Room: ePoster Forums
Purpose: 4D IMRT QA is to delivery of a treatment plan to a dosimeter-embedded phantom with simulated motion to evaluate the dosimetric and motion management accuracy of a given clinical workflow. A typical example of 4D IMRT QA is IROC motion phantom credentialing. Motion phantom setup is challenging for 4D IMRT QA. This study investigates the optimum ways to setup a motion phantom.
Methods: A motion phantom was designed by placing a lung tumor phantom upon a QUASAR motion platform, mimicking the IROC liver phantom setup. Two sinusoidal motion traces with 15mm and 40mm amplitude, 10Hz frequency were prepared. BBs were marked on the phantom and the non-movable baseplate prior to simulation. A free breathing (FB) scan and 4D scans were acquired. The treatment plan was generated on the FB scan with iso-center defined at 10cm above the BBs marked on the baseplate. Iso-center position localized by the following IGRT procedures were compared to the above definition: 1) CBCT matching to the BBs on the FB scan, 2) CBCT matching to the target on the FB scan, 3) gated CBCT matching to the target, 4) gated CBCT matching to the BBs. The CBCT gating window was set around phase 0 and acquired CBCT images were matched to CT phase 0 of the simulation CT.
Results: With 14mm motion, regular CBCTs resulted in 4mm iso-center error when matching to FB scans on both BBs and target. With 40mm motion, 3.3mm and 1.5mm error were observed from matching BBs and target between CBCTs and FB scans. The gated CBCT matching to phase 0 images resulted in 0mm iso-center error regardless the amplitude and matching criteria.
Conclusion: Regular CBCT failed to accurately setup the motion phantom for 4D IMRT QA. Gated CBCT is the optimum IGRT method for 4D IMRT QA.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by the MSK Cancer Center Support Grant/Core Grant (P30 CA008748).