| Summary: | The purpose of this project is to develop a quality assurance (QA) procedure for gated volumetric modulated radiation therapy (VMAT) in stereotactic ablative radiation therapy (SABR) for liver cancer treatments and investigate the gating parameters for acceptable plan delivery in terms of the dose to a moving volume and treatment delivery time.
10 patient plans for VMAT SABR liver were created using the Eclipse??? treatment planning system (TPS). The plans were then transferred to a CT-scanned (computed tomography) Quasar??? phantom (i.e. a water-equivalent, geometrically simplified representation of a patient) and delivered on a TrueBeam??? linear accelerator using a 10FFF (flattening filter free) beam and Varian???s real-time position management (RPM) system for respiratory gating. Two kinds of breathing patterns were used: free breathing (FB) and an interrupted (~5 s pause) end of exhale coached breathing (CB) pattern. Ion chamber and Gafchromic??? film measurements were acquired for a gated delivery while the phantom moved under the described breathing patterns and a non-gated, stationary phantom delivery. The gate window was set to obtain a range of residual target motion from 2-5 mm.
All gated deliveries have been shown to be dosimetrically equivalent to the static deliveries with differences in point dose measurements under 1% and average gamma (2%, 2 mm) agreement above 98.7%. Comparison with the treatment planning system resulted also in good agreement, with differences in point dose measurements under 2.5% and average gamma (3%, 3 mm) agreement of 92%. The use of a CB pattern increases significantly the duty cycle compared with free breathing and allows for shorter treatment times.
Gated VMAT treatments have been delivered successfully to a motorized phantom. FB patterns contain considerable variability and it is difficult to achieve acceptable results even with very small gate windows. However, a CB pattern combined with a sufficiently small gate resulted in acceptable dose distributions that can be delivered in a reasonable amount of time.
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