The clinical application of carbon ions generated by the heavy ion medical accelerator in Chiba (HIMAC) reached its 20th anniversary in 2014. More than 9,000 cancer patients have been treated at the National Institute of Radiological Sciences (NIRS). Carbon-ion radiotherapy has been applied for treating various types of tumors that were considered difficult to control with existing modalities. Our experience to date has indicated that carbon-ion radiotherapy is advantageous for head and neck cancer, non-small cell lung cancer, pancreatic cancer, prostate cancer, bone/soft tissue sarcomas of the pelvis, uterine cervix adenocarcinomas, and other cancers. However, some cancer types (such as those in close proximity to radiosensitive normal organs) require additional treatments to sensitize the target cancer because of limitation of the irradiation dose. Furthermore, systemic combined therapy is also utilized to suppress possible metastasis. Currently, some anticancer agents are utilized with carbon-ion radiotherapy, including dacarbazine, nimustine hydrochloride, vincristine (DAV), gemcitabine, cisplatin, and 5-fluorouracil. Interesting reagents such as PARP and HSP90 inhibitors have been proposed as cancer cell- specific sensitizers for carbon-ion irradiation during basic biological studies, especially those from the Research Project with Heavy Ions at NIRS-HIMAC. In our laboratory, we have focused our studies on the suppression of metastasis. We have proposed the concurrent use of reagents to inhibit the invasive potential of cancer cells under carbon-ion irradiation. Recently, we have also shown that combining carbon-ion radiotherapy with the local injection of dendritic cells inhibits lung metastases in an in vivo murine model.