Background: Research has highlighted the need for evidence-based interventions to improve paediatric advance care planning (pACP) in adolescents with cancer. Although adolescents express the desire and ability to share their values, beliefs and preferences for treatment, there is a lack of structured multicomponent interventions to improve parent-adolescent communication on different ACP themes including those not limited to end-of-life care. The aim of this study is to evaluate the effectiveness and implementation, context and mechanisms of impact of a novel ACP program in paediatric oncology. Methods: We will conduct a multi-centre parallel-group randomised controlled superiority trial with embedded mixed-methods process evaluation in Flanders, Belgium. Adolescents aged 10-18 who have cancer, and their parent(s) will be recruited via all four university hospitals in Flanders, Belgium, and support groups. Families will be randomised to receive care as usual or the multicomponent BOOST pACP program, consisting of three conversation sessions between an external facilitator and the adolescent and parent(s). The primary endpoint is improved parent-adolescent communication from the perspective of the adolescent. Secondary endpoints are adolescents' and parents' attitudes, self-efficacy, intention and behaviour regarding talking about ACP themes with each other, parents' perspective of shared decision making in the last clinical encounter, and the paediatric oncologist's intention and behaviour regarding talking about ACP themes with the family. Measurements will be performed at baseline, at 3 months and at 7 months using structured self-reported questionnaires. We will perform a process evaluation in the intervention group, with measurement throughout and post-intervention, using structured diaries filled out by the facilitators, interviews with facilitators, interviews with involved paediatric oncology teams, and audio-recordings of the BOOST pACP conversations. Discussion: The BOOST pACP program has been developed ...