Proton therapy cancer treatment, coloured computed tomography (CT) scan. Malignant (cancerous) brain tumour (pink) being targeted with a proton beam (red). Front of head is at bottom, showing the eyeballs. Protons are positively charged particles found in the nucleus of an atom. 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.