Mixed-methods process evaluation of ctDNA use to guide decision-making in patients with advanced solid cancers: study protocol for a substudy of the LIQPLAT trial.

INTRODUCTION: There is an urgent need to better understand how information from circulating tumour DNA (ctDNA) can be integrated into routine care for patients with advanced solid cancer. METHODS AND ANALYSIS: The implementation of liquid biopsies in routine care of patients with advanced solid cancer trial (LIQPLAT) is a single-centre, single-arm trial investigating the implementation of ctDNA in the routine care of patients with advanced solid cancer. We present a mixed-methods process evaluation embedded in the LIQPLAT trial, following Medical Research Council guidance and the Reach, Effectiveness, Adoption, Implementation, Maintenance framework. We show a logic model, which details the causal chain and related assumptions from recruiting patients into the trial to the goal of improving quality of life and survival. Data collection is longitudinal and includes: semistructured interviews with healthcare professionals (pathologists, biologists, oncologists; planned n=20) and patients (planned n=15) to identify implementation barriers and facilitators; recordings of molecular tumour board meetings to analyse clinical decision-making; the 23-item Normalisation MeAsure Development survey for healthcare professionals (planned n=20) at four time points. Quantitative data from hospital records will be used to assess implementation outcomes like patient acceptance rates and ctDNA workflow success. Qualitative data will undergo thematic and content analysis, and quantitative data will be analysed using a Bayesian framework. ETHICS AND DISSEMINATION: The LIQPLAT trial was approved by the regional ethics committee of Northwestern and Central Switzerland (BASEC 2024-00358). The qualitative aspects of the process evaluation were exempted from ethics review according to the Swiss Human Research Act. We follow guidelines for data security, confidentiality and information governance. Results will be submitted for publication in peer-reviewed journals and discussed at conferences. TRIAL REGISTRATION NUMBER: NCT06367751, SNCTP000005844.