When proton beam is blasted inside the body, this radiation reaches the cancer cells while normal cells are hardly affected. The blast reaches the DNA inside the nucleus of cancer cells. When proton beam hits the DNA, the DNA of cancer cells are damaged directly or indirectly.
When this occurs, the probability is high that both double helixes of the DNA will be damaged, and significant effectiveness can be expected from proton beam, with respect to cancer cells that may return with traditional radiation.