A system for the measurement of radiation emitted from an in-vivo administered radioactive analyte. The system employs a sensor having a scintillation material to convert gamma radiation to visible light, which enables embodiments of the sensor to be ex vivo. A light detector converts the visible light to an electrical signal. This signal is amplified and is processed to measure the captured radiation. Temperature of the sensor may be recorded along with this radiation measurement for temperature compensation of ex vivo embodiments. The sensor enables collection of sufficient data to support separate application to predictive models, background comparisons, or change analysis.