Sunlight can be used to treat cancer using a method developed by University of Tokyo researchers. Their method focusses sunlight on the cancer-affected areas using optical fibres. The photochemical reaction with a previously injected substance kills cancer cells.

Cancer treatment methods, using lasers in photodynamic therapy, already exist, but the sunlight method is superior in two ways: conventional photodynamic therapy methods need laser equipment whereas sunlight can be used with various substances; and the sunlight method is much cheaper.