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

Purpose: To 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). Methods: The PSD used in this study was made from a 1-mm diameter, 2-mm long BCF60 scintillating fiber (emission peak at 530 nm) 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 x 2048 pixel, uncooled Alta 4020 polychromatic CCD camera. The LS consisted of a 1600 x 1200 pixel Alta 2020 polychromatic CCD camera (cooled to -18 degrees C) with a 50-mm lens with f/# =1. Dose measurements were made under the same conditions for each system (isocentric setup; depth of 1.5 cm in solid water using a 10 x 10 cm(2) field size and 6-MV photon beam). The performance of each system was determined and compared, using the chromatic Cerenkov removal method to account for the stem effects produced in the clear plastic fiber. Results: The 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 degrees 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%. Conclusion: Our 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. (C) 2015 Elsevier Ltd. All rights reserved.