Exhibit Hall | Forum 4
Purpose: To develop a phantom for dosimetric quality assurance (QA) of non-coplanar and mixed beam treatment techniques.
Methods: A homogeneous 1.18 g/cm³ PMMA phantom for gafchromic EBT3 film measurement was designed and fabricated to enable dose measurement from any beam direction. It is a 22.32 x 22.32 x 22.32 cm³ box (bottom 2 cm and walls 1 cm thick) holding four 20.32 x 10.14 x 10.14 cm³ blocks. 20.32 x 20.32 cm² film sheets fit between the blocks (interleaved) and between blocks and box walls/bottom. Therefore, up to three film sheets can be placed simultaneously in orthogonal planes at 1.00, 2.00 or 11.14 cm depths. For dose calculation, the phantom is considered as an equivalent 1.147 g/cm³ density scaled water cube. Its application is demonstrated for dynamic trajectory radiotherapy (DTRT) extending volumetric modulated arc therapy (VMAT) by dynamic table and collimator rotations, colli-DTRT extending non-coplanar VMAT by dynamic collimator rotations, coplanar VMAT, coplanar mixed photon-electron beam radiotherapy (MBRT) and single-electron-field therapy (1E). Both MBRT and 1E collimate electron beams with the multileaf collimator. Absolute Monte Carlo calculated dose of four DTRT plans, two colli-DTRT plans, a VMAT plan, an MBRT plan and a 1E plan were compared to measured dose via gamma analysis (2%(global)/2 mm, 10% threshold) in 2-3 phantom planes.
Results: All plans were successfully delivered in the developer mode of a TrueBeam (Varian Medical Systems) treatment unit. Averaged over all measured planes, Gamma passing rates are 99.2% on average for DTRT (min=98.3%, max=100%), 99.7% on average for colli-DTRT (min=99.6%, max=99.8%), 98.7% for VMAT, 99.6% for MBRT and 99.7% for 1E. Time to setup the phantom including film sheets was between 5 to 10 min.
Conclusion: A phantom suitable for dosimetric QA of non-coplanar and mixed beam treatment techniques was successfully developed and applied for various treatment techniques.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Varian Medical Systems and grant 200021_185366 of the Swiss National Science Foundation.