Purpose: The optimal management of stage II follicular lymphoma (FL) is unclear. Radiation therapy (RT) alone has been the gold standard treatment, but a proportion of patients relapse. We sought to characterize outcomes and prognostic factors for stage II FL treated with RT alone to identify a high-risk subgroup of patients who may benefit from treatment intensification., Methods: This was a population-based, province-wide, retrospective study. Included patients had grade 1-3A, non-mesenteric, stage IIA or IIAE FL diagnosed between 1986 and 2016 and treated with curative-intent (≥20 Gy) RT alone., Results: 102 patients were included. Median follow-up was 10.4 years (range, 0.3-22.3). Median age was 59 years (range, 33-86). Median greatest disease diameter was 3.6 cm (range, 1.5-11.5). Freedom from progression (FFP) was 60.3% at 5 years and 40.7% at 10 years. Overall survival (OS) was 89.2% at 5 years and 81.8% at 10 years. Greatest disease diameter of >3.6 cm was associated with inferior FFP (10-year FFP 34% vs. 47%, p = 0.013) on univariable analysis and inferior FFP (hazard ratio [HR] 1.87, p = 0.019) and inferior OS (HR 2.12, p = 0.027) on multivariable analysis (MVA). Older age was associated with inferior OS (HR 1.08, unit = 1 year, p < 0.001) on MVA., Conclusions: 40.7% of stage II FL patients treated with RT alone remained disease-free at 10 years. Greatest disease diameter >3.6 cm was associated with inferior FFP and OS, representing a novel prognostic indicator in this population that may help in the decision-making process on whether to complement RT with systemic therapy., Competing Interests: The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: David W. Scott has received grants from Roche. Alina S. Gerrie has received grants from Abbvie, AstraZeneca, Janssen, and Roche Canada, consulting fees and honoraria from Abbvie, AstraZeneca, BeiGene, and Janssen, and support for travel expenses from Janssen. Diego Villa has received grants from Roche and AstraZeneca and honoraria from Roche, Merck, BMS/Celgene, Kite/Gilead, Janssen, AstraZeneca, Abbvie, BeiGene, and Novartis. Laurie H. Sehn has received grants from F. Hoffmann-La Roche Ltd, Genentech, Inc., and Teva, and consulting fees and honoraria from F. Hoffmann-La Roche Ltd, Genentech, Inc., AbbVie, AstraZeneca, BMS/Celgene, Gilead Sciences, Janssen, Incyte, Kite Pharma, Merck, Seagen, Teva, and TG Therapeutics. Kerry J. Savage has received grants from Roche and Bristol Myers Squibb, has received consulting fees and honoraria from Bristol Myers Squibb, AbbVie, and Seagen, participates on the data safety monitoring board of Regeneron, and participates on the steering committee of Beigene and AstraZeneca. Yi Xu, Belinda A. Campbell, Matthew Chan, Jessica Chan, Pedro Farinha, Christopher P. Venner, and Andrea C. Lo report no disclosures., (© 2024 The Authors. Published by Elsevier B.V. on behalf of European Society for Radiotherapy and Oncology.)