Some radiation therapy patients report seeing flashes of light in front of their eyes during treatment – even when their eyes are closed. Now this long-standing mystery may have been solved, thanks to this weird effect being caught on camera for the first time.

What's happening, according to a new study, is that enough light is being produced inside the eye to cause these visual sensations. It's what's known as Cherenkov emissions or Cherenkov radiation, the same effect that causes nuclear reactors to glow blue when they're underwater.

Models have shown that as the radiation beam passes through the vitreous fluid or the clear gel of the eye, light is generated, and the researchers have provided the direct evidence.

The discovery could help to improve future radiation treatments – and to put patients' minds at ease about those flashing lights.

"Our newest data is exciting because for the first time, light emission from the eye of a patient undergoing radiotherapy was captured," says biomedical engineer Irwin Tendler, from Dartmouth College.

"This data is also the first instance of evidence directly supporting that there is enough light produced inside the eye to cause a visual sensation and that this light resembles Cherenkov emission."

The idea of Cherenkov emission in radiotherapy had already been put forward by scientists, but only alongside other hypotheses. To find direct evidence, the team behind the new study used a special camera imaging system called CDose.

Specifically designed to capture light emissions during radiotherapy, both in animals and humans, CDose enabled the researchers to see light coming from the eye – something that's usually very hard to detect.

The patient studied did indeed report seeing light flashes during the experiment. Follow-up tests on pig eyes were able to confirm the composition of the light and identify it as Cherenkov radiation.

"Our real-time data rigorously showed that the amount of light produced is sufficient to elicit a visual sensation – a topic that has been debated in the literature," says Tendler.

"By analysing the spectral composition, we also show that this emission can be classified as Cherenkov light – again, another contested point in the literature."

The findings might enable future radiotherapy techniques to be improved – detection of Cherenkov emission could be used as a sign that treatment has or has not hit its intended target, for example.

There's also a relationship between whether patients see light flashes and whether they subsequently experience any vision loss – scientific readings could replace patient reports in this case, for a more accurate method of prediction.

While the researchers say other factors can't be ruled out completely, their specialist camera setup has helped to confirm that what Pavel Cherenkov first discovered in 1934 can indeed be reproduced inside the human eye.

"Although theories regarding direct nerve stimulation, scintillation of the lens, and ultraweak bioluminescent photons cannot be ruled out, it seems clear that Cherenkov light production throughout the eye is quantifiable and significant," conclude the researchers in their published paper.

The research has been published in the International Journal of Radiation Oncology.