Characterization of a fiber-taper charge-coupled device system for plastic scintillation dosimetry and comparison with the traditional lens system

Abstract

PurposeTo compare the signal-to-noise ratio (SNR), dose sensitivity and stability, and reproducibility of a lens-less charge-coupled device (CCD) photon-counting system with those of a traditional CCD + lens photon-counting system for plastic scintillation detectors (PSDs). MethodsThe PSD used in this study was made from a 1-mm diameter, 2-mm long BCF60 scintillating fiber (emission peak at 530nm) coupled to a 2.6-m Eska GH-4001 clear plastic fiber. This PSD was coupled to either a fiber-taper-based photon-counting system (FTS) or a lens-based photon-counting system (LS). In the FTS, the fiber-taper was attached to a 2048×2048 pixel, uncooled Alta 4020 polychromatic CCD camera. The LS consisted of a 1600×1200 pixel Alta 2020 polychromatic CCD camera (cooled to −18°C) with a 50-mm lens with f/#=1. Dose measurements were made under the same conditions for each system (isocentric setup; depth of 1.5cm in solid water using a 10×10cm2 field size and 6-MV photon beam). The performance of each system was determined and compared, using the chromatic Čerenkov removal method to account for the stem effects produced in the clear plastic fiber. ResultsThe FTS increased the light collected by a factor of 4 compared with the LS, for the same dose measurements. This gain was possible because the FTS was not limited by the optical aberration that comes with a lens system. Despite a 45°C operating temperature difference between the systems, the SNR was 1.8–1.9 times higher in the FTS than in the LS, for blue and green channels respectively. Low-dose measurements of 1.0 and 0.5 cGy were obtained with an accuracy of 3.4% and 5.6%, respectively, in the FTS, compared with 5.8% and 15.9% in the LS. The FTS provided excellent dose measurement stability as a function of integration time, with at most a 1% difference at 5 cGy. Under the same conditions, the LS system produced a measurement difference between 2 and 3%. ConclusionOur results showed that the FTS could measure doses more accurately than the LS and that low-dose measurements were feasible without the complexity of a lens-based system. The FTS would therefore be better adapted for routine clinical usage of fiber arrays. The increased SNR in the FTS suggests that water-equivalent PSDs with smaller radii could be used to obtain measurements with greater spatial resolution, further simplifying the use of PSDs for “in-vivo” monitoring and small-field dosimetry.