The role of mediastinal radiotherapy (RT) in primary mediastinal large B cell lymphoma is controversial. We used the Surveillance, Epidemiology and End Results program 18 database to identify the role after rituximab approval. Among 474 patients included, 65.8% were 18–39 years old and 34.2% were 40–59 years old; 45.8% received RT. Univariate analysis showed that disease stage and race could affect survival. After adjusting for stage and race, RT was correlated with prognosis in patients aged 40–59 years (none/unknown vs. RT, hazard ratio = 2.898, p = 0.034). However, in patients aged 18–39 years, this impact was not significant. Omission of RT in selected young patients may be considered.