A Review of RapidArc Standard Deviations
E Brandner*, R Specht, S Bahri, I Poltinnikov, M Huq, University of Pittsburgh Medical Center, Pittsburgh, PASU-E-T-397 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: he purpose of this study is to acquire baseline values for the standard deviations of MU and angle reported by a Varian 23ex linac following the delivery of a RapidArc field.
Methods: During the delivery of RapidArc fields, the Varian linac records the gantry angle and cumulative MU every 50 ms. By comparing this data to the corresponding planned data, the system calculates the standard deviations of the MUs and the gantry angles for each field. These standard deviations are reported to the linac screen at the completion of each field and documented daily for each field along with the patient, plan, treatment site, and date. The data was then entered into Micorsoft Excel for analysis. This data was acquired for every RapidArc patient (88 total patients) over the course of the first year of RapidArc use.
Results: The maximum MU standard deviation was 0.09 and the maximum gantry angle standard deviation was 0.30°. The averages are 0.05 MU and 0.19° respectively. No specific treatment site (prostate/prostate fossa, head and neck, brain, thorax, or other) differed significantly from these averages.
Conclusion: If the MU standard deviation is greater than 0.1 or the gantry angle standard deviation is greater than 0.3°, the RapidArc QA tests of the linac and the plan should be promptly repeated to identify any possible problems. These baseline values may be specific to the linac in question but provide a point of reference for other linacs as well.