Figure 20.

Schematic diagram of proton beam irradiation of the spine. There are several distinct sources of radiation exposure, including therapeutic protons (red), stray neutrons emanating from the treatment apparatus (blue), and neutrons produced by therapeutic proton radiation inside the body. A small-diameter beam of protons enters the treatment apparatus, which spreads the beam to a clinically useful size and collimates it to spare healthy tissues. The stray neutron is created by proton-induced nuclear reactions inside the treatment unit, some of which leak out and irradiate the patient. The stray radiation exposures provide no therapeutic benefit but increase the predicted risk that a patient will develop a radiogenic side effect, such as a second cancer, later in life. (Newhauser and Durante, 2011)