Abstract
Purpose: To characterize and evaluate Octavius 4D system for patient specific quality assurance using 3D gamma index analysis for patients treated with VMAT technique. Methods: Characterization of the 4D system was carried out by following tests. (i) Verification of calibration factor of each chambers, (ii) reproducibility of all the 729 chambers, (iii) linearity for static and arc mode, (iv) output factor dependency for static and arc mode, (v) dosimetric comparison between measured and TPS calculated (AcurosXB 11.0.21) dose distributions for combination of open fields and/or wedge fields and TG 119 benchmark plans for VMAT. Evaluation of Octavius 4D system is carried out by 3D gamma index analysis (3mm/3% DD) of 102 clinically approved VMAT patient plans of different sites. Results: Average percentage deviation between measured and manufacturer specified calibration factors for all the chambers is −0.28%. Reproducibility measurements of 729 chambers showed maximum standard deviation of 0.25% and 0.2% for 6MV and 15MV respectively. Linearity (test) for static and arc mode, output factor dependency (test) for static and arc mode tests agree very well with semiflux ionization chamber measurements. Average passing rates for combination of open fields and/or wedge fields are, 96.13% and 96.08% for coronal and sagittal plane 2D gamma evaluation respectively and 95.84% for volumetric 3D gamma evaluation. Isocentre dose measurements for TG 119 benchmark IMRT AND VMAT plans are within 2% and average volumetric 3D gamma analysis results are 97.77% and 96.25% for 6 and 15MV respectively. Volumetric global gamma evaluation shows 97.13%, 95.39% and 97.11% passing results for Head and Neck, Thorax and Pelvis sites respectively. Conclusion: Characterization and evaluation results indicate that Octavius 4D system is an efficient tool for 3D patient specific quality assurance.
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