Purpose: To track the position of a gold implant in a moving water phantom using Cherenkov light. This was performed to simulate a method of optical fiducial tracking and to measure dose enhancement around a gold implant.
Methods: A cubic glass container of 7.8 in³ was filled with water. A gold thin plate simulating an implant, which was 0.8 x 1.2 cm² with 0.25 mm thickness, was firmly positioned at the center of the phantom. The water phantom was placed on a platform, which moved the phantom sinusoidally with an amplitude of ±2 cm and a period of 60 s. A flattening-filter-free 10 MV photon beam (5x5 cm² and 2400 MU/min) was used to irradiate the angled surface of the implant. A scientific CMOS camera was set up 80 cm from the implant, using a 2.0 f-number. A total of 60 Cherenkov images were taken every one second with 1 second of the exposure time. To calibrate the length scale of the camera image, another set of images was taken under the ambient room light.
Results: Cherenkov light images showed that the beam-source side of the gold plate was brighter than the other side, indicating the increased backscattered electrons. The result of the 10 positions out of 12 images obtained by the Cherenkov light was in good agreement with the positions expected from the motion of the moving platform, with a difference < 1 mm. However, we noticed that the difference was larger when the platform moved to one end of the amplitude, i.e., 2 cm from the center position. The maximum difference was 2.9 mm.
Conclusion: We successfully demonstrated that the Cherenkov light can be used to track the dynamic motion of a gold implant in water.