Classic radio therapy uses photons. These are packages of electromagnetic radiation, just like visible light or radiowaves, but with a lot more energy. The emitted radiation dose of a photon bundle decreases gradually as is progresses deeper into the body.
As a result, a limited amount of radiation dose ends up in healthy tissue and also past the tumour in the radiation bundle. Depending on the sensitivity of these tissues and the exposure, side effects might occur.
Thanks to the substantial progress in radiation techniques in recent years, it has been possible to refine the treatment so that it can be administered more accurately. Proton therapy adds another aspect, i.e. the radiation bundle is completely stopped in the tumour, and there is no radiation dose in the healthy tissue 'downstream'.
Proton therapy uses a bundle of protons, positively charged particles that originate from the core of an atom and that delivers its maximum dose when reaching a precise depth in the body after which the radiation stops completely (in contrast to photons).