BACKGROUND Young to middle-aged breast cancer patients face significant psychosocial challenges. Existing interventions often lack comprehensiveness, timely initiation, and specific tailoring to this population's unique needs. OBJECTIVE To evaluate the impact of a peer-led, nurse-involved, blended online and offline peer support intervention program (PNO2PSP) on psychosocial adjustment in young to middle-aged breast cancer patients. METHODS The PNO2PSP effectiveness was validated through a single-center cluster randomized controlled trial involving 70 newly diagnosed young to middle-aged breast cancer patients (35 in each group). The intervention group received an 8-week PNO2PSP in addition to routine care. Psychosocial adjustment, self-efficacy, social support, and coping modes were assessed pre-surgery and at 4, 8, and 12 weeks post-surgery. Generalized Estimating Equations (GEE) were used for intention-to-treat analysis. In-depth interviews with 9 participants explored their experiences. RESULTS Compared to the control group, the intervention group demonstrated significantly lower psychosocial adjustment scores at 4 weeks (T1) (Wald χ² = 6.466, P = 0.011) and 12 weeks (T3) (Wald χ² = 4.395, P = 0.036); Social support was higher at 8 weeks (T2) (Wald χ² = 8.175, P = 0.004). Confrontation coping scores were higher at T3 (Wald χ² = 4.189, P = 0.041), while avoidance coping scores were lower at T1 (Wald χ² = 7.051, P = 0.008), T2 (Wald χ² = 7.346, P = 0.007), and T3 (Wald χ² = 5.062, P = 0.024). Qualitative findings supported these quantitative results, highlighting the program's role in facilitating psychosocial adjustment, providing vital support, boosting treatment confidence, and fostering positive coping. CONCLUSIONS The PNO2PSP effectively enhances psychosocial adjustment, social support, and positive coping in young to middle-aged breast cancer patients. Its scientifically validated, feasible, and patient-centered design supports its recommendation for wider clinical implementation, with continued training for peer supporters and sustained delivery of peer support. CLINICALTRIAL Registry: Chinese Clinical Trial Registry, TRN: ChiCTR2300076471, Registered 10/10/2023
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