A novel tool for in vivo dosimetry in diagnostic and interventional radiology using plastic scintillation detectors

Abstract

A plastic scintillation detector (PSD) has been developed and optimized to measure dose rate in real-time for low energy beams. The PSD is composed of a scintillating fiber coupled to a clear optical fiber transmitting the scintillator’s light to a photodetector. A spectrometer was used to characterize the scintillator’s spectrum under low and high energy exposures. Seven photodetectors were then evaluated to identify their operating range and potential applications. They comprise a photomultiplier tube (PMT), an avalanche photodiode, two passive diodes of which, and a set of three CCD cameras. The scintillator was exposed to low energy potential beams (120 kVp, 180 kVp, and 220 kVp) of an orthovoltage unit. The source-to-detector distance was varied to explore a broad dose rate range likely to be used in radiology and superficial treatment. Every detector could measure dose rate down to 10 mGy/s while keeping a relative standard deviation below 2 %. The CCD cameras were the less sensitive devices, but they allow multiple fibers to be read simultaneously. Among the photodetectors, the PMT was found to be the most sensitive detector with a relative standard deviation of less than 1 % at the lowest dose rate available. The PMT was then included in the PSD design for an in vivo study in interventional radiology where low dose rate sensitivity is essential. The PSD was located inside a plastic water phantom to measure skin and depth dose from 1 mm down to 24 cm. There was less than a 2 % difference between the PSD measured dose rate and the ion chamber reading located at the same depth. These results indicate that a broad range of photodetectors can be used in the PSD design, but low dose rate measurements require very sensitive devices such as a PMT.