All too often, the most brutal part of a bout with cancer is radiation therapy. X rays are electromagnetic waves that travel with a constant amount of energy, so although they effectively kill cancer cells, they pass through the skin and healthy tissues on their way to and from the tumor. In doing so, they damage the normal cells in their path. Protons, on the other hand, are particles that have a mass and a charge (they are positive). They can thus be targeted with exquisite specificity only to the tumor site, emitting the bulk of their radiation there and there only and sparing patients the terrible side effects that can accompany therapy with X rays.