Comparison between two different patient specific QA systems for detection of errors introduced to VMAT plans

Abstract

Volumetric Arc Therapy (VMAT) technique involves varying the gantry angle, MLC position, and the dose rate as the radiation is delivered. To ensure the integrity of VMAT plans, each plan must be verified through a quality assurance (QA) process, which is known as patient-specific QA. The quality assurance system should detect the planning and machine errors. This study aims to investigate the sensitivity of two different patient specific QA systems to detect errors in VMAT plans. Three types of intentional errors were introduced with different magnitudes to five clinical prostate VMAT plans. These errors include MLC position shift, dose variation and, couch rotation error. Two proposed patient specific QA systems were used to detect VMAT plans errors. Furthermore, global and local γ analysis methods were used to compare the measured and calculated dose distributions using 3%/3mm and 2%/2mm tolerance criteria. The result shows that the two systems have the ability to detect MLC shift with the Octavius 4D showing extra sensitivety to detect smaller magnitude of MLC shift. Moreover, the 2D array 729 detector showed extra sensitivity for detecting increased dose. On the other hand, the 2D array 1500 showed extra sensitivity in detecting underdose and has the ability to also detected overdose. Additionally, the Octavius phantom has the ability to detect a smaller magnitude of couch rotation shift. While the Octavius 4D phantom failed to detect the couch rotation.