Evaluation of AAA Dose Calculation Sensitivity to High-Density Material Artifacts Using a CIRS Thorax Phantom
Authors: Md. Anwarul Islam1, Rajada Khatun2, Moumita Chowdhury3, Md. Mokhlesur Rahman3
Institution: 1Clinical Oncology, United Healthcare Services Limited, Dhaka, Bangladesh. 2Medical Physics Division, Atomic Energy Centre, Dhaka, Bangladesh. 3Department of Medical Physics and Biomedical Engineering
Introduction
High-density materials such as metallic implants can introduce CT artifacts and uncertainties in electron-density assignment, potentially affecting treatment planning system (TPS) dose calculation and monitor-unit (MU) estimation. This study evaluated the dosimetric impact of stainless steel (SS) and bone-equivalent material (BEM) in a thorax phantom.
Methods
A CIRS thorax phantom containing SS and BEM inserts was scanned using a GE LightSpeed 16-slice CT scanner at 120 kV and 200 or 400 mA with Standard and Bone reconstruction. Five phantom configurations were investigated using an IBA Farmer ionization chamber positioned at different locations relative to the high-density inserts. Six-MV AP/PA 3D conformal plans were generated in Eclipse v13.6 using the Anisotropic Analytical Algorithm (AAA). Original-material plans were compared with corresponding water-density reference plans while maintaining identical treatment geometry and prescription. Ionization-chamber measurements were performed using a TRS-398-based dosimetry procedure.
Results
SS produced substantially greater dosimetric effects than BEM. When the chamber was outside the insert, SS-related dose differences were approximately 3.5–3.8%, while differences increased to approximately 5.3–7.4% when the chamber was positioned within the high-density region. Relative to the water-density reference, SS produced dose differences of up to approximately 10%, whereas BEM generally showed much smaller differences. Similar material-dependent effects were observed in MU calculations. Variations in tube current and reconstruction technique produced comparatively smaller changes.
Conclusion
High-density material, particularly SS, can significantly influence TPS-calculated dose and MU, with the magnitude strongly dependent on the spatial relationship between the material and dose-scoring region. Appropriate CT density representation and experimental verification are therefore important when high-density materials are present within or near clinically relevant treatment regions.