External beam irradiation angle measurement using Cerenkov emission I: Signal dependencies consideration

This study introduces a novel hybrid Cerenkov-scintillation dosimeter which is intended to be used for irradiation angle measurements based on the Cerenkov angular dependency. First measurements aimed at validating the ability to account for the Cerenkov electron energy spectrum dependency by simultaneously measuring the deposited dose, thus isolating signal variations resulting from the angular dependency. The Cerenkov probe is composed of a 10-mm long sensitive volume of clear PMMA optical fiber separated by an absorptive filter from a 1-mm diameter transport fiber. Filtered and raw Cerenkov signals from the sensitive volume and transport fiber, respectively, were collected using the Hyperscint RP-200 scintillation dosimetry platform. The total signal was unmixed using a hyperspectral approach. Dose calibration of the detector signal was accomplished with photon and electron beams. Using a solid-water phantom, measurements at fixed incident angles covering a wide range of doses and output factors were realized. For fixed incident angle, signal characterization of the Cerenkov detector displays a linear dose-light relationship. As expected, the sensitive volume signal was found to be energy dependent. Output factors were accurately measured within 0.8 % for field size up to 25 cm x 25 cm with both photons and electrons. First validation of the Cerenkov angular dependency shows a linear dose-light relationship for the whole range of angles tested. As expected, the Cerenkov signal intensity per dose unit varies based on the irradiation angle due to the angular dependency. Results showed that using calibration conditions where the electron energy spectrum is similar to the measurement conditions allows to rely on deposited dose to account for this dependency. These preliminary results constitute a first step toward experimental irradiation angle measurements.