The purpose of this randomized clinical trial is to study anxiety and depression symptoms at diagnosis in patients with colon cancer. Previous studies have shown that so-called. Behavioural activation as therapy for these symptoms has a positive impact on the patient's well-being, but it has not been studied whether it also has a positive impact on the patient's mental health in cancer, and whether it can have a prognostically beneficial effect on cancer recurrence risk and survival.

The investigators will investigate the prevalence of anxiety and depression symptoms as well as quality of life in colon cancer patients at diagnosis and at follow-up using self-estimated validated protocols in a prospective cohort study. Data will be compared with data from a reference population. In the cohort study the investigators will also perform a nested randomized clinical trial of a brief psychological treatment called Behavioural Activation to patients with colon cancer and comorbid anxiety and/or depression.

All participants will answer the questionnaires at diagnosis, after1, 2 and 6 months and after 1 and 3 years.

Colon cancer is common. At diagnosis, about 20% have metastatic disease. The prognosis is determined by the stage of the tumour and the degree of spread, but also by the patient's physical health and other diseases. However, it is not known how the patient's mental health are affected and how common anxiety and depression symptoms are at diagnosis. Depression is known to be one of the most common causes of ill health in the world. It also has been shown that patients with depression and cancer also have a poorer survival rate compared to cancer patients without depression However, previous studies have shown that so-called brief Behavioural activation as therapy for these symptoms has a positive impact on the patient's well-being, but it has not been studied whether it also has a positive impact on the patient's mental health in cancer, and whether it can have a prognostically beneficial effect.

The goal of this randomized study is to increase knowledge about, to detect and treat anxiety and depression symptoms in patients with colon cancer, with so-called brief behavioural activation, which can lead to improved care, and possibly improve the patients' prognosis in the form of reduced risk of cancer recurrence and improved survival.

All participants will continue their care as usual (CAU), and half of the participants will be randomized to receive a five-session telephone-based BA-treatment spread over two months as an add-on to CAU. All participants will answer a questionnaire in the beginning of the study, 4 weeks after diagnosis, after 9 weeks, and after 3-, 6- and 12 months.

The main questions that the trial aims to answer are if the BA-treatment has an effect in the short and long term on:

• Depressive symptoms
• Anxiety symptoms
• Self-rated activation
• Self efficacy
• Mental wellbeing
• Quality of life
• Cancer recurrence and survival

Inclusion Criteria:

• All patients diagnosed with colon cancer in Region Västmanland, will be considered for inclusion. Those who meet the inclusion criteria will be offered to participate in the studies after written approval.

Exclusion Criteria:

• language difficulties that cause patients to be unable to absorb the treatment
• ongoing alcohol or substance use syndrome
• elevated suicide risk
• ongoing psychotic disorder, severe depression
• antidepressant treatment initiated within the last month
• ongoing or previous manic episode
• cognitive illness/dementia
• ongoing psychological treatment.

De-identified outcome measure IPD will be shared upon reasonable requests from academic researchers.

Study protocol and statistical analysis plan to be shared/published before data collection has ended.

Data will be shared upon reasonable requests assessed by the investigators.