Cross verification of independent dose recalculation, log files based, and phantom measurement‐based pretreatment quality assurance for volumetric modulated arc therapy

Abstract Independent treatment planning system (TPS) check with Mobius3D software, log files based quality assurance (QA) with MobiusFX, and phantom measurement‐based QA with ArcCHECK were performed and cross verified for head‐and‐neck (17 patients), chest (16 patients), and abdominal (19 patients) cancer patients who underwent volumetric modulated arc therapy (VMAT). Dosimetric differences and percentage gamma passing rates (%GPs) were evaluated and compared for this cross verification. For the dosimetric differences in planning target volume (PTV) coverage, there was no significant difference among TPS vs. Mobius3D, TPS vs. MobiusFX, and TPS vs. ArcCHECK. For the dosimetric differences in organs at risks (OARs), the number of metrics with an average dosimetric differences higher than ±3% for TPS vs Mobius3D, TPS vs MobiusFX, and TPS vs ArcCHECK were 1, 1, 7; 2, 1, 4; 1, 1, 5 for the patients with head‐and‐neck, abdomen, and chest cancer, respectively. The %GPs of global gamma indices for Mobius3D and MobiousFX were above 97%, while it ranged from 92% to 96% for ArcCHECK. The %GPs of individual volume‐based gamma indices were around 98% for Mobius3D and MobiousFX, except for γPTV for chest and abdominal cancer (88.9% to 92%); while it ranged from 86% to 99% for ArcCHECK. In conclusion, some differences in dosimetric metrics and gamma passing rates were observed with ArcCHECK measurement‐based QA in comparison with independent dosecheck and log files based QA. Care must be taken when considering replacing phantom measurement‐based IMRT/VMAT QA.

As a novel IMRT delivery technique, volumetric modulated arc therapy (VMAT) has more degrees of freedom by simultaneously moving multileaf collimators (MLCs) and gantry, as well changing the dose rate, which also renders it sensitive to calculation and delivery errors and requires more intensive QA procedures. 3 Traditionally, the pretreatment IMRT QA was carried out by irradiating a phantom-detector combination to measure the consistency between delivered and calculated dose distribution. 1 However, studies pointed out that phantom measurement QA may not be able to detect some types of failures in the IMRT process, such as dose calculation errors, plan transfer errors, etc. 4 Additionally, the use of water equivalent phantoms for dose recalculation and delivery oversimplifies the QA processes, because water equivalent phantoms do not represent patients' real geometry and tissue heterogeneities. 5 Another shortcoming of measurement-based QA is its labor-intensive and time-consuming characteristics, and requires access to the treatment machine.
A growing interest in using machine log files and independent treatment planning system (TPS) dose recalculation for IMRT QA has been proposed. [6][7][8][9] It has been reported that linear accelerator (Linac) log files based QA is able to provide insight into machine parameters that was not possible with phantom-based QA and to improve the efficiency of patient-specific QA. 10,11 Additionally, log files based QA could assess the actual delivered dose by reconstructing the dose on patients' original computer tomography (CT) image sets. 12 However, the accuracy of log files based QA has also been questioned and concerned. 13 It has been reported that in some cases the recorded MLC position in the log files did not agree with the observed positions. 14 Currently, it is still no consensus on whether the Linac log files and independent TPS dose checks are effective enough to be alternative to phantom measurement-based QA. The purpose of this study was to investigate the dosimetric agreement among independent TPS dose recalculation, log file-based, and phantom measurement-based QA in patients who underwent VMAT at different tumor sites.

2.A | Patients and treatment planning
Patients who underwent VMAT treatments from January 2019 to June 2019 were randomly selected and enrolled in this study. Onearc or two-arc VMAT plans were optimized with the SmartArc algorithm in the Pinnacle TPS (Philips Healthcare,Fitchburg, WI) for a 6-MV X-ray beam. One-arc plans were optimized with a gantry angle from 181°to 180°. For two-arc plans, the first arc rotated clockwise from 181°to 180°, and the second arc rotated counterclockwise from 180°to 181°. The collimator was set 15°for all plans. A maximum leaf motion constraint of 0.46 cm/deg and a final arc space of 4 degree were set for both one-arc and two-arc VMAT plans with a dose grid of 3 mm × 3 mm × 3 mm during the VMAT optimization.

2.B | Independent dose check
Mobius3D software (Mobius Medical Systems, Houston, TX) was applied in this study to verify the VMAT plans generated by Pinnacle TPS independently using a collapsed cone convolution/superposition algorithm. 18,19 Before the clinical application, the Mobius3D software was commissioned with measured percent depth doses     20 Tyagi et al also observed dosimetric differences up to −21.4% between TPS and independent dose check with 3DVH for VMAT plans. 23 The relative high errors in Dmax of lens may be due to the relative small volume of lens (0.03 cc) which renders it very sensitive to small spatial deviation between two calculation systems. Similarly, V100 is in a region of great dose gradient which also renders it sensitive to small spatial deviations.
The dosimetric differences resulted from independent dose check and log files based QA were quite consistent in this study.
There were no significant differences between TPS vs Mobius3D

| CONCLUSIONS
Cross verification of independent dose check, log files based QA and phantom measurement-based QA showed reasonable accuracy for VMAT in the head-and-neck, chest and abdominal cancer patients.
Some differences in dosimetric metrics and gamma passing rates were observed with ArcCHECK measurement-based QA in comparison with independent dose check and log files based QA.
Care must be taken when considering replacing the phantom measurement-based QA for IMRT/VMAT.

ACKNOWLEDG MENTS
This work was partially funded by Wenzhou Municipal Science and Technology Bureau (2018ZY016 and Y20190181), and National Natural Science Foundation of China under Grant (No. 11675122).

CONFLI CT OF INTEREST
The authors have declared that no competing interest exists.