Radiation therapy trends following breast-conserving surgery in early-stage estrogen receptor-positive breast cancer: particularly in older patients more frequently omitted.

OBJECTIVES: To evaluate nationwide trends in administration of (partial) breast radiation therapy (RT) following breast-conserving surgery (BCS) in women with early-stage, ER + breast cancer (BC). METHODS: Women aged ≥50 years with ER+, HER2-, non-metastatic BC, treated between 2014 and 2024 with BCS were included. Eligible tumours were ≤1 cm grade 1 or 2, or ≤2 cm grade 1. Descriptive statistics and multivariable logistic regression were performed. RESULTS: In total 21,418 patients were included. Endocrine therapy was administered in 3.5%. Age related differences in RT administration were observed. In younger patients (50-69 years), RT omission increased from 1.8% (2014-2016) to 8.7% (2023-2024, p < 0.001). Among older patients (≥70 years), omission increased from 4.6% (2014-2016) to 55.3% (2020-2022) and 33.8% (2023-2024, p < 0.001). RT type was comparable across age groups. Whole breast RT (WBRT) without boost was most common (>65%), followed by partial breast irradiation (PBI) (>21%). Over time, application of WBRT without boost decreased from 81.2% (2014-2016) to 46.5% (2023-2024, p < 0.001), while PBI increased from 4.3% (2014-2016) to 51.9% (2023-2024, p < 0.001). Age (OR 10.1) and year of diagnosis were strongly associated with RT omission. CONCLUSION: Younger patients received RT more often than older patients. Over time, WBRT remained the most common RT type, although its use decreased over time, while PBI increased. Omission of RT became more frequent, most markedly in older patients, partially driven by participation in a national trial on RT omission (TOP-1 study). Ultimately, RT was omitted in about one third of older patients, indicating an ongoing shift towards treatment de-escalation, individualized medicine or premature trial implementation.