Author: Xiaodong Wu, Xingchen Zhai, Weizhao Zhao 👨🔬
Affiliation: University of Miami 🌍
Purpose: The ZAP-X is a self-contained, self-shielded radiosurgery system utilizing a 3MV LINAC. Its self-shielded design, poses challenges for conventional beam quality assurance (QA) methods. This study investigates the feasibility of developing a robust QA system using Cherenkov radiation.
Methods: Both simulation and experiment studies were conducted for 3MV ZAP-X beam with a 25mm cone. The FLUKA flair Monte Carlo platform was used to simulate x-ray beam with nominal accelerating potential (NAP), ranging from 2.85MeV to 3.15MeV in 0.03 MeV increments, irradiating an octagonal-column-shaped water tank at a 45cm SSD. In the experiment, a Canon EOS Rebel T3i camera with Canon EF-S 18-55mm f lens was used to capture Cherenkov light emitted from the same water tank housed in a dark box. The exposure time, ISO speed (sensitivity) and signal-to-dose response were investigated using an f/3.5 aperture. A 20x20 median filter was applied to all captured images (3456x5148 uint8 RGB) to reduce random noise while maintaining submillimeter resolution.
Results: For simulation, the ratios of Cherenkov light fluence at 20cm depth to 10cm depth was found proportional to NAP. For the experiment, maximum image intensity and average background noise were measured. The optimal signal-to-noise ratio (SNR) was observed at 800 ISO with a 20-second exposure without background subtraction. A monatomic degression in SNR was observed with longer exposure times ranging from 5 to 30 seconds. When using background subtraction with a 20-second exposure and 1600 ISO, the linearity of maximum intensity to dose (50 MU to 400 MU) correlation was calculated, with an R square value of 0.995.
Conclusion: Cherenkov radiation can be utilized to achieve high sensitivity in detecting beam energy fluctuations and other characteristics of the 3MV photon beams of the ZAP-X system. An optimal camera configuration was determined and a high linearity of signal-to-dose response was observed.