Recent advances in high-precision photon radiotherapy represented by stereotactic body radiotherapy (SBRT) and intensity-modulated radiotherapy (IMRT) have helped improve the treatment outcomes of lung cancers, both in disease control and side effect reduction, for normal tissues, including lung and heart, compared with conventional radiotherapy. However, photon radiotherapy is inherently limited by the physical properties of the photon beam, which result in unavoidable low to moderate irradiation to normal tissues, even at substantial distances from the tumor. The unavoidable spread of radiation sometimes leads to acute or late side effects. Efforts have been made to develop charged particle radiotherapy as an effective and less-invasive new radiation modality. Charged particle radiotherapy has the physical advantage of the dose being focused on the tumor with only minimal exposure of the surrounding normal tissues. This physical advantage of improved dose localization helps to reduce the toxicity of normal tissue, such as lung and heart, even further. Carbon ion, which is categorized as a heavy ion, not only has favorable physical properties but also a biologic advantage compared with photons and protons. Carbon ion has a strong cell-killing effect, and this effect is not markedly influenced by the oxygen concentration or the cell cycle. These physical and biological advantages of carbon-ion radiotherapy have been confirmed by a number of clinical studies, although many of these are single-institutional studies. Many multi-institutional studies for carbon-ion radiotherapy are now being vigorously conducted, including studies in lung cancer patients. The clinical superiority of carbon-ion radiotherapy to conventional techniques is expected to become clearer in the near future.
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