Proton therapy cancer treatment, coloured computed tomography (CT) scan. Malignant (cancerous) brain tumour (red) being targeted with a proton beam (outlined in yellow). The eyes and optic nerves can be seen at top (blue and green) and the brain stem can be seen at bottom (green). Protons are positively charged particles found in the atomic nucleus. Proton therapy uses beams of protons to irradiate tumours and kill the cancerous cells. Proton beams allow precise targeting of diseased tissues, with little damage to surrounding areas. Proton therapy is therefore useful to treat cancers near to delicate or vital organs, such as brain and eye tumours.