Since S-1 (combined drug of tegafur, 5-chloro-2,4-dihydroxypyridine and potassium oxonate) was approved in Japan in 1999 after a large-scale clinical trial, adjuvant chemotherapy with S-1 has been the standard treatment for stage II/III gastric cancer. However, for far-advanced gastric cancer, various adjuvant chemotherapies, including S-1, have not achieved satisfactory results. Therefore, Japan Clinical Oncology Group (JCOG) is currently performing phase 2 and 3 clinical trials of neoadjuvant chemotherapy (NAC) for locally far-advanced gastric cancer. However, previous studies of NAC have reported a pathological response rate of 15-50% for the main lesion, indicating no significant effect on local tumor control.
On the other hand, other studies have reported that anticancer drugs, including S-1, have a radiosensitizing effect, and that neoadjuvant chemoradiotherapy (NACRT) achieves superior local tumor control compared with NAC. Therefore, we conducted a feasibility trial of NACRT with S-1 for locally far-advanced gastric cancer combined with adjacent tissue invasion.