Cerenkov-free scintillation dosimetry in external beam radiotherapy with an air core light guide.

Plastic scintillators have many advantages for dosimetry in external beam radiotherapy. The current method of transmitting the scintillation light to a remote detector is through a solid core optical fibre. When exposed in a high energy therapeutic radiotherapy beam this fibre is subject to an unwanted background signal from Cerenkov light which can exceed the scintillation signal at characteristic angles. We have constructed a plastic scintillation dosimeter that uses an air core light guide to transport the light from the scintillator to the light detector. We show that there is sufficient signal propagation in the air core light guide to allow the scintillator signal to be carried outside the primary beam of a radiotherapy linear accelerator and for a dosimeter to be constructed using a scintillator inserted into the end of the light guide. Studies of the background light generated in the air core light guide, as a function of the angle between the beam and the fibre axis, show that there is no characteristic Cerenkov peak generated in the air core. Depth dose measurements using the air core scintillation dosimeter with no correction for Cerenkov are compared to ionization chamber measurements for a 6 MV photon beam and a 9 MeV electron beam.