An estimate of the perturbing effect of Cerenkov radiation to the signal from a liquid scintillator volume irradiated by a megavoltage photon beam

Detecting the scintillation light emitted from a volume filled with liquid scintillator (LS), combined with tomographic reconstruction is a promising approach for 3D dosimetry in radiotherapy. Cerenkov radiation and direct x-ray interaction with the detector dominate background signal while using scintillation light detection in megavoltage dosimetry measurements. Spikes can be eliminated using a multiple image technique and Poisson distribution parameters. The perturbing effect of Cerenkov radiation has been evaluated and suppressed for small plastic scintillator detectors using various techniques. We provide experimental evidence for the Cerenkov light contribution to the signal when LS is used as a registration medium. We recorded the light images of a volume irradiated by 6 and 18 MV photon beams, 1 cm in diameter, and filled with: 1) LS, 2) the LS solvent only, and 3) water. The light is detected at 90 degrees with respect to the beam using a low noise cooled CCD camera. We compare the light emission intensities obtained from the three media and account for the refractive index differences. The contribution of Cerenkov radiation was estimated to be between 2 and 6% for 6 MV photons and between 3 and 10% for 18 MV photons. In our experiment, the detection angle was the only Cerenkov light reducing factor. Therefore, after appropriate Cerenkov light subtraction, a 3D dosimetry method using liquid scintillator has the potential for high measurement accuracy.