Background: Despite recent advances in chemotherapy and radiotherapy, prognosis of unresectable locally advanced pancreatic cancer (LAPC) patients remains poor. To improve the outcome, a phase I/II clinical trial of carbon-ion radiotherapy (CIRT) with concurrent gemcitabine for LAPC was started in National Institute of Radiological Sciences in Japan.When compared to photon beams, carbon ion beams offer improved dose distribution, enabling us to concentrate a sufficient dose within a target volume while minimizing the dose in the surrounding radiosensitive normal tissue. In addition, carbon ions being heavier than protons provide a higher biological effectiveness, which increases with depth reaching the maximum at the end of the beam’s range. Although CIRT is expected to offer powerful local effect, whereas the potential risk of distant metastasis represent a critical problem for LAPC. To control distant metastases and subsequently prolong patient survival, a more effective systemic treatment might be essential. Recently, S-1 has shown favorable antitumor activity in several clinical trials for pancreatic cancer. Oral S-1 also has a great clinical advantage because the risks of complications associated with intravenous administration are avoided.We considered oral S-1 to be an attractive alternative to gemcitabine in the CIRT for LAPC. Thus, we planned a phase II clinical trial to evaluate the efficacy and safety of CIRT with concurrent S-1, which is the first trial in the world. Methods: Eligibility included pathological confirmation of pancreatic invasive ductal carcinomas and radiographically unresectable disease without metastasis. Concurrent S-1 is administered orally twice a day at a dose of 80mg/m2/day for 28 days every 6 weeks. Carbon-ion radiotherapy is prescribed with 55.2GyE at 12 fractions in 3 weeks. The primary endpoint is two year overall survival. Clinical trial information: UMIN000013803.