Measurement accuracy and Cerenkov removal for high performance, high spatial resolution scintillation dosimetry

Abstract

With highly conformal radiation therapy techniques such as intensity-modulated radiation therapy, radiosurgery, and tomotherapy becoming more common in clinical practice, the use of these narrow beams requires a higher level of precision in quality assurance and dosimetry. Plastic scintillators with their water equivalence, energy independence, and dose rate linearity have been shown to possess excellent qualities that suit the most complex and demanding radiation therapy treatment plans. The primary disadvantage of plastic scintillators is the presence of Cerenkov radiation generated in the light guide, which results in an undesired stem effect. Several techniques have been proposed to minimize this effect. In this study, we compared three such techniques-background subtraction, simple filtering, and chromatic removal-in terms of reproducibility and dose accuracy as gauges of their ability to remove the Cerenkov stem effect from the dose signal. The dosimeter used in this study comprised a 6-mm(3) plastic scintillating fiber probe, an optical fiber, and a color charge-coupled device camera. The whole system was shown to be linear and the total light collected by the camera was reproducible to within 0.31% for 5-s integration time. Background subtraction and chromatic removal were both found to be suitable for precise dose evaluation, with average absolute dose discrepancies of 0.52% and 0.67%, respectively, from ion chamber values. Background subtraction required two optical fibers, but chromatic removal used only one, thereby preventing possible measurement artifacts when a strong dose gradient was perpendicular to the optical fiber. Our findings showed that a plastic scintillation dosimeter could be made free of the effect of Cerenkov radiation.

Measuring output factors of small fields formed by collimator jaws and multileaf collimator using plastic scintillation detectors

“…Thus, Cerenkov light can be effectively removed. 19,29 The chromatic removal technique was used for all measurements discussed below.…”

“…14 Plastic scintillators fit this description well, as they possess close water equivalence ͑i.e., are nonperturbing͒ when compared to other small-volume detectors such as small air-filled ion chambers, diodes, and metal-oxide-semiconductor fieldeffect transistors ͑MOSFETs͒; can be made with smaller volumes than ionization chambers ͑providing high spatial resolution͒; respond linearly with dose; and are practically energy and dose-rate independent over the range of energies used in radiotherapy. [15][16][17][18][19][20][21] Plastic scintillation dosimetry in the setting of small-field irradiation has been studied by several investigators using a variety of systems. [22][23][24][25] However, no close examination has been made of the dependence of the measurement of output factors on the way in which the small fields are defined.…”

Intra-Operative Radiotherapy with Electron Beam

“…This contribution is very small and negligible compared to the scintillation light in the scintillating fibers. However the Cerenkov light becomes dominant, in the light guides, connecting the scintillating fibers to the electronic readout, when the guides are subject to radiation (Archambault et al, 2006).…”

Monte Carlo study of the energy and angular dependence of the response of plastic scintillation detectors in photon beams

“…2,16,17 Various methods have been described to remove the Čerenkov radiation from the detector's signal. 2,4,[18][19][20] An elegant method of circumventing the production of Čerenkov radiation in the optical fiber has also been proposed. 21,22 Since air has a refractive index very close to 1, those investigators used a hollow optical fiber ͑filled with air͒ instead of one with a solid fiber core to minimize the generation of Čerenkov radiation.…”

“…Plastic scintillation detectors ͑PSDs͒ have many advantages for ionizing radiation dosimetry, [1][2][3][4] e.g., water equivalence, photon beam-quality independence, dose-rate independence, linearity with dose, instantaneous readout, and high spatial resolution. All these merits make PSDs preferable to other detectors for real-time in vivo dosimetry.…”