Light flashes during proton and photon radiotherapy: A multicenter prospective observational study

•A total of 88 patients experienced light flashes among 416 who received photon radiotherapy and 205 who received proton beam therapy in our prospective study.•The frequency of light flashes during radiotherapy was associated with the retina dose and younger age, and were commonly blue or white light for brain or head and neck irradiation.•Light flashes were seen by only 35/524 patients (6.7%) without retinal irradiation, but by 13/33 (39.4%) and 41/64 (64.1%) with retinal maximum isodose lines of 10–50% and 60–100%, respectively.•Totals of 52, 15, 15, 9, 16 and 8 patients sensed light of blue, purple, yellow, red, white and other colors, respectively. Patients who receive radiation therapy sometimes complain of a light flash during irradiation. The details of the characteristics of this light have not been described. To evaluate light flashes during photon and proton radiotherapy. A prospective observational study was performed in all adult patients (≥20 years old) who received photon and proton therapy at two centers between January 2019 and August 2020, except for patients who could not communicate and those with visual abnormality. Evaluations were obtained for the presence or absence of light flashes, light darkness (7 levels), light intensity (5 levels), frequency, light movement, light flashing, and time seeing the light, using a weekly checklist. A total of 650 courses were examined for 621 patients, of whom 416 received photon radiotherapy and 205 received proton beam therapy. The checklist indicated that 88 patients (16.1%) sensed light during photon or proton radiotherapy. In multivariate logistic regression analysis, the factors that were significantly associated with a light flash were a higher retina dose and younger age (p