Cerenkov-free Scintillation Dosimeter for use in External Beam Radiotherapy

Abstract

Scintillation dosimetry has the advantages of tissue equivalence, fast response and a small sensitive volume, making them potentially suitable for IMRT dosimetry. Application of scintillation dosimeters to external beam radiotherapy has been hampered by the generation of an unwanted Cerenkov background signal in the optic fiber. Despite numerous approaches aimed at the subtraction of this signal, Cerenkov remains a challenge. We propose an innovative solution, using an air core light guide, which avoids the generation of Cerenkov light and therefore the need to remove it by complex signal processing. The generation of Cerenkov light in an air cored light guide was measured over a range of angles between the radiation beam central axis and the guide axis, that include the characteristic angles for maximum Cerenkov capture in a conventional solid core optical fiber. The new dosimeter was constructed by inserting a scintillator into the light guide at one end. The other end was coupled to a solid core PMMA fiber for conveying the signal outside the bunker into the light detection system. The dosimeter was used to measure the percentage depth dose in water for photon and electron beams. There was no detectable Cerenkov light generated in the air core fiber at any angle of beam incidence. For the air cored dosimeter the background signal was reduced to 2% of the scintillator signal. In the photon percentage depth dose measurements the dosimeter agreed with ionization chamber readings to within 1.5%, with an uncertainty of 0.3%. The readout time was 0.5 second and greater accuracy could be achieved for longer integration times. The air core dosimeter has sufficient sensitivity for use as a dosimeter for external beam radiation therapy. This novel dosimeter design has favorable characteristics for measurements in IMRT fields, where high spatial resolution and fast readout are essential.