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Abstract
The gated RapidArc may produce a dosimetric error due to the stop-and-go motion of heavy gantry which can misalign the gantry restart position and reduce the accuracy of important factors in RapidArc delivery such as MLC movement and gantry speed. In this study, the effect of stop-and-go motion in gated RapidArc was analyzed with varying gating window time, which determines the total number of stop-and-go motions. Total 10 RapidArc plans for treatment of liver cancer were prepared. The RPM gating system and the moving phantom were used to set up the accurate gating window time. Two different delivery quality assurance (DQA) plans were created for each RapidArc plan. One is the portal dosimetry plan and the other is MapCHECK2 plan. The respiratory cycle was set to 4 sec and DQA plans were delivered with three different gating conditions: no gating, 1-sec gating window, and 2-sec gating window. The error between calculated dose and measured dose was evaluated based on the pass rate calculated using the gamma evaluation method with 3%/3 mm criteria. The average pass rates in the portal dosimetry plans were 98.72±0.82%, 94.91±1.64%, and 98.23±0.97% for no gating, 1-sec gating, and 2-sec gating, respectively. The average pass rates in MapCHECK2 plans were 97.80±0.91%, 95.38±1.31%, and 97.50±0.96% for no gating, 1-sec gating, and 2-sec gating, respectively. We verified that the dosimetric accuracy of gated RapidArc increases as gating window time increases and efforts should be made to increase gating window time during the RapidArc treatment process.
References
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Fig. 1.
System for the DQA process with the MapCHECK2 measurement and the dynamic moving phantom for the accurate control of gating window time.
Fig. 4.
The comparison of DQA analysis results with portal dosimetry plan in three different gating cases.
Table 1.
The pass rates calculated using the gamma evaluation in the DQA process of gated RapidArc plans with a portal dosimetry and a MapCHECK2 measurement.
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