Application of plastic scintillating fibres to surface dosimetry in megavoltage photon and electron beams: considerations for Cerenkov correction.

This work investigated the application of plastic scintillation detectors (PSDs) in dosimetry measurements on the surface of a phantom in megavoltage photon and electron beams. Emission spectra associated with fluorescence in the PMMA optical fibre, Cerenkov in the optical fibre, and the signal from the scintillator were characterized with a spectrophotometer. The angular dependence of the PSD response was studied with the PSD both in the plane and perpendicular to the plane of gantry rotation. Two methods of stem correction were investigated: stem subtraction and full spectral correction. It was found that with the PSD in the plane of gantry rotation, the stem subtraction method overestimates the dose particularly in electrons. The magnitude of overestimation is dependent on the beam energy, scintillator type and the angle of incidence. In this study a maximum error of 12% was observed for 16 MeV with a gantry angle of 45°. It was shown that the cause of this error was a non-negligible emission of Cerenkov radiation from the scintillator itself. This error was not observed when the PSD was oriented perpendicular to the plane of rotation. Similarly, the full spectral method was effective in isolating the scintillation-only signal regardless of PSD orientation.