• 18 years of age or older

• Irresectable stage III or metastatic melanoma

• Treated with at least one dose of first-line ipilimumab-nivolumab and considered to be a candidate for maintenance treatment with nivolumab:

‣ previous systemic treatment, including immune-checkpoint inhibitors, in (neo)adjuvant setting for resectable melanoma is allowed

⁃ in this protocol, nivolumab maintenance is interchangeable with pembrolizumab maintenance therapy.

• Response evaluation according to RECIST v1.1 30 using a diagnostic CT documenting target lesions every 12 (-2/+6) weeks from the start of ipilimumab-nivolumab:

‣ for patients with CR on a diagnostic CT at response evaluation, a low-dose CT (which is usually part of 18FDG-PET/CT) is allowed at baseline

⁃ for patients with PR on a diagnostic CT at response evaluation, a low-dose CT (which is usually part of 18FDG-PET/CT) is allowed if sufficient target lesions are measurable for response evaluation according to RECIST v1.1 criteria 30

⁃ in case of asymptomatic brain metastases prior to start of first-line ipilimumab-nivolumab, intracerebral tumor response should be confirmed using an MRI for response evaluation prior to inclusion in this study.

• Patients should be included after first CR/PR or first confirmed CR/PR according to RECIST v1.1 30:

‣ inclusion should take place no later than 5 weeks after first confirmed CR/PR

⁃ in case of SD at first response evaluation, confirmed CR/PR is required for inclusion

⁃ planned and willing to discontinue nivolumab within 4(+1) weeks after inclusion, i.e. first CR/PR or first confirmed CR/PR

⁃ no later than 9 months after start of treatment with ipilimumab-nivolumab

• Presence of MRI brain for the screening of brain metastases (prior to discontinuation of ipilimumab-nivolumab)

• Participants with previously locally treated brain metastases may participate in case they meet the following criteria:

‣ completely asymptomatic brain metastases at inclusion

⁃ MRI of brain at baseline and for response evaluation during treatment

• Signed and dated informed consent form