Ionizing radiation therapy is one of the primary modalities for treating cancers. Ideally, the particle selected to deliver ionizing radiation for routine therapy should control the disease, cause minimal side effects, and be affordable. Two major properties for judging the utility of a particle, physical controllability and selective cell destruction, influence the decision for selection. The proton, at present, has the best combination of capabilities for routine radiation therapy. Heavier ions require further study to determine their role in patient treatment.

Key words: Proton; Heavy charged particles; Radiotherapy; Ionization; LET; RBE.