International Journal of Radiation Oncology*Biology*Physics
Impact of Beam-hold Interruption to Dosimetry and Delivery Efficiency of Respiratory Gated VMAT with a High-output Linac
Section snippets
Purpose/Objective(s)
Varian's high-output linear accelerator (TrueBeam) delivers respiratory gated VMAT (RapidArc) treatment by holding beam until the gantry angle, rotation speed, MLC leaf position, and gating-window are all synchronized. The current study evaluated the impact of beam-hold interruption to the dosimetry and delivery efficiency of gated VMAT.
Materials/Methods
Respiratory signals were generated by moving the marker block using an adjustable motor. Beam-hold interruptions were tested for respiratory rate of 12, 15, 20, and 30 cycle/min with a gating threshold of 33.3% duty cycle; corresponding to beam-on window of 1.67, 1.33, 1, and 0.67 sec. Two delivery modes were examined: (1) Flattening Filter Free (FFF) mode with maximum dose rate of 2400 MU/min, tested with an 1-arc plan for SBRT lung cancer for 12Gy/fraction; (2) standard-beam mode with maximum
Results
The difference between the gated and non-gated ion chamber measurements was within 0.16% for 1-arc FFF plan and within 0.24% for 2-arc standard-beam plan. The fraction of pixels with γ > 1 ranged from 0%-0.01% and 0.03%-0.46% for 3%/3mm and 2%/2mm, respectively, for the 1-arc FFF plan, and from 0.96%-2.15% and 6.03%-12.65% for 3%/3mm and 2%/2mm, respectively, for the 2-arc standard-beam plan. While there was no clear association between beam-hold number and dosimetric differences, actual dose
Conclusions
In clinical respiratory rate range, gated RapidArc dose deliveries were in agreement with non-gated RapidArc deliveries in both FFF mode and standard mode. Delivery efficiency significantly decreased at high respiratory rate.
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Author Disclosure: S. Shen: None. R.A. Popple: None. J. Duan: None. J.B. Fiveash: None. X. Wu: None. I.A. Brezovich: None.