Full‑Angle Psqa Using a Rotational Detector Phantom: Advancing Toward Proton Arc Therapy QA
Abstract
Purpose
To introduce and evaluate a rotational phantom that enables full‑angle patient‑specific QA (PSQA) in proton therapy using a standard, commercially available ion‑chamber‑based detector. The focus of this work is to develop an angle‑dependent PSQA workflow suitable for advanced delivery techniques, including IMPT and emerging proton arc therapy approaches.
Methods
We inserted an ion‑chamber array detector (PTW Octavius 1500XDR) into a rotational phantom (PTW Octavius 4D). The rotational phantom is controlled by an inclinometer attached to the gantry, allowing synchronous rotation with the gantry and keeping the detector array perpendicular to the proton beam. Detector alignment to isocenter was verified using orthogonal kV imaging. Two clinical PSQA fields, consisting of brain and thorax plans, were delivered daily over three consecutive days at angles of 0°, 45°, and 90°. Gamma analysis was used to evaluate daily reproducibility and angular dependence of measured dose distributions.
Results
Dose measurements demonstrated high reproducibility across all days and gantry angles. At 3%/3 mm criterion, gamma passing rates for brain and thorax averaged 95.0% (SD=0.96) and 98.5% (SD=0.75), respectively, while at 2%/2 mm criterion averages were 89.1% (SD=2.07) and 88.4% (SD=3.31). These results were in very close agreement with corresponding clinical PSQA records. No systematic degradation was observed with gantry rotation, and all angles produced consistent results between measurement days.
Conclusion
This study demonstrates that full‑angle PSQA can be achieved using a single setup of a rotational phantom integrated with a standard ion‑chamber array. The approach proved operationally practical and provided an efficient, clinically consistent framework, indicating that routine PSQA can be extended beyond fixed gantry orientations without added complexity. By incorporating custom depth capability, this framework can enable smooth verification for a wide range of angles and depths, laying the groundwork for QA methodologies required for emerging proton arc therapy techniques.