Visual sensations during megavoltage radiotherapy to the orbit attributable to Cherenkov radiation

Abstract

During megavoltage photon and electron beam radiotherapy treatment involving the eye, patients commonly report visual sensations; "nerve stimulation" is the conventional explanation. We propose that the phenomenon can be attributed to Cherenkov radiation inside the eye. The threshold electron energy for Cherenkov radiation in water is 260 keV. The human retina is able to perceive approximately 5-14 visible photons in 0.001 s. A single 500 keV electron traversing 1 mm of water will induce nearly 15 Cherenkov visible range photons. We propose that a portal image involving the eye will produce sufficient Cherenkov radiation to be detected by the retina.

Experimentally Observed Cherenkov Light Generation in the Eye During Radiation Therapy

“…9,12 Using a model of light production in the eye during radiation therapy, it has been shown that 2.08 Â 10 4 Cherenkov photons are generated in 0.001 seconds during 6-MV irradiation. 13 Differences in experimental results (8.13 Â 10 3 and 2.5 Â 10 3 in 0.001 seconds for ex vivo animal and in vivo human data, respectively) versus these model calculations can be attributed to experimental setup-specific characteristics such as the light collection efficiency of the imaging system and solid angle diversion of light exiting the eye. Nonetheless, both theoretical and measured data show that the threshold for visual perception is exceeded by Cherenkov light generation in the eye during radiation therapy; the human eye requires 5 to 14 photons exciting the retina in 0.001 seconds in completed dark conditions to elicit a visual sensation.…”

“…Previous review studies have indicated that patient perception of Cherenkov light can be enhanced when ambient lighting conditions are dim; furthermore, these visual phenomena can be substantially suppressed if the patient is staring directly into a bright light. 13,39 The patient presented in this study was treated with only sconce lights illuminated in the linear accelerator vault. Similar to our patient, other patients undergoing radiation therapy have often reported that light flashes are blue in color, spark-like in nature, and move across their range of vision.…”

“…[9][10][11] Specifically, it has been stated that Cherenkov light generation in the eye is the dominant mechanism responsible for perceived phosphenes during electron and photon radiation therapy. 9,12,13 Despite this diversity of possible causes, there has been little direct mechanistic measurement of this phenomenon.…”

“…14,15 Quantitative models have shown that Cherenkov light can also be generated within the human eye when ionizing radiation transects the vitreous humor; the retina acts a light sensor, and the brain is able to, in turn, perceive Cherenkov emission created inside the eye. 13 Astronauts in space, as well as patients with cancer undergoing radiation therapy, have reported seeing blue flashes of light when encountering high-energy radiation. 16 The study completed here provides the first direct evidence that perceived x-rayeinduced phosphenes result, at least in part, from production of Cherenkov light emission.…”

Instrumentation in Diffuse Optical Imaging

“…A very recent advance in Cherenkov imaging is to use external high-energy radiation sources, such as the linear accelerators, to induce Cherenkov luminescence in tissue [146, 147]. Because the medical accelerators have high radiation energy (e.g., 6-18 MV for x-ray photon beams and 6-21 for electron beams) and dose rate (e.g., 1-200 MU/min), penetration and SNR of Cherenkov imaging are significantly improved.…”