Embodiments apply light energy in medical treatments. To enhance or control the effect of the light energy, embodiments apply the light energy after tissue has been treated, e.g., with a photosensitizing agent. For example, embodiments may treat target tissue with riboflavin before exposure to ultraviolet light. For example, a system for cataract surgery includes a removal system configured to remove a first lens from an eye, wherein a capsular bag remains in the eye after removal of the first lens. The system includes an application system configured to treat lenticular epithelial cells in the capsular bag with the photosensitizing agent. The system includes a delivery system with a light source and an optical device. The optical device delivers light to the treated lenticular epithelial cells. Energy from the light destroys the lenticular epithelial cells in the capsular bag to reduce the growth of epithelial cells that cause posterior capsule opacification.
