PurposeDefinitive radiotherapy (RT) of 30 Gy or higher is commonly recommended to treat Helicobacter pylori-independent gastric mucosa-associated lymphoid tissue (MALT) lymphoma with an excellent disease control rate. However, the efficacy of reduced-dose RT has not yet been evaluated in a prospective cohort study. This multi-institutional study aimed to determine the role of reduced-dose RT in the treatment of stage IE gastric MALT lymphoma. MethodsBetween March 2017 and June 2022, 62 patients with histologically confirmed stage IE gastric MALT lymphoma without evidence of H. pylori infection were enrolled. The patients were treated with reduced-dose RT at a total dose of 24–25.5 Gy to the entire stomach. The response to therapy was evaluated by endoscopy with a biopsy of suspicious lesions if necessary. The primary endpoints were 6-month complete remission (CR) and local failure-free survival (LFFS). ResultsAmong 62 patients, 32 (51.6%) were previously treated for H. pylori eradication. Radiotherapy was delivered using 3D-conformal (n=20, 32.3%) or intensity-modulated radiotherapy (n=42, 67.7%). The median follow-up duration was 34.5 months (range, 9.6–68.8 months). The 6-month CR rate was 96.7%. The 5-year LFFS and progression-free survival rates were 92.0% and 90.4%, respectively. None of the patients experienced grade 3 or worse acute toxicities, and grade 2 acute toxicities were reported in 17 patients (27.4%). ConclusionReduced-dose RT exhibited excellent response rates in stage IE gastric MALT lymphoma, comparable to historical controls of standard dose (≥ 30 Gy) radiotherapy, with a minimal toxicity profile. Current prospective evidence strongly supports the use of definitive radiotherapy (24–25.5 Gy) for the treatment of H. pylori-independent stage IE gastric MALT lymphoma.