Purpose or ObjectiveAs a useful 3D quality assurance (QA) tool, ArcCHECK has gained wide application in helical tomotherapy (HT), but most of the studies were based on old models.This study is to report our experience in ArcCHECK based QA for HT with TomoEdge technique including the establishment of the action level. Material and MethodsA total number of 303 clinical plans in different treatment regions in our hospital were retrospectively studied.The tomotherapy plan verification was conducted using the ArcCHECK diode array with an acrylic insert for placing an A1SL ionization chamber.DQA plans were created by situating the target at the center of ArcCHECK for point dose measurement and meanwhile, making sure the electronic part of the device was not under the main beam.Gamma analysis method was then used to quantitatively compare the dose measured by ArcCHECK and that calculated from the treatment planning system (TPS).The criteria of 3mm distance to agreement (DTA), 3% dose difference, 10% threshold and absolute dose comparison were chosen.According to AAPM TG 119, the recommended action level of gamma passing rate was calculated as AL=Gm-1.96σ,where Gm was the mean of the gamma passing rate and σ was the standard deviation (SD). ResultsDetails of our data are shown in table 1.A value pitch of 0.287 was used for almost all cases except for some brain treatments with values of 0.143 and 0.215.Jaw width of 2.5cm and 5cm with dynamic technique took up 95.7% of total cases and only fixed setting was used for 1cm jaw width.Good point dose agreements between measurements and TPS data were obtained with a mean deviation of 0.75%.Absolute gamma comparison gave an averaged passing rate of 96.6% with the SD of 4.7%, which resulted in an action level of 87.4%.Cases with lower gamma passing rate appeared to be in thorax and abdomen regions, which were suspected to be related to high MF and off-axis induced diode over-response.Our limited data showed that the gamma passing rate could be larger than 92% after a new DQA plan was created by locating the ArcCHECK close to the isocenter.
