The present invention provides a prophylactic light activated treatment method for subjects at risk of a plaque-mediated thrombotic event, such as acute coronary syndromes. In one embodiment, the subject receives a dose of photosensitizer sufficient to result in a concentration of photosensitizer in the neovascularization associated with atherosclerotic disease to permit effective photodynamic therapy. Light of a wavelength that excites the photosensitizer, but that does not to any significant extent penetrate to an adventitial layer of the blood vessel, is applied to the region of the blood vessel. The therapeutic light dose, which may be applied in sequential stages, is sufficient to cause closure of neovasculature leakage and lead to plaque stabilization and reduced adverse clinical outcomes. The present invention further provides light transmission devices and systems useful in carrying out the methods of the present invention.