Design and Validation of a MR-Compatible Deformable and Multi-Detector Phantom for QA and End-to-End Validation
Abstract
Purpose
Validation of online plan adaptation is complex, especially in the context of a MR-Linac where there are very few commercial phantoms available. The purpose of this study is to design and manufacture a modular, simple and low-cost phantom for End-to-End validation of the adaptive workflow and machine QA on a Unity (Elekta, UK).
Methods
The phantom is made of a water-filled transparent plastic jar which is sealed with a 3D-printed detector-specific holder. Compatible detectors include ionization chamber (IC) CC13 (IBA, Belgium), and plastic scintillation detector (PSD) (Medscint, Canada). A silicone insert can be placed over the detector to provide contrast in the MR image. Multiple inserts can be used to simulate changes between simulation and treatment, thus testing the dosimetric accuracy of the adaptive workflow. CT-scans of the phantom with PSD and IC were acquired. Geometric validation was achieved by measuring the offset between the center of the detectors and the center of the jar on the 3D images. Dosimetric validation was done by comparing calculated dose from the treament planning system (TPS) Monaco (Elekta, UK), on the corresponding CT-scan, to dose measured with the PSD calibrated in a conventional Linac. Also, MR images of the phantom with the silicone insert were acquired on the Unity.
Results
CT-scans show that both detectors positions are within 1 mm from the phantom center. Dose calculated in Monaco TPS showed good agreement to PSD measurements <1% for simple fields (10x10, 5x5, AP and PA). The phantom MR images showed bright contrast between the silicone insert and the water, making it easy to delineate.
Conclusion
Geometric and dosimetric testing show that this new phantom has the potential to assess the whole adaptive workflow. Furthermore, the integration of a multi-probe scintillator could allow dose measurement in the target volume and in the OAR simultaneously.