Cherenkov emission-based quality assurance for linear accelerators

When electrons exceed the speed of light in a medium, they emit low-intensity visible light, known as Cherenkov emission (CE). This study proposes a novel CE-based quality assurance (QA) test for linear accelerators. A CE-based QA (C-QA) phantom incorporating a mock tumor and four CE observation plates (top, bottom, left, and right) was developed. After tumor-based alignment using cone-beam computed tomography (CBCT), lateral and posterior fields were used for irradiation. A C-Dose camera was employed to measure the treatment position, gantry angle, photon energy (TPR20,10), and CE counts for both fields. The treatment position and TPR20,10 were determined by analyzing the changes in the CE profile, while the gantry angle was calculated based on the tilt between the entry and exit field positions. Confidence limits were evaluated over a three-month period, during which long-term testing demonstrated favorable results. The standard deviations (σ) for CBCT-based positional accuracy and gantry angle were within ±1 mm in all three directions and within 1°, respectively. The mean ± σ for TPR20,10 was 0.631 ± 0.004, closely matching the 0.629 measured using an ionization chamber. Detected CE counts exhibited a higher variation (σ = 2.7%). CE-based QA appears to be an effective and reliable method for radiotherapy. Treatment position could be directly measured without conventional dosimetric devices, while CE imaging simultaneously evaluated positional accuracy, gantry angle, and photon energy (TPR20,10). However, accurate assessment of linear accelerator dose output remains a challenge, and the quantification of CE counts requires further investigation.