Individual volume‐based 3D gamma indices for pretreatment VMAT QA

Abstract Although gamma analysis is still a widely accepted quantitative tool to analyze and report patient‐specific QA for intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT), the correlation between the 2D percentage gamma passing rate (%GP), and the clinical dosimetric difference for IMRT and VMAT has been questioned. The purpose of this study was to investigate the feasibility of individual volume‐based 3D gamma indices for pretreatment VMAT QA. Percentage dosimetric errors (%DE) of dose‐volume histogram metrics (includes target volumes and organ at risks) between the treatment planning system and QA‐reconstructed dose distribution, %GPs for individual volume and global gamma indices, as well their correlations and sensitivities were investigated for one‐ and two‐arc VMAT plans. The %GPs of individual volumes had a higher percent of correlation with individual 15 %DE metrics compared with global %GPs. For two‐arc VMAT at 2%/2 mm, 3%/3 mm, and 4%/4 mm criteria, individual volume %GPs were correlated with 9, 12, and 9 out of 15 %DE metrics, while global %GPs were correlated with only 2 out of 15 %DE metrics, respectively. For one‐arc VMAT at 2%/2 mm, 3%/3 mm, and 4%/4 mm criteria, individual volume %GPs were correlated with 18, 16, and 13 out of 23 %DE metrics, and global %GPs were correlated with 19, 12, and 1 out 23 %DE metrics, respectively. The area under curves (AUC) of individual volume %GPs were higher than those of global %GPs for two‐arc VMAT plans, but with mixed results for one‐arc VMAT plans. In a conclusion, the idea of individual volume %GP was created and investigated to better serve for VMAT QA and individual volume‐based %GP had a higher percent of correlation with DVH 15 %DE metrics compared with global %GP for both one‐ and two‐arc VMAT plans.


2.A | Study design
As shown in Fig. 1 the flowchart for the overall study design, Oneand two-arc VMAT plans were verified with model-based and measurement-based QA to acquire the percent dose errors (%DE) between planed and QA-reconstructed dose distributions, as well as the global gamma passing rate (%GP) and individual volume-based gamma passing rate (%GP). Statistical correlations between %GP and %DE were investigated using Pearson's correlation coefficient. The sensitivities of individual volume-based %GP and global %GP were then investigated and compared.

2.B | Patients and planning
Thirty-one nasopharyngeal cancer (NPC) patients who underwent two-arc VMAT treatment and 33 prostate cancer patients who underwent one-arc VMAT treatment were enrolled in this study.
VMAT plans were optimized with the SmartArc algorithm in Pinnacle treatment planning system (TPS) (Philips Healthcare, Fitchburg, WI) for a 6-MV photon beam on an Elekta Synergy â linac (Elekta Ltd, Crawley, UK) equipped with an 80-leaf MLC (MLCi2 TM , Elekta Ltd, Crawley, UK). Two-arc VMAT objective settings and optimization parameters for NPC patients have been reported in previous studies. 14,15 Briefly, prescription doses were 70 Gy and 56 Gy for gross tumor volume (GTV) and planning target volume (PTV) in 28 fractions, respectively. Organ at risks (OARs) consisting of the brainstem, spinal cord, left and right parotids, as well as lens were constrained for optimization. The first arc rotated clockwise from 181°to 180°, and the second arc rotated counterclockwise from 180°to 181°.
For one-arc VMAT plan of prostate cancer patients, target delineation was done by one radiation oncologist according to the contouring guidelines of Radiation Therapy Oncology Group (RTOG) Trial 0126. 16 GTV encompassed the prostate gland, CTV F I G . 1. Flowchart for the whole study design. YI ET AL.
| 29 encompassed GTV plus the proximal bilateral seminal vesicles. PTV was generated by adding a surrounding margin of 7 mm to CTV. A total of 78 Gy dose was prescribed to PTV at 39 fractions. OARs were outlined according to the Male RTOG Normal Pelvis Atlas. 17 For one-arc VMAT optimization, at least 95% of PTV must be covered by 95% of the prescription dose. OAR constraints included rectum, bladder, peritoneal cavity or bowel, femoral heads, and unspecified tissue. A start angle of 181°and a stop angle of 180°w ere applied for one-arc plans using clockwise (CW) rotation direction. A leaf motion of 0.46 cm/deg and a final arc space degree of 4 were employed for both one-arc and two-arc VMAT.

2.C | Model-based QA and measurement-based QA
In this study, model-based and measurement-based QA were conducted with COMPASS system (version 1.2, IBA Dosimetry, Schwarzenbruck, Germany), 18 which includes an two-dimensional ion chamber (IC) array (MatriXX, IBA Dosimetry) and dose reconstruction software based on a beam model describing the characteristics of the accelerator (e.g., energy spectrum, lateral beam quality variations) and a collapsed-cone convolution/superposition (CCC/S) algorithm. 19 A strict commissioning of the whole system, including the validation of accuracy for 2D-IC array measurement, beam modeling, and dose reconstruction, was performed in advance according to the same standards as the clinic used TPS. 20 A model-based dose reconstruction was performed by importing all the DICOM files (RT plan, RT dose, RT structures, and computed tomography [CT] images) of patients from Pinnacle into COMPASS.
A 3D dose deposition on the patients' CT dataset was reconstructed without measurement using CCC/S algorithm based on the commissioned fluence model and the dose engine to provide an independent dose verification for TPS calculation.
A measurement-based QA was conducted by using MatriXX IC array with 5 cm RW3 (water-equivalent phantom) (PTW, Germany) buildup slabs and gantry angle sensor placed on a gantry holder mount. VMAT plans were firstly delivered to MatriXX and the doseresponse was measured. Predicted dose-response was calculated by

2.E | Correlation and sensitivity analysis
Statistical correlation between 3D %GP of individual volume and % DE, as well as correlation between global %GP and %DE were investigated using Pearson's correlation coefficient (r) with SPSS 17.0 (spss Inc., Chicago, IL, USA). %DE was assumed to be correlated with a determined %GP when P < 0.05, which was obtained from r. In order to compare the sensitivities of 3D %GP of individual volume and global %GP, the number of "false negative" (FN) cases (cases with high QA passing rates but with large errors in DVH dose metrics) and "true positive" (TP) cases (cases with low QA passing rates and with large errors in DVH dose metrics) were calculated. In particular, we considered all those structures "FN" that had DVH metrics errors > 3% among those patients with %GP > 95%. We considered all the cases "TP" that had DVH metrics errors > 3% and %GP < 95%. From the FN and TP rates, receiver operating characteristic (ROC) curves were generated to investigate the ability of individual volume %GP and global %GP to identify accurately the plan with dose errors > 3%. 6 3 | RESULTS

| CONCLUSION
In this study, the feasibility of 3D individual volume c-analysis was investigated for both model-based and measurement-based VMAT QA with COMPASS 3D verification system. Individual volume-based 3D %GP had a higher percent of correlation with DVH-based 15 %DE metrics compared with global %GP for both two-arc VMAT NPC and one-arc VMAT prostate cancer patients, indicating the sensitivity of using individual volume-based 3D gamma indices for VMAT QA.

ACKNOWLEDGMENTS
The study was partially supported by a grant from National Natural

CONFLI CTS OF INTEREST
The authors declare no conflict of interest.