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Dosimetric validation of Acuros® XB algorithm for RapidArc™ treatment technique: A post software upgrade analysis


1 Department of Applied Science and Humanities, Dr. A.P.J Abdul Kalam Technical University, Lucknow, Uttar Pradesh; Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, India
2 Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, India
3 Department of Applied Science and Humanities, Bundelkhand Institute of Engineering and Technology, Jhansi, Uttar Pradesh, India
4 Department of Radiotherapy, Mahatma Gandhi Memorial Medical College, Indore, Madhya Pradesh, India

Date of Submission23-Dec-2019
Date of Decision15-Jul-2020
Date of Acceptance12-Aug-2020
Date of Web Publication03-Aug-2021

Correspondence Address:
Lalit Kumar,
Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, Sector-5, Rohini, New Delhi - 110 085
India
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.JCRT_1154_19

 > Abstract 


Aim: To validate the Acuros® XB (AXB) algorithm in Eclipse treatment planning system (TPS) for RapidArc™ (RA) technique following the software upgrades.
Materials and Methods: A Clinac-iX (2300CD) linear accelerator and Eclipse TPS (Varian Medical System, Inc., Palo Alto, USA) was used for commissioning of AXB algorithm using a 6 megavolts photon beam. Percentage depth dose (PDD) and profiles for field size 2 cm × 2 cm, 4 cm × 4 cm, 6 cm × 6 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm to 40 cm × 40 cm were taken. AXB calculated PDDs and profiles were evaluated against the measured and analytical anisotropic algorithm (AAA)-calculated PDDs and profiles. Test sites recommended by American Association of Physicists in Medicine task group (AAPM TG)-119 recommendation were used for RA planning and delivery verification using AXB algorithm.
Results: Dosimetric analysis of AXB calculated data showed that difference between calculated and measured data for PDD curves were maximum <1% beyond the depth of dose maximum and computed profiles in central region matches with maximum <1% for all considered field sizes. Ion-chamber measurements showed that the average confidence limit (CLs) was 0.034 and 0.020 in high-gradient and 0.047 and 0.042 in low-gradient regions, respectively, for AAA and AXB calculated RA plans. Portal measurements show the average CLs were 2.48 and 2.58 for AAA and AXB-calculated RA plans, with gamma passing criteria of 3%/3 mm.
Conclusions: AXB shows excellent agreement with measurements and AAA calculated data. The CLs were consistent with the baseline values published by TG-119. AXB algorithm has the potential to perform photon dose calculation with comparable fast calculation speed without negotiating the accuracy. AAPM TG-119 was successfully implemented to access the proper configuration of AXB algorithm following the TPS upgrade.

Keywords: Acuros XB, American Association of Physicists in Medicine task group 119, analytical anisotropic algorithm, RapidArc, software upgrade



How to cite this URL:
Kumar L, Bhushan M, Kishore V, Yadav G, Gurjar OP. Dosimetric validation of Acuros® XB algorithm for RapidArc™ treatment technique: A post software upgrade analysis. J Can Res Ther [Epub ahead of print] [cited 2021 Dec 6]. Available from: https://www.cancerjournal.net/preprintarticle.asp?id=322900




 > Introduction Top


The objective of the radiotherapy is to precisely deliver the prescribed dose to the tumor without violating the surrounding normal tissue tolerance. The American Society for Radiation Oncology said that “nearly two-third of all cancer patients receives radiation therapy during their illness.”[1]

In contemporary radiotherapy planning, the use of beam modulation involves numerous small beam-lets of varying intensities passing through the region of low and high densities depending on the location of the target and surrounding normal tissues. This necessitates a highly precise dose calculation algorithm to accurately model the radiation transport through a heterogeneous medium. The dose calculation algorithm assumes an essential role in determining the outcome of radiation therapy treatment. The International Commission on Radiation Units and Measurements[2] has advocated an overall dose accuracy within 5%. Considering the uncertainties resulting from the machine calibration, dose computation, and patient setup, it is indispensable to own a dose computation algorithm that will anticipate the dose distribution within 3% accuracy.

The Radiological Physics Center's credentialing and review programs detailed that nearly 30% of institutions had failed to meet the criteria of 7% for dose in low gradient region and/or 4 mm distance to agreement (DTA) in high gradient region of the 250 irradiation of a head and neck phantom after espousal of intensity-modulated radiotherapy for a decade.[3] This turns out due to poor commissioning and inadequate appraisal of dose computation in standard and well-controlled conditions, lack of knowledge of algorithm in performing the calculation for clinical cases with an intrinsic degree of accuracy. This imposes the demand to validate a dose calculation algorithm in local conditions to evaluate the uncertainties in computations and potential limitations of the algorithm.

The Acuros XB (AXB) algorithm was developed by Transpire, Inc. (GibHarbor, WA, USA) from a general deterministic grid-based Boltzmann solver code (employed for nuclear fusion and optical tomography), known as the Attila.[4],[5] The AXB algorithm is inclusively blended into eclipse distributed calculation framework and employs the same multiple-source model originally derived for the analytical anisotropic algorithm (AAA). For AXB beam model configuration, AAA beam modal data can be directly imported in the AXB beam model and entails the only reconfiguration for executing the dose calculations.[6]

The AXB exploits the solutions of linear Boltzmann transport equation (LBTE) to account for the heterogeneities present in the patient dose calculations. The LBTE is the linearized form of the Boltzmann transport equation, which oversees the macroscopic behavior of the radiation particles as they travel through and interact with the matter. In general, there are two perspectives to obtain the solution to the LBTE. The primary aspect is Monte-Carlo methods they do not explicitly solve the LBTE, but obtain indirect solution to the LBTE. The second aspect is to explicitly solve the LBTE by means of numerical methods such as AXB. The AXB rigorously solves the LBTE to compute the final dose.[7],[8]

Radiation therapy is a rapidly evolving modality in cancer management. Software patches and upgrades are very common in the contemporary clinical environment. It includes modification in the features of dose computation algorithms and available tools in the treatment planning systems (TPSs). Software upgrades in TPS may lead to disparity in planning solutions and needs to be evaluated before implementing it to the clinic. Therefore, it is alluring to play out extensive quality assurance (QA) at planning and dose delivery level, which is nearly impossible to perform at every stage due to complexity, involved in RapidArc (RA) treatment planning and delivery following a software upgrade. Recently, Eclipse software Version 10 (Varian Medical System, Inc., Palo Alto, CA, USA) was upgraded to version 11 in our clinic. The present study aimed to validate the commissioning of the AXB algorithm version 11 for RA technique using a photon beam of 6 megavolts (MV) energy in Eclipse TPS. Furthermore, this study also validates the AXB version 11 algorithm performance against AAA version 11 algorithm for RA treatment technique following the above-mentioned TPS upgrade.


 > Materials and Methods Top


For AXB algorithm commissioning, a photon beam of 6 MV energy was triggered using a Varian Clinac-iX (2300 CD) linear accelerator (Linac) (Varian Medical System, Inc., Palo Alto, CA, USA). Radiation field analyzer-300 (Blue Phantom, IBA Co-operation, Germany) was employed for generating all required data to configure the AXB algorithm in Eclipse TPS (Varian Medical System, Inc., Palo Alto, CA, USA). Scanditronix-wellhofer CC13 and ionization chamber (IC) 15 cylindrical IC were employed for scanning the radiation fields.

Linac calibration was performed under the reference conditions prescribed by the technical series report no. 398 recommended by the International Atomic Energy Agency.[9] Linac was calibrated at 1cGy per MU for reference source to surface distance (SSD) of 100 cm and at the depth of dose maximum, i.e., dmax for photon beam of 6 MV energy.

Percentage depth dose (PDD) was measured for field sizes ranging from 2 cm × 2 cm, 4 cm × 4 cm, 6 cm × 6 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm to 40 cm × 40 cm. Profiles were also measured for above fields in cross-line and in-line direction at the depth of dmax, 5 cm, 10 cm, 20 cm, and 30 cm, respectively. All PDDs and profile scans were performed at 100 cm SSD.

American Association of Physicists in Medicine task group 119 evaluation

The test sites advocated by the American Association of Physicists in Medicine (AAPM) task group (TG)-119[3],[10] were used to validate the commissioning and performance of the AXB in RA treatment planning compared to AAA algorithm. The AAPM TG employs specific test Radiotherapy (RT) structure that can be downloaded from http://www.aapm.ogr/pubs/tg119/default.asp. These RT structures were imported on the computed tomography (CT) images of local I'mRT phantom (made of solid water, RW3 white-polystyrene material by Scanditronix-wellhofer) using Eclipse TPS for following test sites: multi-target, prostate, head and neck, C-Shape (easy), and C-shape (hard). The commissioning was demonstrated by planning these advocated test cases on the I'mRT phantom (Scanditronix-wellhofer) using the eclipse TPS and delivering these plans to the phantom.

The RA planning was executed for test cases as prescribed by AAPM TG-119 report. The RA plans were optimized using a progressive resolution optimizer algorithm and the final dose calculation was executed using AXB. RA plans were also re-calculated for AAA algorithm in order to compare the AXB calculated results. A grid size of 2.5 mm × 2.5 mm × 2.5 mm was used for dose calculation. The collimator rotation of 30° from nominal angle was used in order to mitigate the leakage due to tongue and groove effect of multi-leaf collimator in RA planning. All plans were normalized to an isodose line that ensured coverage of the target volume to meet TG 119 objective. The planning objectives and dose prescription were used in accordance with the TG-119 goals, as detailed in [Table 1].
Table 1: The treatment planning results for analytical anisotropic algorithm and Acuros XB algorithm and their comparison with American Association of Physicists in Medicine TG-119

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To verify the dose calculation, all the RA plans were delivered on the I'mRT phantom. Point dose measurements were performed using CC 13S (IBA Dosimetry, Germany) ion chamber and Dose 1 (IBA Dosimetry, Germany) electrometer in conjunction with the I'mRT phantom. Point dose verifications were done in high dose gradient and low dose gradient regions as prescribed in TG 119 report. Portal dose measurements were also performed for fluence verification of the RA plans. The dose difference ratio and confidence limit (CL) were calculated to evaluate the systematic and random errors using the following formula;[10],[11],[12]








 > Results Top


The AXB and AAA algorithm employs the same multiple source model for configuration. This model accounts for the primary source, extra focal second source, electron contamination source, and the photon scattered from the hard wedge. [Table 2] demonstrates the open beam parameters used for the configuration of AAA and AXB beam model using 6 MV photon beam energy in Eclipse TPS. [Figure 1] illustrates the (a) mean radial energy curve, (b) energy spectrum curves, (c) intensity profile, and (d) electron contamination curve for AAA and AXB algorithm for a 6 MV photon beam triggered from 2300 CD medical electron accelerator. These parameters have been computed by TPS in the configuration of AXB and AAA algorithm.[13] The energy spectrum accounts for the initial photon spectrum from bremsstrahlung due to electrons impinging on the target, mean energy curve accounts for the beam hardening effect of the flattening filter on photon spectrum, intensity profile accounts for varying photon fluence (due to flattening filter) as a function radial distance from the central axis of the beam and electron contamination curve accounts for the energy spectrum of contaminated electron originated in flattening filter, in collimating jaws and in air.[14]
Table 2: Open beam parameters used for the configuration of analytical anisotropic algorithm and Acuros XB algorithm in Eclipse treatment planning system

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Figure 1: Illustrates the (a) mean radial energy curve, (b) energy spectrum curves, (c) Intensity profile and (d) electron contamination curve for analytical anisotropic algorithm and Acuros XB algorithm for a 6MV photon beam from 2300CD medical electron accelerator

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Percentage depth doses and profile evaluation

Data postprocessing were executed as per the recommendation prescribed by AAPM TG-106 report.[15] PDD curves were normalized at depth of maximum dose (dmax) and profiles were normalized at the central axis of the respective field. PDD curves were scrutinized into two parts before dmax region and after dmax region. Profiles were scrutinized into three regions, namely, central region, penumbra region, and outside field region, respectively.

[Figure 2]a illustrates the difference between AAA-calculated PDD and measured PDD curves and [Figure 2]b illustrates the difference between AXB-calculated PDD and measured PDD curves after the depth of maximum dose (dmax). AXB calculated result shows good agreement with measurement, and were comparable to AAA calculated results. Results have shown <1% difference between measurements and calculated PDD by AAA and AXB algorithm for all the field size.
Figure 2: Percentage difference between calculated and measured percentage depth dose as a funtion of field size for (a) analytical anisotropic algorithm and (b) Acuros XB algorithm, respectively

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[Figure 3]a illustrates the percentage difference between measured and AAA-calculated profiles and [Figure 3]b illustrates the percentage difference between measured and AXB-calculated profiles, averaged in the central beam region of open beam as a function of field size at depth of dmax, 5 cm, 10 cm, 20 cm, and 30 cm. The AXB results show good agreement with measurement, and were comparable to AAA algorithm. Result for all field sizes, shown <1% difference between measurements and profiles calculated by AAA and AXB algorithm in the central region of the beam profiles.
Figure 3: Percentage dose difference of profiles averaged in central region of open fields as a function of the field size and at depths of dmax, 5 cm, 10 cm, 20 cm and 30 cm for (a) analytical anisotropic algorithm (b) Acuros XB algorithm, respectively

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Gamma evaluation

[Figure 4] illustrates the gamma histograms for open beams calculated by TPS for different depth dose and profiles curves using (a) AAA and (b) AXB algorithm, respectively. The histogram of gamma index were evaluated from (a) depth dose curve in two regions, before dmax and after dmax and (b) from profiles in-field (in central region), out-field (the tail from 1 cm outside of the geometric field size), and penumbra region.[16] The gamma evaluation was executed with DTA equal to 1 mm and △D (dose difference) equal to 1%.[13]
Figure 4: Gamma histograms for DTA = 1 mm and △D =1% criteria for open beams calculated by treatment planning system in different depth dose and profiles curves for (a) analytical anisotropic algorithm and (b) Acuros XB algorithm, respectively

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TPS calculated gamma error histogram values, average gamma error in PDD curves were; before dmax (AXB: 0.28, and AAA: 0.24) and after dmax (AXB: 0.15, and AAA: 0.17), respectively. The average gamma error in profile curves in the central region, penumbra region, and outside field region were 0.17, 0.21, and 0.42 for AXB and 0.10, 0.22, and 0.35 for AAA algorithms, respectively.

American Association of Physicists in Medicine task group 119 evaluation

RapidArc plan evaluation

[Table 1] details the statistic of RA plans calculated using AAA and AXB algorithms along with the objective advocated by AAPM report TG-119. All the objectives were achieved or exceeded the requirements of TG-119 for RA plans calculated using AAA and AXB algorithms, respectively. [Figure 5] illustrates the structure sets for (a) prostate (b) C-shape (c) head and neck and (d) multi-target, prescribed by TG-119 on the CT-images of I'mRT phantom along with arc geometry used for planning. [Table 3] reveals that AXB-computations demonstrate a slight increase in numbers of monitor units (MUs) compared to AAA-computations. [Table 3] also reveals a remarkable decrease in final dose calculation time for AXB-computations contrast to AAA-calculations for RA plans.
Figure 5: Structures (a) prostate, (b) C-shape, (c) head and neck and (d) Multi-target with planned isodose and treatment planning setup used for RapidArc planning

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Table 3: Number of monitor units and calculations time comparison between analytical anisotropic algorithm and Acuros XB algorithms

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 > Ion-chamber results Top


[Table 4] details the IC measured results in high-dose and low-dose regions, for RA plans computed using AAA and AXB algorithms, respectively. The dose difference ratio and CLs were calculated using TG-119 definitions. For AAA-calculated results, the dose difference ration were 0.004 ± 0.015 in high-dose low-gradient regions and 0.011 ± 0.018 in low-dose high-gradient regions, corresponding to the average 95% CLs of 0.034 and 0.047, respectively. For AXB-calculated results, the dose difference ration were 0.001 ± 0.010 in high-dose low-gradient regions and 0.015 ± 0.014 in low-dose high-gradient regions, corresponding to the average 95% CLs of 0.020 and 0.042, respectively. The CLs were in accordance with prescribed limits in TG 119, i.e., average CLs for overall tests and institutions were 0.045 and 0.047 for the high-dose region and low-dose region, respectively.
Table 4: The ion chamber measurement in high-dose and low-dose regions for RapidArc plans calculated using analytical anisotropic algorithm and Acuros XB algorithms, respectively

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Fluence evaluation

Fluence dosimetry was carried out using machine mounted PortalVision aS1000 MV imaging system (Varian Medical system, Palo Alto, CA, USA). This electronic portal imaging system is a flat panel detector, consists of a solid-state matrix of 1024 × 748 pixels of amorphous silicon. The gamma passing criteria of 3 mm DTA and 3% DD were used for fluence evaluation, with threshold value 10%.[17] [Table 5] details the fluence dosimetry results for both AAA and AXB-calculated RA plans in comparison to TG-119 report. The overall mean gamma passing rate was 98.5 (standard deviation [SD]: 0.5) and 98.4 (SD: 0.5) for AAA and AXB-calculated RA plans. The CLs were 2.48 and 2.58 for AAA and AXB-calculated RA plans, corresponding to 97.6% and 97.5% average percent points passing the gamma criteria of 3%/3 mm. The CLs were in accordance with TG-119 CL, i.e., 1.13 corresponds to the 93% average percent points passing the gamma criteria of 3%/3 mm.
Table 5: The fluence dosimetry measurements for analytical anisotropic algorithm and Acuros XB -calculated RA plans in comparison to TG-119 report.

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 > Discussion Top


This investigation confirms the commissioning beam data for AXB version 11 algorithm using 6MV photon beam triggered from a Clinac-iX (2300CD) linac machine following TPS upgrade and appraise the capabilities of AXB-algorithm in executing the dose calculations for RA technique. This study also benchmarks the AXB version 11 calculations using TG-119 recommendations in contrast to AAA version 11.

The primary requirement for a TPS is to acquire a decent concurrence with the essential measurement data that have been used for deriving the beam model parameters. Dosimetric analysis of AXB calculated data shows <1% contrast among the measured and calculated data for PDD curves beyond the depth of dose maximum and profiles within the central region for all considered field size. Fogliata et al. had also validated the AXB algorithm in water and reported similar findings.[7] Venselaar et al. reported that acceptable maximum deviation in PDD beyond the dmax is <2% and the maximum acceptable deviation in profiles in central region is <3%.[18],[19] Hoffman et al. had additionally detailed that AXB calculation was in acceptable concurrence with both measurements and the AAA algorithm in the homogeneous medium.[20]

All the goals were accomplished or surpass the prerequisites of the TG-119 for RA plans calculated using AAA and AXB algorithms, respectively. The CLs were in accordance with the benchmark esteems advocated by TG-119 for RA plans, calculated using AAA and AXB algorithm. Results show that the performance of these two algorithms was similar for RA plans on a homogenous I'mRT phantom for different test sites prescribed by AAPM TG-119. This sort of investigation guarantees the best possible configuration of AXB beam model in the TPS and offers confidence in the accuracy of dose calculation and delivery in local conditions following system upgrade. The availability of a test environment is decisive for a fruitful up-gradation of such comprehensive radiation therapy software. The AAPM TG-119 advocates an end to end benchmarking procedure, which ensures that all the systems are configured properly. In an instance of any significant error occurs during data collection or beam modeling, results won't be comparative as expressed in AAPM TG-119.[21] The disparity in result may happen because of error in beam input data, sub-optimal beam modeling, error in beam calibration, an inherent limitation of algorithm, and various other reasons. McVicker et al. examined the sensitivity of the TG119 in finding the error associated with the commissioning process by introducing deliberate errors in original beam data and concluded that the TG119 commissioning criteria are effective in detecting errors.[22]

The study uncovers that AXB requires lesser calculation time in contrast to AAA algorithm for RA plans. This can be clarified utilizing the fact that AXB has less reliance on the number of beams employed in a radiotherapy treatment plan. For AAA, the majority of the calculation time is spent in solving the scattered photon and electron fluence. On the contrary, AXB employed scattered calculation phase only once for all the beams utilized in a RA plan. Consequently, AXB requires less time on calculations with the increase in number of beams for a RA plan in contrast to AAA. A comparative finding has been accounted for by Failla et al.[6] This examination uncovers that AXB-calculated results show a slight increase in numbers of MUs in comparison to AAA-calculated results. The number of MUs relies upon the size of the target being dealt with, and the degree of modulation introduced in dose optimization for sparing the organs at risk. A comparative finding has been accounted for by Zifodya et al.,[23] and Fogliata et al.[24] Ojala et al.[25] detailed the difference between AXB version 11 and AXB version 10 and concluded that there was insufficient contrast found in dose calculation between both versions of AXB algorithm, aside from air cavities.

The present study not only validates the upgraded version of AXB, but also benchmarks it performance against its counterpart AAA algorithm. It is very tedious to perform QA for every checkpoint following a software upgrade; the study gives an overview for evaluating the TPS following software upgrade in a limited time frame in the contemporary clinical scenario, where it is very frequent to upgrade the TPSs.

The major limitation of this study is that, its reliance on the measurements in the homogenous medium like water for beam data generation, configuration part, and homogenous I'mRT phantom, to validate the RA planning and delivery results for AXB algorithm. This was done to identify the sources of errors or uncertainties in validating the intrinsic behavior of AXB algorithm before introducing it to complex inhomogeneous and sophisticated clinical environment.


 > Conclusions Top


Dosimetric validation of AXB algorithm was executed for photon dose calculation in radiotherapy environment. The AXB shows excellent agreement with measurement and calculated data by AAA. The CLs were in accordance with the baseline values published by TG-119. Thus, AXB algorithm has the potential to perform photon dose calculation with comparable fast calculation speed without negotiating the accuracy, which is more desirable for the contemporary radiotherapy environment. AAPM TG-119 was successfully implemented to access the proper configuration of AXB algorithm following the TPS upgrade.

Acknowledgments

The authors thank the management of Rajiv Gandhi Cancer Institute and Research Centre in New Delhi, India, for their continued support and encouragement to complete this research work.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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[PUBMED]  [Full text]  
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