Autonomous motivation and adherence intention in long-term breast cancer treatment: a self-determination theory model

Background: Non-adherence to long-term oral therapies in breast cancer, adjuvant endocrine therapy and oral targeted therapies in metastatic disease, affects 30–50% of patients and is associated with increased recurrence risk or poorer disease control. Socio-economic and clinical determinants are well documented, but the motivational mechanisms linking them to adherence remain insufficiently modelled. Methods: Cross-sectional observational study in two phases. Adult women with breast cancer treated for ≥6 months were recruited via Seintinelles (France). Phase 1 (n = 89) validated an SDT instrument. Phase 2 (n = 412) tested a structural motivational model using partial least squares structural equation modelling (PLS-SEM). An exploratory multigroup analysis compared patients with <3 vs ≥ 3 treatment lines. Results: The model supported a fully mediated motivational chain: social relatedness → competence → autonomy → autonomous motivation → treatment intention. Autonomous motivation was the sole significant predictor of intention (β = 0.687); controlled motivation had no significant effect (β = 0.016, ns). The oncologist's attitude was the strongest contextual predictor (β = 0.764 on social relatedness). Side effects acted indirectly, via psychological burden eroding perceived competence. In patients with ≥3 treatment lines, the effect of autonomy on autonomous motivation was stronger (β = 0.902 vs 0.653; p = 0.002). Conclusions: What sustains adherence is not how strongly patients are motivated, but whether their motivation is autonomous. Autonomy-supportive communication by the oncologist and proactive management of symptoms and their psychological burden emerge as the most actionable clinical levers, especially as disease burden accumulates.