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Evaluation of Segment Delivery Accuracy of Gated IMRT

H Alahmad

H Alahmad*, J Park , G Yan , B Lu , C Liu , J Li , University of Florida, Gainesville, FL


SU-I-GPD-T-470 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate the segment delivery accuracy of gated step-and-shoot IMRT of a commercial gating system (Response, Elekta Limited).

Methods: A 6 MV lung step-and-shoot IMRT plan with six beams was delivered on a linac (Versa HD, Elekta Limited) with a dose rate of 600 MU/min. The gating was achieved by using the Response interface and an in-house Optical Tracking System. A moving phantom (Quasar, Modus QA) was used to simulate breathing motion. A diode array (Mapcheck2, Sunnuclear Corp) was used and the movie files were saved. In addition, a farmer-type ion chamber was placed on the central axis for simultaneous measurement of the integrated signal for each delivery. Each of the six beams was delivered with no gating, then with gating using duty cycles of 20%, 30%, and 50%. From the movie files, cumulative signals for each segment from gated delivery were compared with those from non-gated delivery. In addition, cumulative signals for each beam from both the diode array and the chamber were compared for delivery accuracy.

Results: Relatively large discrepancy in the cumulative dose, up to 2.3%, was observed in beams delivered using 20% duty cycle. Beams delivered using 30% and 50% duty cycles showed less discrepancies (up to 1% and 0.5%, respectively). However, the highest average segmental dose discrepancy was found to be less than 1.3% for 20% gated beams, and less than 1% for both 30% and 50% gated beams.

Conclusion: Gated IMRT beams using 30% and 50% duty cycles shows acceptable dosimetric accuracy. The accuracy lessens when using 20% duty cycle, and the error becomes more prominent in segments with high monitor unites where more interruptions occur. For the 20% gated beams, the error calculated based on total dose comparison is slightly higher than the combined error based on segmental dose comparison (1% difference).

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