Assessing Dosimetric Quality of Respiratory-Gated VMAT Plan Delivery Using the Varian Rpm System
Abstract
Purpose
To assess the dosimetric impact of respiratory gating on VMAT plan delivery using the Varian Real-time Position Management (RPM) system.
Methods
This study was performed using a Varian TrueBeam linac equipped with the RPM system for respiratory gating, along with a Quasar pRESP QA phantom. The phantom consists of an acrylic body oval into which movable cylindrical inserts are placed, which can be driven by the phantom motor to mimic patient breathing. A custom acrylic insert was used which allows Gafchromic film to be placed in the coronal plane. A sample VMAT plan was selected and delivered to film while the phantom was static (no gating) and while the phantom was moving in a DIBH-like pattern (gated). Three phantom motion patterns were developed to represent perfect, good, and poor DIBH breathing. These patterns consisted of artificially perfect square breathing, typical patient DIBH breathing, and poor patient DIBH breathing including coughing and short breath hold periods, respectively. All other gating parameters were set using institutional standards. Dose delivered to film was compared to the TPS plan using FilmQA Pro software, with gamma criteria of 3%/2 mm and a 10% threshold.
Results
Film dose agreed well with the TPS plan for the static phantom reference delivery (γ=99.8%). For gated delivery, perfect, good, and poor DIBH breathing all also agreed well with the TPS plan (γ=99.8%, 99.6%, 99.6%, respectively). These results are well within the 95% tolerance limit recommended by TG-218 for QA verification of dose distributions. By comparison, ungated deliveries resulted in worse agreement (γ<<95%).
Conclusion
The RPM system is able to accurately control the delivery of a DIBH-gated VMAT plan, with all gated deliveries resulting in delivery of comparable quality to the reference static delivery. Quality of DIBH breathing had little impact on agreement between planned and delivered dose distributions.