Purpose: A novel plastic scintillator and spectral analysis approach (Hyperscint™) was investigated in a 74 MeV proton beam for stem effect, dose and dose rate response, and suitability to provide a Cerenkov-free reference spectrum for use in MV x-ray dose calibrations.
Methods: Irradiations were performed in a passively scattered 74 MeV horizontal proton beam line with a 2.5 cm circular field aperture. The plastic scintillator detector (PSD) was 1 mm in diameter and 1 mm in length, coupled to the spectrometer with PMMA optical fibre. The probe was positioned at the centre of a 2 cm spread out Bragg peak in polystyrene phantom 7 cm distal to the proton nozzle. Doses from 200 cGy to 5000 cGy were delivered using the nominal 6 nA beam current, followed by comparison with 1 nA, 2 nA, and 10 nA beam currents. The optical emission spectrum in the proton beam was compared with that acquired in a 150 kVp ortho-voltage x-ray beam, to assess suitability for use as a Cerenkov-free reference spectrum. Stem effect was further investigated in the proton beam by irradiating the fibre cable alone.
Results: The 74 MeV proton beam and 150 kVp x-ray beam produced indistinguishable Cerenkov-free optical emission spectra in the PSD. Integrated area under the spectral curve was linear with dose and independent of dose rate. Optical fibre emission was observed to increase with length of cable irradiated at 0.1 % of the PSD signal per cm of irradiated cable.
Conclusion: The Hyperscint™ dosimetry system shows potential for dose measurement in a 74 MeV proton beam with negligible stem effect. The Cerenkov free spectrum may be used to facilitate calibration of the device in MV x-ray beams to improve Cerenkov removal and performance in small field dosimetry.