Assessment of Short Immunotherapy After Radical Surgery of High-risk Malignant Melanoma

Inclusion Criteria: 1. Provision of written informed consent for participation. 2. ≥ 18 years of age. 3. Performance status ECOG/WHO 0-1. 4. Adequate organ functions as per standards for immunotherapy. 5. Radical surgery for CMM (including acral) stage IIb-c, III (including in transit) and IV. Stage III CMM patients with unknown primary and stage IIb-c CMM patients who have not undergone sentinel node procedure are eligible. 6. A complete physical examination within 28 days prior to randomization 7. Previous adjuvant treatment with BRAF + MEK inhibitors is allowed. 8. Neoadjuvant treatment with immunotherapy for two months (currently pembrolizumab every third weeks three times or nivolumab every fourth week two times) is allowed providing that a complete or near complete pathological response was not achieved and patients with clear progressive disease according to the pathology report are not eligible. 9. All participants who have not received neo-adjuvant treatment must have disease-free status documented by radiological assessment within 28 days prior to randomization while 6 weeks is sufficient for neo-adjuvant treated patients. 10. Participants must be off immunosuppressive doses of systemic steroids (\>10 mg/day prednisone or equivalent) for a minimum of 14 days prior to study drug administration. 11. Sufficient renal function for radiological assessments with i.v. contrast. 12. Peri-operative radiation therapy is allowed. 13. Patients who experience a locoregional lymph node relapse, i.e. stage III disease or operable stage IV at a time-point later than primary diagnosis are welcome to participate. Exclusion Criteria: 1. The patient is, in the opinion of the investigator, assessed as unfit to receive systemic adjuvant treatment. 2. Serious and/or uncontrolled medical disorder that in the opinion of the investigator is contraindicated. 3. An active, known, or suspected autoimmune disease. Participants with type I diabetes mellitus, hypothyroidism requiring hormone replacement only and skin disorders (such as vitiligo, psoriasis, or alopecia) not requiring systemic treatment are eligible. 4. Life-expectancy less than 2 years due to concurrent disease (e.g., cardiac disease and liver cirrhosis). 5. Inability to provide informed consent or refusal to do so. 6. Inability to comply with the study protocol. 7. Participation in other clinical trials interfering with the current study protocol. 8. Existing or previous malignancies within the past 5 years (except for in situ breast and in situ cervical cancer, melanoma in situ, malignant melanoma, non-melanoma skin cancer and low risk prostate cancer). 9. Pregnancy or planned pregnancy. 10. Ocular and mucosal melanoma.