Introduction A cancer diagnosis not only profoundly impacts individuals but also the very core of their families, reshaping their lives in many ways. However, there is a lack of focus on the well-being and health of the entire family across adult cancer research. This is concerning given that one-third of the Danish population will get a cancer diagnosis before the age of 75, suggesting that many Danes will become caregivers during their lifetime. In addition, identifying vulnerable families is challenging, and the determinant factors for their vulnerability are unknown.

Aims The principal aim of this study is to investigate family health during cancer treatment. This will be done by gathering information on various parameters such as perceived support, quality of life and self-efficacy in patients with cancer and families across the cancer trajectory. Additionally, the study seeks to pinpoint particularly vulnerable families and investigate contributing factors to their vulnerability.

Methods and analysis This mixed-methods study follows a sequential explanatory design, combining patient-reported outcomes in a longitudinal, prospective multicentre survey with interviews conducted with a nested sampling of the participants from the survey. A total of 240 patients diagnosed with prostate-, breast-, gastrointestinal- and lymphoma cancer, and designated adult family caregivers will be recruited from six different sites for the survey. Variables such as family health, needs and perceived support, quality of life, self-efficacy, depression, stress and resilience will be explored. Survey data will be collected at baseline, 3, 6, 12 and 18 months. The interviews (n=12–15) will be conducted twice with patients and caregivers jointly: once during the treatment phase (3 months) and once after completion of treatment (12 months). For the survey part, we estimated a sample size with 90% power and 5% significance to detect a minimal clinically important change in the Family Health Scale. Assuming an SD of √2×22 = 31, based on a cross-sectional SD of 22, 44 patients per group were required; to allow for dropout, 60 per group (240 total) were included. Patient and caregiver characteristics will be summarised descriptively. Longitudinal patient-reported outcomes will be analysed with linear mixed regression, separately for patients and caregivers. Changes will be reported as mean differences with 95% CIs and compared with published minimal clinically important differences or, if unavailable, 0.3×baseline SD. For the qualitative part, thematic analysis by Braun and Clarke is chosen to extract data, identify patterns and analyse data and themes from the interviews. NVivo will be used for coding interview data.

Ethics and dissemination The study will be conducted in accordance with the Helsinki Declaration. Measures will be taken to ensure confidentiality, data protection and participant safety throughout the study. The results will be published in peer-reviewed journals and conference presentations.

Trial registration number ClinicalTrials.gov: NCT06433349. Protocol version 2.0, June 2024.