Development and Validation of a 3D Printed Phantom for Off-Axis Dosimetry for External Beam Radiotherapy
Abstract
Purpose
To develop and validate a cost-effective 3D printed phantom for off-axis dosimetry using ion chambers
Methods
The phantom was designed using Autodesk Fusion and printed on a Prusa XL fused deposition modelling 3D printer using polylactic acid (PLA) filament. The phantom was designed to allow for water-filling and be large enough to accommodate a variety of clinically relevant off-axis positions. The phantom comprises 25 cavities that allow for ion chamber placement up to 8cm off-axis in the lateral and anterior-posterior directions and 25cm in the superior-inferior direction. An external wall thickness of 5mm provides support for water filling and an internal cavity wall thickness of 3mm minimizes the amount of PLA in the interior of the phantom. Solid plugs were printed to fill the hollow cavities, and a single plug printed to accommodate a CC13 ion chamber (which can be printed to accommodate any superior-inferior chamber position). The phantom was printed in two pieces (a cap, and a base) which were epoxied together. The interior of the phantom was coated with a thin layer of 2-part silicon for watertightness. The phantom was CT scanned and imported into the TPS. Point dose measurements at various depths on axis were acquired within the phantom to determine the optimal HU density override of PLA in the TPS needed to best match measurement.
Results
The phantom was successfully printed, and the use of epoxy and silicone was sufficient to provide watertightness. By minimizing the RMSE, the optimal density override was determined to be 225 HU, providing agreement between calculated and measured point doses within 1%.
Conclusion
The phantom provides a cost-effective option for performing off-axis dosimetry with ion chambers and will be used for applications such as single-isocenter multi-metastases brain treatments or characterization of novel MLC models.