C-ion RT has a benefit in terms of dose distribution, which is even better than stereotactic body irradiation (SBRT) and intensity-modulated radiation therapy (IMRT) using high-energy X-rays. In addition, C-ion RT has higher biological effectiveness as well, resulting in higher local tumor control and a lesser degree of adverse effects to normal tissues. Although more than 7,000 patients have been treated with C-ion RT at NIRS in the last 19 years, a breast cancer study protocol has not been performed until now. Clinical trials with C-ion RT have two major purposes, one being the treatment of advanced tumors that are hard to cure with conventional treatment and the other the treatment of common cancers with the aim of improving survival and QOL of patients. The reason we did not treat breast cancer is that cases treatable by partial irradiation were difficult to identify.