Log file-based quality assurance method for respiratory gating system

As medical linear accelerator technology advances, enabling higher dose rate deliveries, hypofractionation regimens has increased. This necessitates respiratory gating systems that synchronize radiation delivery with tumor position, requiring simple rigorous quality assurance (QA) to ensure treatment accuracy and patient safety.

This study aimed to propose log-based QA for respiratory-gated radiation therapy using the respiratory gating system and treatment machine.

4D CT scans were performed with a Varian motion phantom using a Varian Respiratory Gating for Scanner (RGSC). A treatment plan using 25%-75% respiratory phases with 100 MU was created and delivered to a solid water phantom. Treatment logs containing respiratory signals, beam on/off flags, and frame information were extracted from the treatment planning system's offline review. Log file analyses were conducted using in-house softwares to assess temporal synchronization between respiratory phases and beam triggers. Output measurements using a calibrated ion chamber (FC65G) were performed to evaluate dosimetric accuracy. Additionally, EPID images were acquired in cine mode and analyzed frame-by-frame to independently verify beam delivery timing.

Log file analysis revealed precise temporal synchronization, with mean time differences of 0.03 s ± 0.05 s between the planned 25% phase and beam-on, and -0.04 s ± 0.05 s between 75% phase and beam-off. The log-derived beam-on duration (2.61 s ± 0.02 s) closely matched the planned duration (2.66 s ± 0.00 s). Three-month log data showed consistent temporal accuracy, with trigger-on times remaining stable at 2.60 s ± 0.01 s across all measurements. Supporting ion chamber measurements confirmed dosimetric agreement between gating and non-gating modes (difference: 0.05 cGy ± 0.09 cGy) CONCLUSIONS: The proposed log file-based QA method demonstrated high accuracy and reproducibility in assessing respiratory gating performance. This approach provides an efficient, objective method for standardizing QA procedures in respiratory-gated radiation therapy, enhancing treatment accuracy and patient safety.