201, and 1.175 g/cc for racemosa and PMMA, respectively. AXB results were compared against anisotropic analytical algorithm (AAA) and ion chamber (IC) measured data for 3 cm × 3 cm and 10 cm × 10 cm field size of 6 megavolts beam. AXB results were in better agreement with IC measured data at all measuring points in comparison to AAA. The discrepancies between AXB and IC measured data were 1.3%–2.2% for 3 cm × 3 cm, −1.5%–−0.9% for 10 cm × 10 cm at low-density region, and −3.6%–−1.6% for 3 cm × 3 cm, and −1.4%–−0.8% for 10 cm × 10 cm at secondary buildup region, whereas discrepancies between AAA and measured data were 1.6%–3.6% for 3 cm × 3 cm, −4.6%–−3.4% for 10 cm × 10 cm at low-density region, and within −5.3%–−2.1% for 3 cm × 3 cm and −1.5%–1.0% for 10 cm × 10 cm at the secondary build-up region. Therefore, AXB is more appropriate in dealing with low-density heterogeneity in comparison to AAA.

Abstract ID: 377 The photon dose calculation algorithms

Purpose An overview of the photon dose calculation algorithm is given according to the classification in type “a” (no lateral electron transport included), type “b” (with lateral electron transport estimation), type “c” (true Monte Carlo simulations or Boltzmann transport equation solvers). Methods Performances of the different algorithms for common clinical treatment sites were presented: prostate, head and neck, breast and lung, planned with conformal technique. To better focus on the heterogeneities, different phantom studies were proposed. Finally, the overview focused on the most critical case of small treatment fields on low density tissue. Results The dose calculation models belonging to type “a” (pencil beam based) presented quite high inaccuracies in the small fields and low density regions, while type “b” were more congruent in the clinical situations with the type “c”, where Monte Carlo is considered the benchmark for dose estimation. This congruence is lowered for the critical clinical case of e.g. a stereotactic lung treatment. Additional point of discussion, especially for type “c” algorithms, concerned the report of the estimated dose as dose to medium, or dose to water in different human tissues. Conclusions The clinical use of a dose calculation system should be correlated with the degree of accuracy of the algorithm in each specific clinical situation.

The accuracy of treatment planning system dose modelling in the presence of brass mesh bolus.

This work assesses the dosimetric accuracy of three commercial treatment planning system (TPS) photon dose calculation algorithms in the presence of brass mesh used as a bolus. Bolus material is used in radiotherapy to provide dose build-up where superficial tissues require irradiation. They are generally water equivalent but high density materials can also be used. Dose calculations were performed on Monaco and Masterplan TPS (Elekta AB, Sweden) using phantoms defined by the three DICOM CT image sets of water equivalent blocks (no bolus, 1 layer and 2 layers of brass mesh) exported from the CT scanner. The effect of the mesh on monitor units, build-up dose, phantom exit dose and beam penumbra were compared to measured data. Dose calculations for 6 and 15MV photon beams on plain water equivalent phantoms were seen to agree well with measurement validating the basic planning system algorithms and models. Dose in the build-up region, phantom exit dose and beam penumbra were poorly modelled in the presence of the brass mesh. The beam attenuation created by the bolus material was overestimated by all three calculation algorithms, at both photon energies, e.g. 1.6% for one layer and up to 3.1% for two layers at 6MV. The poor modelling of the physical situation in the build-up region is in part a consequence of the high HU artefact caused by the mesh in the CT image. CT imaging is not recommended with the brass mesh bolus in situ due to the poor accuracy of the subsequent TPS modelling.

What should we know about photon dose calculation algorithms used for radiotherapy? Their impact on dose distribution and medical decisions based on TCP/NTCP

The dose calculation algorithms, integrated in a radiotherapy treatment planning system, use different approximations to swiftly compute the dose distributions. Any biological effect is somehow related to the dose delivered to the tissues. Thus, the optimization of treatment planning in radiation oncology requires, as a basis, the most accurate dose calculation to carry out the best possible prediction of the Normal Tissue Complication Probability (NTCP), as well as Tumor Control Probability (TCP). Presently, a number of bio-mathematical models exist to estimate TCP and NTCP from a physical calculated dose using the differential dose volume histogram (dDVH). The purpose of this review is to highlight the link between any change of algorithms and possible significant changes of DVH metrics, TCP, NTCP and even more of estimated Quality-adjusted life years (QALY) based on predicted NTCP. The former algorithms, such as pencil beam convolution (PBC) algorithm with 1D or 3D density correction methods, overestimated the TCP while underestimating NTCP for lung cancer. The magnitude of error depends on the algorithms, the radiobiological models and their assumed radiobiological parameters setting. The over/under estimation of radiotherapy outcomes can reach up to 50% relatively. Presently, the anisotropic analytical algorithm (AAA), collapsed cone convolution algorithm (CCC), Acuros-XB or Monte Carlo are the most recommended algorithms to consistently estimate the TCP/ NTCP outcomes and QALY score, to rank and compare radiotherapy plans, to make a useful medical decision regarding the best plan. This paper points out also that the values of the NTCP radiobiological parameters should be adjusted to each dose calculation algorithm to provide the most accurate estimates.

Comparison of 3DCRT Dose Distribution in Radiotherapy for Lung Cancer Patient by Using AAA and PBC Algorithms

The Pencil Beam Convolution (PBC) algorithm in radiation treatment planning system is widely used to calculate the radiation dose distribution in radiotherapy planning. A new photon dose calculation algorithm known as Anisotropic Analytical Algorithm (AAA) by Varian Medical Systems is applied to investigate the difference of dose distribution by using AAA and PBC algorithms for the lung cancer with an inhomogeneity of its low density. In the present work, radiotherapy treatment planning of 10 lung cancer patients are designed with 6 MV photon beam using three-dimensional conformal radiation therapy (3DCRT) and dose distribution was calculated by the AAA and the PBC Algorithms. The dose distribution performance is evaluated by dose profile curve along transversal slice of PTV and Dose Volume Histogram (DVH) covered by the 95% isodose of PTV. The mean dose of organ at risks did not changed significantly but the volume of the PTV covered by the 95% isodose curve was decreased by 6% with inhomogeneity due to the algorithms. The dose distribution and the accuracy in calculating the absorbed dose of the AAA algorithm of the Varian Eclipse treatment planning system is analyzed and discussed.

Comparison of 3DCRT Dose Distribution in Radiotherapy for Lung Cancer Patient by Using AAA and PBC Algorithms

The Pencil Beam Convolution (PBC) algorithm in radiation treatment planning system is widely used to calculate the radiation dose distribution in radiotherapy planning. A new photon dose calculation algorithm known as Anisotropic Analytical Algorithm (AAA) by Varian Medical Systems is applied to investigate the difference of dose distribution by using AAA and PBC algorithms for the lung cancer with an inhomogeneity of its low density. In the present work, radiotherapy treatment planning of 10 lung cancer patients are designed with 6 MV photon beam using three-dimensional conformal radiation therapy (3DCRT) and dose distribution was calculated by the AAA and the PBC Algorithms. The dose distribution performance is evaluated by dose profile curve along transversal slice of PTV and Dose Volume Histogram (DVH) covered by the 95% isodose of PTV. The mean dose of organ at risks did not changed significantly but the volume of the PTV covered by the 95% isodose curve was decreased by 6% with inhomogeneity due to the algorithms. The dose distribution and the accuracy in calculating the absorbed dose of the AAA algorithm of the Varian Eclipse treatment planning system is analyzed and discussed.

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.

The photon dose calculation algorithm used in breast radiotherapy has significant impact on the parameters of radiobiological models.

The comparison of the pencil beam dose calculation algorithm with modified Batho heterogeneity correction (PBC‐MB) and the analytical anisotropic algorithm (AAA) and the mutual comparison of advanced dose calculation algorithms used in breast radiotherapy have focused on the differences between the physical dose distributions. Studies on the radiobiological impact of the algorithm (both on the tumor control and the moderate breast fibrosis prediction) are lacking. We, therefore, investigated the radiobiological impact of the dose calculation algorithm in whole breast radiotherapy. The clinical dose distributions of 30 breast cancer patients, calculated with PBC‐MB, were recalculated with fixed monitor units using more advanced algorithms: AAA and Acuros XB. For the latter, both dose reporting modes were used (i.e., dose‐to‐medium and dose‐to‐water). Next, the tumor control probability (TCP) and the normal tissue complication probability (NTCP) of each dose distribution were calculated with the Poisson model and with the relative seriality model, respectively. The endpoint for the NTCP calculation was moderate breast fibrosis five years post treatment. The differences were checked for significance with the paired t‐test. The more advanced algorithms predicted a significantly lower TCP and NTCP of moderate breast fibrosis then found during the corresponding clinical follow‐up study based on PBC calculations. The differences varied between 1% and 2.1% for the TCP and between 2.9% and 5.5% for the NTCP of moderate breast fibrosis. The significant differences were eliminated by determination of algorithm‐specific model parameters using least square fitting. Application of the new parameters on a second group of 30 breast cancer patients proved their appropriateness. In this study, we assessed the impact of the dose calculation algorithms used in whole breast radiotherapy on the parameters of the radiobiological models. The radiobiological impact was eliminated by determination of algorithm specific model parameters.PACS numbers: 87.55.dh, 87.55.dk

Clinical dosimetric impact of Acuros XB and analytical anisotropic algorithm (AAA) on real lung cancer treatment plans : review

Photon dose calculation algorithms in treatment planning system could affect the accuracy of dose delivery when tissue heterogeneity is involved along the beam path. Treatment planning for lung cancer is challenging, especially in the case of treatment plan involving small fields. The combination of low-density (air) medium and small fields cause charge particle disequilibrium nears the air/tissue interface. Beam modeling within the dose calculation algorithms must also employ an accurate method of accounting tissue heterogeneity corrections in order to avoid dose overestimation or underestimation. Analytical anisotropic algorithm (AAA) is one of the widely tested and validated dose calculation algorithms in external beam photon radiation therapy. Recently, Acuros XB (AXB) was made available for photon dose calculations, and several studies have demonstrated better dose prediction accuracy of the AXB over AAA. This article reviews the results from the treatment planning studies, which have investigated the clinical dosimetric impact of the AXB and AAA on real lung cancer treatment plans. -------------------------------------- Cite this article as: Rana S. Clinical dosimetric impact of Acuros XB and analytical anisotropic algorithm (AAA) on real lung cancer treatment plans: review. Int J Cancer Ther Oncol 2014; 2 (1):02019. DOI : http://dx.doi.org/10.14319/ijcto.0201.9

Importance of Treatment Planning Systems in Radiation Therapy for Cancer Treatment - A Brief Report

External beam radiation therapy is one of popular modalities for cancer treatment. The advancement in treatment technologies and computerized treatment planning systems have made it possible to deliver high dose to the tumor. Dose calculations implemented in treatment planning systems, however, can be inaccurate when radiation beam travels through heterogeneities before reaching the target. The purpose of this article is to summarize the latest findings on the accuracy of most commonly used photon dose calculation algorithms in commercial treatment planning systems. PubMed and Google Scholar were used to do the literature search, and the results from the relevant articles are reviewed.Current literature suggests that Acuros XB is more accurate for dose calculations in external beam photon radiation therapy.

Evaluation of Acuros XB algorithm based on RTOG 0813 dosimetric criteria for SBRT lung treatment with RapidArc

The Radiation Therapy Oncology Group (RTOG) 0813 protocol requires the use of dose calculation algorithms with tissue heterogeneity corrections to compute dose on stereotactic body radiation therapy (SBRT) non‐small cell lung cancer (NSCLC) plans. A new photon dose calculation algorithm called Acuros XB (AXB) has recently been implemented in the Eclipse treatment planning system (TPS). The main purpose of this study was to compare the dosimetric results of AXB with that of anisotropic analytical algorithm (AAA) for RTOG 0813 parameters. Additionally, phantom study was done to evaluate the dose prediction accuracy of AXB and AAA beyond low‐density medium of different thicknesses by comparing the calculated results with the measurements. For the RTOG dosimetric study, 14 clinically approved SBRT NSCLC cases were included. The planning target volume (PTV) ranged from 3.2‐43.0 cc. RapidArc treatment plans were generated in the Eclipse TPS following RTOG 0813 dosimetric criteria, and treatment plans were calculated using AAA with heterogeneity correction (AAA plans). All the AAA plans were then recalculated using AXB with heterogeneity correction (AXB plans) for identical beam parameters and same number of monitor units. The AAA and AXB plans were compared for following RTOG 0813 parameters: ratio of prescription isodose volume to PTV (R100%), ratio of 50% prescription isodose volume to PTV (R50%), maximal dose 2 cm from the PTV in any direction as a percentage of prescription dose (D2cm), and the percentage of ipsilateral lung receiving dose equal to or larger than 20 Gy (V20). The phantom study showed that the results of AXB had better agreement with the measurements, and the difference ranged from −1.7% to 2.8%. The AAA results showed larger disagreement with the measurements, with differences from 4.1% to 12.5% for field size 5×5 cm2 and from 1.4% to 6.8% for field size 10×10 cm2. The results from the RTOG SBRT lung cases showed that, on average, the AXB plans produced lower values for R100%, R50%, and D2cm by 4.96%, 1.15%, and 1.60%, respectively, but higher V20 of ipsilateral lung by 1.09% when compared with AAA plans. In the set of AAA plans, minor deviation was seen for R100% (six cases), R50% (nine cases), D2cm (four cases), and V20 (one case). Similarly, the AXB plans also showed minor deviation for R100% (one case), R50% (eight cases), D2cm (three cases), and V20 (one case). The dosimetric results presented in the current study show that both the AXB and AAA can meet the RTOG 0813 dosimetric criteria.PACS number: 87.55.D‐, 87.55.dk, 87.55.kd, 87.55.km

Dosimetric evaluation of photon dose calculation under jaw and MLC shielding

The accuracy of photon dose calculation algorithms in out-of-field regions is often neglected, despite its importance for organs at risk and peripheral dose evaluation. The present work has assessed this for the anisotropic analytical algorithm (AAA) and the Acuros-XB algorithms implemented in the Eclipse treatment planning system. Specifically, the regions shielded by the jaw, or the MLC, or both MLC and jaw for flattened and unflattened beams have been studied. The accuracy in out-of-field dose under different conditions was studied for two different algorithms. Measured depth doses out of the field, for different field sizes and various distances from the beam edge were compared with the corresponding AAA and Acuros-XB calculations in water. Four volumetric modulated arc therapy plans (in the RapidArc form) were optimized in a water equivalent phantom, PTW Octavius, to obtain a region always shielded by the MLC (or MLC and jaw) during the delivery. Doses to different points located in the shielded region and in a target-like structure were measured with an ion chamber, and results were compared with the AAA and Acuros-XB calculations. Photon beams of 6 and 10 MV, flattened and unflattened were used for the tests. Good agreement between calculated and measured depth doses was found using both algorithms for all points measured at depth greater than 3 cm. The mean dose differences (± 1SD) were -8% ± 16%, -3% ± 15%, -16% ± 18%, and -9% ± 16% for measurements vs AAA calculations and -10% ± 14%, -5% ± 12%, -19% ± 17%, and -13% ± 14% for Acuros-XB, for 6X, 6 flattening-filter free (FFF), 10X, and 10FFF beams, respectively. The same figures for dose differences relative to the open beam central axis dose were: -0.1% ± 0.3%, 0.0% ± 0.4%, -0.3% ± 0.3%, and -0.1% ± 0.3% for AAA and -0.2% ± 0.4%, -0.1% ± 0.4%, -0.5% ± 0.5%, and -0.3% ± 0.4% for Acuros-XB. Buildup dose was overestimated with AAA, while Acuros-XB gave results more consistent with measurements. From RapidArc plan analysis the average difference between calculation and measurement in the shielded region was -0.3% ± 0.4% and -2.5% ± 1.2% for AAA and Acuros-XB, respectively, relative to the mean target dose value (1.6% ± 2.3%, -12.7% ± 4.0% if relative to each local value). These values were compared with the corresponding differences in the target structure: -0.7% ± 2.3% for AAA, and -0.5% ± 2.3% for Acuros-XB. The two algorithms analyzed showed encouraging results in predicting out-of-field region dose for clinical use.

SU-E-T-562: Do the Differences Between Photon Dose Distributions in the Breast Have Significant Impact On the Parameters of Radiobiological Models?

Purpose: The comparison of advanced photon dose calculation algorithms in breast radiotherapy has focused on the physical dose distributions (DDs). Studies on their radiobiological impact are restricted to the normal tissue complication probability (NTCP) of the ipsilateral lung and the heart. The comparisons of the tumor control probability (TCP) and of the NTCP of moderate breast fibrosis are lacking. This study quantifies the radiobiological difference between four photon DDs in the breast. As the clinical outcome should be independent of the photon dose calculation algorithm, significant differences are eliminated by optimization of the model parameters found in literature. Methods: Thirty breast cancer patients, irradiated with two wedged tangential half-beams, were included in the study. Each DD, calculated with the Pencil Beam Convolution algorithm with modified Batho heterogeneity correction (PBC-MB) was recalculated using the Analytical Anisotropic Algorithm (AAA), AcurosXB (dose to medium) and AcurosXB (dose to water) with fixed monitors units (Eclipse, Varian Medical Systems, Inc.). The TCP and the NTCP of each DD were calculated with the Poisson model and the relative seriality model, respectively. The endpoint for the NTCP was moderate breast fibrosis. The differences were checked for significance with the paired t-test. Significant difference was eliminated by minimizing the difference between both the TCPs and the NTCPs. Results: AAA-TCP and AAA-NTCP are significantly lower than calculated for PBC-MB: 1.1% and 3.1%, respectively. AAA-TCP and AAA-NTCP are significantly higher than AcurosXB (dose to medium): 0.5% and 1.5%, respectively, and than AcurosXB (dose to water): 1.4% and 2.9%, respectively. New TCP and NTCP model parameters were determined for the radiobiological analysis of the PBC-MB and the AAA DD, respectively. Conclusion: This study shows that the differences between the photon DDs of advanced dose calculation algorithms have radiobiological significance which can be eliminated by determination of algorithm specific model parameters.

On the Clinical and Dosimetric Accuracy of Recently Released Advanced Dose Calculation Algorithm and Analytical Anisotropic Algorithm (AAA) Photon Dose Calculation Algorithms for SBRT Lung Treatment With Volumetric Modulated Arc Therapy (VMAT)

Advanced radiation therapy techniques, such as VMAT, require accurate dose calculations. This study evaluates advanced dosed calculation algorithm (ADCA) and Analytical Anisotropic Algorithm (AAA) photon dose calculation algorithms for Stereotactic Body Radiation Therapy (SBRT) of lung tumors.

Clinical validation of the Acuros XB photon dose calculation algorithm, a grid-based Boltzmann equation solver

Introduction. A new algorithm that uses a grid-based technique to solve the linear Boltzmann transport equation (LBTE) has been developed to improve the accuracy and speed of external photon beam treatment planning calculations. The aim of this study was to test the accuracy of this algorithm in both heterogeneous and homogeneous media. Material and methods. Output factors, depth dose curves and profiles for symmetric fields were measured in water using diamond and ionization chamber detectors. Furthermore, asymmetric fields, fields collimated with the multi-leaf collimator, enhanced dynamic wedge fields as well as fields with different source-skin distances were measured. Various test plans were created on a CIRS thorax phantom including tissue-equivalent inserts and corresponding dose distributions within the phantom were measured with radiochromic films. The new grid-based LBTE solver, Acuros XB (Eclipse version 10.0, Varian Medical Systems, CA, USA) was used to calculate dose distributions for all field configurations and plans, for both 6 MV and 15 MV photons. Calculations were also performed with AAA, a standard convolution algorithm. Results. Compared to measurements, the output factors were within 1% for Acuros XB. For the depth doses, the average deviations were within 1% in dose and 1 mm in distance to agreement (DTA). For the profiles, the deviations were within 2%/1 mm except near the penumbra. Similar results were obtained for the other field configurations. Good agreement with AAA was also found. For the plans calculated on the CIRS phantom, the number of points meeting the gamma criterion of 3% in dose and 3 mm in DTA was higher with Acuros XB (98% for 6 MV; 100% for 15 MV) than with AAA (94% for 6 MV; 96% for 15 MV). Conclusion. Dose calculations with the Acuros XB algorithm in homogeneous media are in good agreement with both measurements and the AAA algorithm. In heterogeneous media, the Acuros XB algorithm is superior to AAA in both lung and bony material.

Accuracy of Acuros XB and AAA dose calculation for small fields with reference to RapidArc® stereotactic treatments

To assess the accuracy against measurements of two photon dose calculation algorithms (Acuros XB and the Anisotropic Analytical algorithm AAA) for small fields usable in stereotactic treatments with particular focus on RapidArc(®). Acuros XB and AAA were configured for stereotactic use. Baseline accuracy was assessed on small jaw-collimated open fields for different values for the spot sizes parameter in the beam data: 0.0, 0.5, 1, and 2 mm. Data were calculated with a grid of 1 × 1 mm(2). Investigated fields were: 3 × 3, 2 × 2, 1 × 1, and 0.8 × 0.8 cm(2) with a 6 MV photon beam generated from a Clinac2100iX (Varian, Palo Alto, CA). Profiles, PDD, and output factors were measured in water with a PTW diamond detector (detector size: 4 mm(2), thickness 0.4 mm) and compared to calculations. Four RapidArc test plans were optimized, calculated and delivered with jaw settings J3 × 3, J2 × 2, and J1 × 1 cm(2), the last was optimized twice to generate high (H) and low (L) modulation patterns. Each plan consisted of one partial arc (gantry 110° to 250°), and collimator 45°. Dose to isocenter was measured in a PTW Octavius phantom and compared to calculations. 2D measurements were performed by means of portal dosimetry with the GLAaS method developed at authors' institute. Analysis was performed with gamma pass-fail test with 3% dose difference and 2 mm distance to agreement thresholds. Open square fields: penumbrae from open field profiles were in good agreement with diamond measurements for 1 mm spot size setting for Acuros XB, and between 0.5 and 1 mm for AAA. Maximum MU difference between calculations and measurements was 1.7% for Acuros XB (0.2% for fields greater than 1 × 1 cm(2)) with 0.5 or 1 mm spot size. Agreement for AAA was within 0.7% (2.8%) for 0.5 (1 mm) spot size. RapidArc plans: doses were evaluated in a 4 mm diameter structure at isocenter and computed values differed from measurements by 0.0, -0.2, 5.5, and -3.4% for Acuros XB calculations (1 mm spot size), and of -0.1, 0.3, 6.7, and -1.2% for AAA, respectively for J3 × 3, J2 × 2, J1 × 1H, J1 × 1L RapidArc plans. Gamma Agreement Index from 2D dose analysis was higher than 95% for J3 × 3 and J2 × 2 plans, being around 80% for J1 × 1 maps. Sensitivity with respect to the dosimetric leaf gap and transmission factor MLC parameters was evaluated in the four RapidArc plans, showing the need to properly set the dosimetric leaf gap for accurate calculations. Acuros XB and AAA showed acceptable characteristics for stereotactic small fields if adequate tuning of configuration parameters is performed. Dose calculated for RapidArc stereotactic plans showed an acceptable agreement against point and 2D measurements. Both algorithms can therefore be considered safely applicable to stereotactic treatments.