Heavy Charged Particles: Does Improved Precision and Higher Biological Effectiveness Translate to Better Outcome in Patients?

Protons are the most common charged particles used in oncology. Acceleration of heavier ions requires larger accelerators and is more expensive, yet heavy nuclei share the same advantageous dose-depth profile characteristics of protons and have potential additional advantages. These advantages are related to the physical characteristics of the beam, owing to reduced lateral scattering and sharper lateral penumbra. In addition, heavy ions produce an increased biological response. In fact, in the target region heavy ions behave as densely ionizing radiation, which produce distinct biological effects compared to sparsely ionizing x-rays and protons. The translation of the putative radiobiological advantages into clinical advantages remains to be demonstrated. Eleven centers worldwide are currently using carbon ions for treatment of different solid tumors. Phase-II trials in Japan and Germany show very promising results for selected tumors, such as chordomas, large sarcomas, and pancreatic adenocarcinoma. Phase-III trials are under way to compare carbon ions to protons or x-rays.