Feasibility study of a scintillation sheet-based detector for fluence monitoring during external photon beam radiotherapy

•A pioneering study of a transmission detector using a scintillation sheet and camera.•Monitoring entrance fluence distribution with high-spatial resolution (0.25 mm/pixel).•Attenuation of the beam intensity with no appreciable effect on beam characteristics.•Possible use in the independent verification of external photon beam radiotherapy.