Advanced characterization of an optical fibre sensor system based on an MPPC detector for measurement of X-ray radiation in clinical linacs

[Display omitted] •A reliable, accurate and in-vivo dosimetry system for measuring the radiation dose and profiling the X-ray beam during radiotherapy is reported. None of the currently available commercial dosimeters are capable of detecting radiation dose truly in-vivo during radiotherapy treatment.•The system has been previously fully characterised using a novel in-house developed light emitting diode based system designed to fully replicate the light signal in the clinical setting.•The use of this system has allowed a rigorous investigation of the characteristics of the multi pixel photon counting (MPPC) detector to establish the correct exposure conditions, which is essential for accurate dose measurement in the clinical setting.•The use of the Optical Fibre Sensor (OFS) system is the first reported instance of ultra-high time resolution measurement (sub-microsecond) of the X-ray irradiation from a clinical based linac being used in the clinical setting.•The system utilises both the digital count and analogue voltage outputs of the (MPPC) detector in a unique combination that allows the identification of a constant ratio (4.68) of the average incident photon number to the average output voltage exists which is universal across a wide range of OFS and can be used to determine the existence of correct exposure conditions. A reliable, accurate and in-vivo dosimetry system for measuring the radiation dose and profiling the X-ray beam during radiotherapy is reported. Its dynamic range is investigated using an accurately controlled pulsed light emitting diode (LED) system. Highly resolved temporal analog and digital signals were captured from the analog and digital outputs of a multi-pixel photon counter (MPPC) detector when exposed to the LED system. The photon distribution of a low intensity pulsed LED light source was observed and is found to obey a Poisson distribution with changing light intensity. The average number of photons was obtained using the digital MPPC output signals which in turn allowed the appropriate intensity of the light source to be determined for the correct light exposure conditions for the detector. The average analog output voltage over a single 3 μs pulse is determined to indicate the intensity of the detected light. The MPPC detector output analog signal is limited to a narrow range (0.6 V–1.4 V) to ensure adequate signal detection level (the lower limit) and prevention of entry into saturation (the upper limit) which also corre