National dosimetric audit network finds discrepancies in AAA lung inhomogeneity corrections

We have developed a fast and accurate in‐house Monte Carlo (MC) secondary monitor unit (MU) check method, based on the EGSnrc system, for independent verification of volumetric modulated arc therapy (VMAT) treatment planning system dose calculations, in accordance with TG‐114 recommendations. For a VMAT treatment plan created for a Varian Trilogy linac, DICOM information was exported from Eclipse. An open‐source platform was used to generate input files for dose calculations using the EGSnrc framework. The full VMAT plan simulation employed 107 histories, and was parallelized to run on a computer cluster. The resulting 3ddose matrices were converted to the DICOM format using CERR and imported into Eclipse. The method was evaluated using 35 clinical VMAT plans of various treatment sites. For each plan, the doses calculated with the MC approach at four three‐dimensional reference points were compared to the corresponding Eclipse calculations, as well as calculations performed using the clinical software package, MUCheck. Each MC arc simulation of 107 particles required 13–25 min of total time, including processing and calculation. The average discrepancies in calculated dose values between the MC method and Eclipse were 2.03% (compared to 3.43% for MUCheck) for prostate cases, 2.45% (3.22% for MUCheck) for head and neck cases, 1.7% (5.51% for MUCheck) for brain cases, and 2.84% (5.64% for MUCheck) for miscellaneous cases. Of 276 comparisons, 201 showed greater agreement between the treatment planning system and MC vs MUCheck. The largest discrepancies between MC and MUCheck were found in regions of high dose gradients and heterogeneous densities. By parallelizing the calculations, point‐dose accuracies of 2‐7%, sufficient for clinical secondary checks, can be achieved in a reasonable amount of time. As computer clusters and/or cloud computing become more widespread, this method will be useful in most clinical setups.

Section: Discussionmentioning

confidence: 99%

Secondary monitor unit calculations for VMAT using parallelized Monte Carlo simulations

Bhagroo

French

Mathews

et al. 2019

“…Nevertheless, a study aiming to investigate agreement between TPS calculated dose and measured dose should take into account possible limitations of the dose calculation algorithm. It has been shown that the AAA algorithm overestimates dose for heterogeneities in lung tissue . Since our measurements were performed with a centrally located ion chamber, this possible underdosage around edges of lung heterogeneities was not revealed.…”

Section: Discussionmentioning

confidence: 86%

Dose deviations induced by respiratory motion for radiotherapy of lung tumors: Impact of CT reconstruction, plan complexity, and fraction size

Sande

Roa

Hellebust

2020

A thorax phantom was used to assess radiotherapy dose deviations induced by respiratory motion of the target volume. Both intensity modulated and static, non-modulated treatment plans were planned on CT scans of the phantom. The plans were optimized using various CT reconstructions, to investigate whether they had an impact on robustness to target motion during delivery. During irradiation, the target was programmed to simulate respiration-induced motion of a lung tumor, using both patient-specific and sinusoidal motion patterns in three dimensions. Dose was measured in the center of the target using an ion chamber. Differences between reference measurements with a stationary target and dynamic measurements were assessed. Possible correlations between plan complexity metrics and measured dose deviations were investigated. The maximum observed motion-induced dose differences were 7.8% and 4.5% for single 2 Gy and 15 Gy fractions, respectively. The measurements performed with the largest target motion amplitude in the superiorinferior direction yielded the largest dosimetric deviations. For 2 Gy fractionation schemes, the summed dose deviation after 33 fractions is likely to be less than 2%.Measured motion-induced dose deviations were significantly larger for one CT reconstruction compared to all the others. Static, non-modulated plans showed superior robustness to target motion during delivery. Moderate correlations between the modulation complexity score applied to VMAT (MCSv) and measured dose deviations were found for 15 Gy SBRT treatment plans. Correlations between other plan complexity metrics and measured dose deviations were not found. K E Y W O R D S complexity, interplay, lung, radiotherapy, SBRT, VMAT

“…In situations like this, the AAA of Eclipse has been shown to underpredict dose5, 20 and CCCS overpredict dose,21 both relative to gold standard Monte Carlo calculations. These publications are consistent with local observations whereby Mobius3D systematically calculates more dose to lung targets than Eclipse , where the reality is most likely to be somewhere in between.…”

Section: Discussionmentioning

confidence: 98%

“…Generally speaking, radiotherapy centers worldwide will implement a commercial treatment planning system (TPS) from a small number of vendors that employ one of several different methods of dose calculation. Although this commercialization of software has undoubtedly improved consistency across centers, the above factors still exist alongside clinically significant variations, as shown by the results of large‐scale independent auditors1, 2, 3, 4, 5 and publication of interinstitutional comparisons 6. The systems themselves are also highly dependent on user‐defined reference data that is not immune to inadvertent change and/or corruption.…”

Section: Introductionmentioning

confidence: 99%

A clinical database to assess action levels and tolerances for the ongoing use of Mobius3D

Jolly

Dunn

Kenny

2016

In radiation therapy, calculation of dose within the patient contains inherent uncertainties, inaccuracies, limitations, and the potential for random error. Thus, point dose‐independent verification of such calculations is a well‐established process, with published data to support the setting of both action levels and tolerances. Mobius3D takes this process one step further with a full independent calculation of patient dose and comparisons of clinical parameters such as mean target dose and voxel‐by‐voxel gamma analysis. There is currently no published data to directly inform tolerance levels for such parameters, and therefore this work presents a database of 1000 Mobius3D results to fill this gap. The data are tested for normality using a normal probability plot and found to fit this distribution for three sub groups of data; Eclipse, iPlan and the treatment site Lung. The mean (μ) and standard deviation (σ) of these sub groups is used to set action levels and tolerances at μ ± 2σ and μ ± 3σ, respectively. A global (3%, 3 mm) gamma tolerance is set at 88.5%. The mean target dose tolerance for Eclipse data is the narrowest at ± 3%, whilst iPlan and Lung have a range of −5.0 to 2.2% and −1.8 to 5.0%, respectively. With these limits in place, future results failing the action level or tolerance will fall within the worst 5% and 1% of historical results and an informed decision can be made regarding remedial action prior to treatment.