Poor adherence to cancer treatment is commonly seen [Citation1–3] with consequences including worse overall survival and quality of life [Citation2]. In line with this, almost half (46%) of lung cancer patients did not receive optimal first-line treatment in a Danish nationwide cohort study [Citation4], which was associated with comorbidity, and socioeconomic resources amongst others. Depression has been identified as a risk factor for non-compliance to non-cancer medical treatment in a meta-analysis [Citation5]. Psychological symptoms like depression and anxiety are prevalent among cancer patients [Citation6] and studies show that this is especially so for lung cancer patients [Citation7–9]

However, research on the link between anxiety and depression on treatment adherence among lung cancer patients is scarce. Two small prospective studies of non-small cell lung cancer patients (NSCLC) (n = 50–82) reported, that depression was associated with reduced treatment adherence [Citation2] and that anxiety symptoms were associated with reduced adherence to chemotherapy defined as a dose delay or reduction [Citation3]. Greer et al. [Citation3] concluded, that further research is needed to clarify the underlying mechanisms of the association between psychological factors and chemotherapy adherence.

The aim of this prospective cohort study was to investigate the associations of anxiety and depression symptoms, measured by The Hospital Anxiety and Depression Scale (HADS) [Citation10], with the completion of first-line treatment in newly diagnosed lung cancer patients in Denmark.