Age-dependent relative and absolute benefit of radiotherapy after breast-conserving surgery: a population-based analysis using SEER data.

Among 532,118 eligible patients, 409,652 (77.0%) received radiotherapy and 122,466 (23.0%) did not.

Radiotherapy was associated with improved breast cancer-specific survival across all age groups.

Absolute risk reduction also decreased with age, from approximately 5.2-5.6% in younger patients to 2.9-3.7% in patients aged 70-84 years.

Restricted cubic spline analysis showed a gradual attenuation of the radiotherapy-associated survival advantage with age, without evidence of an abrupt threshold.