Introduction Ensuring patient safety in radiation oncology is crucial for delivering high-quality healthcare. Patient safety indicators (PSIs) provide a mechanism for identifying, quantifying and evaluating risks and the effectiveness of safety measures. However, there is currently no specialised set of PSIs tailored for radiation oncology in Germany. This study seeks to: (1) create PSIs specifically designed for radiation oncology settings, (2) develop and psychometrically validate an instrument for assessing safety in German radiation oncology facilities and (3) evaluate the feasibility of implementing this instrument in routine clinical practice. The finalised questionnaire will serve as a self-assessment instrument for radiation oncology departments, aiding them in evaluating their efficacy in ensuring patient safety, prioritising safety interventions and tracking performance over time.Methods and analysis We are undertaking a 3-year, mixed methods study to address our objectives. For the identification of PSIs, we will conduct a comprehensive review on the PubMed database, along with reviewing national and international guidelines and recommendations. To refine the initial set of indicators, we will consult with experts, including physicians, medical physicists, nurses, administrators and radiation therapists through focus groups. We will employ a Delphi study for the final consensus and selection of indicators. Additionally, the perspectives of patients will be incorporated by formation of a project patient’s committee which meets throughout the project phases. We will reformulate the identified PSIs into questionnaire items. The questionnaire’s clarity and comprehensibility will be validated through cognitive interviews, followed by psychometric testing in a pilot group of over 150 participants from German radiation oncology departments. The final version of the questionnaire will then be implemented in routine healthcare settings and we will interview individual users about their experiences with the questionnaire in semistructured interviews. We will convene a subsequent expert workshop to discuss the study results and explore avenues for the questionnaire’s broader implementation. The finalised questionnaire will be made accessible via a web app. We hereby present the study potocol as a pre-results report.Ethics and dissemination Ethical approval for this study was granted by the Hamburg Ethics Committee (Approval Number: 2023-101018-BO-ff). This trial is registered by the ARO (Arbeitsgemeinschaft Radioonkologie /working group for radiation oncology of the German Cancer Society), protocol number 2023-03 and in the German register for clinical trials with the number DRKS00034690. Study results will be published in conference papers and talks as well as journal papers with focus on open access journals. The results will be also disseminated during the implementation workshop in phase III, which will involve a diverse group of stakeholders.Trial registration number DRKS00034690