The clinical results after carbon ion radiotherapy for the metastatic lung tumors believed to be in the state of oligo-recurrence were evaluated. One hundred and sixteen lesions in 91 patients with lung cancer metastasis were treated with carbon ion radiotherapy at our institute from April 1997 to February 2011. Regarding the prescribed dose, total dose ranged between 40 gray equivalents (GyE) and 80？GyE, and fraction size ranged from 1 to 16 fractions. After a median followup period of 2.3 years (range, 0.3–13.1 years), the statistical overall survival rate and local control rate were 71.2% and 91.9% at 2 years after treatment, respectively. Treatment-related side effects were not a clinical problem. When classified by the primary organ, there were 49 cases of lung cancer, 20 cases of colorectal cancer, and 22 cases of others. The overall survival rate and local control rate for lung metastasis cases from lung cancer at 2 years after treatment were 81.5% and 92.4%, respectively, and 65.0% and 92.0% regarding lung metastasis from colorectal cancer. Carbon ion beam therapy for the metastatic lung tumors is a safe therapy, and the therapeutic effect is comparable to the outcome obtained from reported surgical resections. 1. Introduction Radiotherapy is the principal treatment option for patients with early stage lung cancer and contraindications to receive surgery. The outcome from using conventional therapeutic techniques has been a 40–70% 5-year local control rate, but a local control rate equivalent to surgery is being reported due to recent advancements in irradiation techniques [1–4]. These irradiation techniques include SBRT, proton beam therapy, and carbon ion radiotherapy (CIRT). Clinical trials for various types of tumors were initiated at the National Institute of Radiological Sciences (NIRS) from June 1994 using carbon ion beams, and dose fractionation suitable for individual diseases and irradiation techniques, such as a respiratory-gated radiotherapy and so forth, were developed. As a result, the healing of refractory cancers such as sarcoma of the bone and soft tissue, for which surgery is difficult, postoperative local recurrence of rectal cancer, and so forth, were achieved, and it was found that safe treatment is possible in a further shorter period regarding cancers of the prostate gland, the head and neck, lungs, and liver [1]. Treatment for nonsmall cell lung cancer was initiated in November 1994. Regarding peripheral stage I lung cancer, the fractionation number was gradually reduced from 9 times [5] to 4 times [6] while confirming
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