Well, lasers currently are already being used to treat some kinds of cancer! Lasers can be used to help destroy cancers on the skin, in combination with special medicines that are used in something called photodynamic therapy. Also, some people have developed special tiny particles that heat up when laser light of a particular colour is shone on them. If you can get these particles inside a tumour, cut open the patient and use a fiber optic laser to heat up the particles, you can burn away the tumour.
However, these approaches don’t work for all types of cancer (for instance, some cancers are too deep in the brain to reach with a fiber optic probe). I think lasers are going to be one tool in a range of things available to us, to help fight cancer.