The impact of working conditions on breast cancer outcomes: a study protocol for a population-based cohort study using UK Biobank data.

INTRODUCTION: Breast cancer is the most common cancer among women globally. While the impact of lifestyle factors like smoking and obesity on breast cancer risk and survival is well documented, the effect of working conditions is not fully understood. Moreover, breast cancer can reduce employability, making it crucial to identify factors that facilitate return to work and improve life satisfaction. Since breast cancer is affected by sleep and lifestyle, which are related to working conditions, understanding how they affect breast cancer outcomes is key. This study aims to explore the relationship between working conditions and breast cancer outcomes, including incidence, mortality and survival within a causal framework. Our specific aims are to understand the relationship between (1) working conditions and occupational groups and breast cancer outcomes, including the extent to which sleep, lifestyle and breast cancer screening uptake explain these relationships and (2) prediagnosis working conditions, sleep and lifestyle and their effect on return to work and life satisfaction among breast cancer survivors. METHODS AND ANALYSIS: We will use data from the UK Biobank, a large-scale cohort study with data on 273 825 women between 40 and 69 years old at baseline, followed from 2006 to 2022. The data has been linked with death and cancer registries and includes 8309 incident breast cancer cases. To quantify the effect of working conditions on breast cancer outcomes (aim 1) and their effect on return to work and life satisfaction (aim 2), we will implement g-methods to estimate the average causal effect and employ counterfactual-based mediation analysis to quantify how much mediating factors, such as sleep and lifestyle, explain this effect. ETHICS AND DISSEMINATION: UK Biobank received ethical approval from the North West Multi-Centre Research Ethics Committee. No further ethical approval was required for the proposed research project. In line with the two aims, four original research manuscripts will be published in open-access peer-reviewed journals to disseminate the findings. In addition, findings will be disseminated at international conferences and scientific meetings.