Background: The aims of this study were to determine whether a radiation therapist-led patient education intervention (RT Prepare) reduced breasts cancer patients' psychological distress (primary endpoint); anxiety, depression and concerns about radiotherapy, and increased knowledge of radiotherapy and preparedness (secondary endpoints). Patient health system usage and costs were also assessed. Methods: A multiple-baseline study across three sites. The RT Prepare intervention comprised two consultations with a radiation therapist: prior to treatment planning and on the first day of treatment. Radiation therapists focused on providing sensory and procedural information and addressing patients' pre-treatment anxiety. Usual care data were collected prior to intervention commencement. Data collection occurred: after meeting their radiation oncologist, prior to treatment planning, first day of treatment and after treatment completion. Multilevel mixed effects regression models were used. Results: In total, 218 usual care and 190 intervention patients participated. Compared with usual care, intervention participants reported lower psychological distress at treatment commencement (p = 0.01); lower concerns about radiotherapy (p < 0.01); higher patient knowledge (p < 0.001); higher preparedness for procedural concerns (p < 0.001) and higher preparedness for sensory-psychological concerns at treatment planning (p < 0.001). Mean within-trial costs per patient were estimated at $AU159 (US$120); mean ongoing costs at $AU35 (US$26). Conclusion: The RT Prepare intervention was effective in reducing breast cancer patients' psychological distress and preparing patients for treatment. This intervention provides an opportunity for radiation therapists to extend their role into providing patients with information and support prior to treatment to reduce psychological distress.