Assessment of using an Octavius 4D Measuring System for Patient-specific VMAT Quality Assurance in Togo

Abstract

Purpose and objective: Quality assurance (QA) programs are designed to improve the quality and the safety of radiation treatments, including machine- and patient-specific QA (PSQA). The objective of this study was to evaluate the current state of PSQA practice and identify the area for potential improvement for VMAT delivery. Materials and methods: The Octavius 4D (O4D) system accuracy was evaluated using an O4D homogeneous phantom for different field sizes. The system response to dose linearity, field sizes, and PDD difference tests were performed against the calculated dose of the treatment planning system (TPS) for a 6 MV photon beam. The deviation of the delivered dose was evaluated at the isocenter and different depths. Moreover, pretreatment verification of 40 VMAT plans was performed including prostate cancer (PC), head and neck cancer (HNC), uterine and cervical cancer (UCC), and breast cancer (BC). The PTW VeriSoft software was used to perform the local and global 3D gamma analysis by comparing the reconstructed 3D dose against the calculated dose using criteria 2%/2 mm and 3%/3 mm, 20% of low-dose threshold, and 95% of gamma passing rate (%GP) tolerance level. In the clinical scenario, the sensitivity of the O4D system in detecting VMAT delivery and setup errors has been investigated by measuring the variation of %GP values before and after the simulated errors using one of the VMAT plans related to each treatment site. Results: The O4D system reported good agreement for linearity, field size, and PDD differences with TPS dose being within ± 2% tolerance for a 6 MV photon beam. Output factors were consistent between the ionization chamber and the O4D detector 1500 array down to 4 x 4 cm2 field size with a maximum deviation of less than 1%. The introduction of deliberate errors caused the decrease of %GP values. In most scenarios, the %GP value of simulated errors was detected with 2%/2 mm and ranged between the detection threshold and gamma passing threshold. Conclusion: The results indicate that the O4D system is sensitive to detect delivery and setup errors with restrictive criteria of 2%/2 mm for routine pretreatment verification. Moreover, this system should be used in combination with kV-CBCT to improve dosimetry accuracy and treatment reproducibility.