<p>Participant characteristics.</p>

Background Implementing a prognostic model in clinical practice requires assessing not just its accuracy but also how it may impact patient and healthcare outcomes. Evaluating these impacts requires clinical trials that are carefully designed to select, collect, and report key outcomes. However, there is currently no consensus on which outcomes are key to evaluating the impact of prognostication. Core Outcome Sets can standardise outcome selection, collection, and reporting, and thereby improve the comparability of studies. Our study aimed to develop a preliminary Core Outcome Set for evaluating the impact of prognostication on people living with advanced cancer. Methods Following the Core Outcome Measures in Effectiveness Trials initiative’s recommended methodology, we first conducted a systematic review of prognostic studies and in-depth interviews with patients, informal caregivers, and clinicians. Drawing on these findings, we produced a comprehensive list of potential outcomes, which we refined and prioritised through a two-round modified Delphi survey with an international expert panel. An online consensus meeting finalised the preliminary Core Outcome Set. Results The systematic review identified 70 outcomes, and the interviews contributed another 16. Consolidation produced 67 outcomes for review in the Delphi survey. A total of 49 experts from 14 countries; participated in the Delphi survey, with 31 (63%) completing both rounds. After two rounds, 34 outcomes were rated as critically important. These were discussed further in the consensus meeting, attended by 12 participants, who reached agreement on 9 core outcomes spanning five domains: physiological/clinical outcomes, global quality of life, care delivery, perceived health status, and personal circumstances. Conclusions This preliminary Core Outcome Set offers a standardised approach for selecting, collecting, and reporting outcomes in prognostic impact studies in advanced cancer, which could enhance the comparability of future prognostic impact studies, and thereby potentially improve clinical practice and patient care outcomes.