Background and objective: Phosphenes are visual light phenomena that are experienced when there is no apparent light that stimulates the eye. In oncology, phosphenes are also present during radiation therapy for patients with tumors of the central nervous system, eyes, head and neck. Due to the discomfort of patients treated with irradiation to the head regions, research is conducted to determine whether the dose to the ocular structures is predictive for the occurrence of phosphenes. The objective was to demonstrate the relationship between the doses of the retina and vitreous humor with the appearance of phosphenes. Material and method: A descriptive study was carried out in a prospective cohort in 25 patients older than 18 years, with malignant tumors located at the level of the brain, both of primary and secondary origin, subjected to irradiation in 6 MV linear accelerators, during February 2020 to January 2021. As independent variables: Retinal dose and vitreous humor dose, and as dependent variables: Light flashes and stable light. Logistic regression analysis was used for prediction, using the SPSS statistical program (version 26.0). Result: A final date of 380 external radiotherapy treatments. The presence of any of the events in a prevalence of 58.7% of the total of fractions. The distribution for the presence of both events, flash of light and stable light, was 69.1%, 20.6% and 10.3% respectively. In the logistic regression analysis, for the light flare, only the dose factor in vitreous was significant (OR: 1.74; 95% CI: 1.059-2.419; p: 0.001). For stable light, the dose in the retina (OR: 1.73; 95% CI: 1.121-2.341; p: 0.005), and dose in the vitreous humor (OR: 1.82; 95% CI: 1.335-2.315; p: 0.003). Conclusions: There is a predictive relationship between the doses of irradiation of the retina and vitreous humor, for the generation of phosphenes. These results help radiotherapy centers take these anatomical structures into account to reduce the presence of phosphenes in patients. Likewise, it would help to reduce phosphenes, keeping the bunker area illuminated during the treatment.