Radiotherapy Trends and Variations in Invasive Non-metastatic Breast Cancer Treatment in the Netherlands: A Nationwide Overview From 2008 to 2019

Aims: This nationwide study provides an overview of trends and variations in radiotherapy use as part of multimodal treatment of invasive non-metastatic breast cancer in the Netherlands in 2008–2019. Materials and Methods: Women with invasive non-metastatic breast cancer were selected from the population-based Netherlands Cancer Registry. Treatments trends were presented over time. Factors associated with (1) boost irradiation in breast-conserving therapy and (2) regional radiotherapy instead of axillary lymph node dissection (ALND) in N+ disease were identified using multilevel logistic regression analyses. Results: Radiotherapy use increased from 61% (2008) to 70% (2016), caused by breast-conserving therapy instead of mastectomy, increased post-mastectomy radiotherapy, and increased regional radiotherapy (32% in 2011 to 61% in 2019) instead of ALND in N+ disease. Omission of radiotherapy after breast-conserving surgery (BCS) in 2016–2019 (4–9%, respectively), mainly in elderly, decreased overall radiotherapy use to 67%. Radiotherapy treatment was further de-escalated by decreased boost irradiation in breast-conserving therapy (66% in 2011 to 37% in 2019) and partial (1% in 2011 to 6% in 2019) instead of whole breast irradiation following BCS. Boost irradiation was associated with high-risk features: younger age (OR>75 vs <50:0.04, 95%CI:0.03–0.05), higher grade (OR grade III vs I:11.46, 95%CI:9.90–13.26) and residual disease (OR focal residual vs R0-resection:28.08, 95%CI:23.07–34.17). Variation across the country was found for both boost irradiation use (OR South vs North:0.58, 95%CI:0.49–0.68), and regional radiotherapy instead of ALND (OR Southwest vs North:0.55, 95%CI:0.37–0.80). Conclusion: Overall radiotherapy use increased in 2008–2016, while a decreasing trend was observed after 2016, caused by post-BCS radiotherapy omission. Boost irradiation in breast-conserving therapy became omitted in low-risk patients, and regional radiotherapy use increased as an alternative for ALND in N+ disease.