Sensitivity in error detection of patient-specific QA tools for IMRT plans

Abstract

Currently, there are many patient specific QA tools that are available in the commercial. The configuration of each device and their efficiencies are also difference from each other. Thus the patient specific QA tools that used in the clinical fields need to be chosen according to their efficiency and daily routine needs. The purpose of this study is to investigate the error detection capability of the portal dosimetry system, MapCHECK 2 system and MatriXX system in IMRT plans. These three devices used for the 2D patient specific QA tools in this study. The 4 head and neck plans (the complicated plan) and 4 prostate plans (the simple plan) were studied for error detection of QA devices. The 6 MV beams were employed with 9 field arrangement for head and neck IMRT cases while 10 MV beams with 7 fields were optimized and calculated for prostate IMRT plans. Measurements were undertaken for the original plan and the modified plans with errors introduced in order to check the sensitivity of the QA tools system. The intentional errors composed of increasing and decreasing of prescribe dose (±2, ±4 and ±6%) and position shifting in X-axis and Y-axis ( ±1, ±2, ±3 and ±5 mm) both representing the setup uncertainty. After measurement, the results were compared between calculated and measured values of the original plan using gamma analysis at 3%/3mm criteria. Then the gamma pass between original and the intentional error were compared. The average gamma pass of original head and neck plans analyzed by 3%/3mm were 96.9, 98.6 and 98.8 for portal dosimetry, MapCHECK 2 system and MatirXX system, respectively. The average gamma pass for prostate plans were 99.4, 99.0 and 99.7 for portal dosimetry, MapCHECK 2 system and MatirXX system, respectively. In head and neck plans, the shift error detections were 1mm for portal dosimetry, 2mm for MapCHECK 2 and 3mm for MatriXX system, respectively. In prostate plan, the shift error detections were 2mm for portal dosimetry, 3mm for MapCHECK 2 and 5mm for MatriXX system, respectively. For the dose error detection, the portal dosimetry could detect 2% dose in head and neck plan and 4% dose in prostate plan. The other two devices could detect 4% dose in both head and neck and prostate plans. The error detection depends on the detector resolution and the type of plan. If the QA device has higher resolution, it can have more sensitivity to small error detection than less resolution detector.