Background and purpose: Radiation oncology guidelines favour hypofractionated whole-breast radiotherapy (HWBRT) over more conventional schemes in the conservative treatment of breast cancer, but its adoption still varies in clinical practice. This study assessed the patterns of HWBRT adoption in Catalonia (Spain). Material and methods: We used a mixed-methods approach based on an explanatory sequential design, first collecting and analysing quantitative data on HWBRT use (>2.5 Gy per fraction) in 11 public radiotherapy centres (2005-2015) and then performing 25 semi-structured interviews with all department heads and reference radiation oncologist/s. Results: Of the 34,859 patients fulfiling the study criteria over the study period, just 12% were hypofractionated, reaching a percentage of 29% in 2015 (p < 0.001). Our analysis showed a narrowing age gap between patients receiving conventional fractionation and hypofractionation in centres leading adoption. However, there were important differences in clinicians' interpretation of evidence (e.g. regarding the perceived risk of long-term toxicity) and selection of patients for specific indications, both within and between departments. Conclusions: Differences observed in the rate of adoption of HWBRT could not be tackled only using a rational, evidence-based approach. Factors related to the management of radiotherapy departments play a major role in the diffusion of therapeutic strategies.