Experimentally Observed Cherenkov Light Generation in the Eye During Radiotherapy

Abstract Purpose Humans have reported sensations of seeing light flashes during radiotherapy, even with their eyes closed. These observations have been attributed to either direct excitation of retinal pigments or generation of Cherenkov light inside the eye. Both in vivo human and ex vivo animal eye imaging was utilized to confirm light intensity and spectra to determine its origin and overall observability. Methods & Materials: A time-gated and intensified camera was used to capture light exiting the eye of a stereotactic radiosurgery patient in real-time, thereby verifying the detectability of light through the pupil. These data were compared with follow-up mechanistic imaging of ex vivo animal eyes with thin radiation beams to evaluate emission spectra and signal intensity variation with anatomical depth. Angular dependency of light emission from the eye was also measured. Results Patient imaging showed that light generation in the eye during radiotherapy can be captured with a signal-to-noise ratio of 68. Irradiation of ex vivo eye samples confirmed that the spectrum matched that of Cherenkov emission and signal intensity was largely homogeneous throughout the entire eye, from the cornea to the retina, with a slight maximum near 10 mm depth. Observation of the signal external to the eye was possible through the pupil from 0o – 90o, with a detected emission near 2,500 photons per millisecond (during peak emission of the ON cycle of the pulsed delivery), which is over two order of magnitude higher than the visible detection threshold. Conclusions By quantifying the spectra and magnitude of the signal, we now have direct experimental observations that Cherenkov light is generated in the eye during radiotherapy and can contribute to perceived light flashes. Furthermore, this technique can be used to further study and measure phosphenes in the radiotherapy clinic.