Room: ePoster Forums
Purpose: To compare the results of patient-specific IMRT quality assurance performed with the integral quality monitor (iRT systems IQM) and the electronic portal imaging device (Varian-TrueBeam aS1000).
Methods: Ten retrospective plans consisting of 24 fields were used, including eight VMAT plans and two static IMRT plans, some of which were highly modulated or contained small fields. The IQM was evaluated using the ratio of the measured to calculated reference signal. The EPID was evaluated using gamma analysis (1%/1mm, 2%/2mm, and 3%/3mm) and the ratio of the measured to predicted reference portal dose.
Results: Values for the ratio of the IQM measured to reference cumulative signal range from 0.97 to 1.04 for all fields (median=1.02, SD=0.01). Gamma analysis data showed 76.60% to 100.00% (median=99.45%, SD=5.62%) of the area had a gamma value less than 1.00 depending on the criteria used. Values for the ratio of the EPID measured to reference portal dose range from 0.99 to 1.08 (median=1.03, SD=0.02). Although there is no statistically significant correlation (P-values>0.20) in the comparison of the calculated and measured signals for the two modalities, similar trends were observed. Exclusion of small fields increased the correlation (P-values>1.0). Small field size and greater field modulation did not affect the IQM measured signal, while EPID pass rates significantly decreased for both factors.
Conclusion: The IQM is more sensitive to small dose differences and provides more information regarding machine output than gamma analysis with the EPID. Unlike the EPID, the IQM does not provide information regarding the spatial dose distribution. Although, dissection of the IQM signal may provide spatial information. Preliminary results show no correlation between the two modalities; however, an improvement may be observed in the statistics with a larger sample size. Furthermore, each systemâ€™s error detection capability can be evaluated by introducing errors in the treatment plans.