Low inclusion rate; Change of position of principal investigator.

This study aims to evaluate anatomical and functional changes during RT for patients receiving fractionated RT for brain tumors. Anatomical changes during RT will be registered and analyzed and if needed the radiotherapy plan will be modified for the individual patient. This means that the "to be irradiated volume" will be modified according to the shape changes of the tumor. The functional MRI sequences will be used to evaluate what parameters, and at which time point, are important for radiotherapy outcome.

Inclusion Criteria:

• Primary brain tumor (WHO IV, GBM); treated with fractionated radiotherapy with or without preceding surgery. Systemic treatment is allowed before and during radiotherapy.
• Brain metastasis (from a solid extracranial tumor) eligible for treatment with fractionated radiotherapy with or without preceding surgery. Systemic treatment is allowed before and during radiotherapy.

Also, postoperative patients with a macroscopic residual tumor lesion can be included (the diagnosis of residual diseases is defined by the treating neurosurgeon or on the MRI executed within 24 hours after the surgery, the diagnosis of residual macroscopic disease should always be confirmed during the neuro-oncology multidisciplinary meeting).

Exclusion Criteria:

• Patients receiving Whole Brain RT (WBRT)
• Patient with a poor kidney function (GFR < 30 mL/min; CKD 4 and 5; patients on dialysis and patients with acute kidney insufficiency)
• None, doubtful or very small (<5 mm in its largest dimension) contrast enhancing lesion
• Patients with a contraindication for MRI

Patients with metastasis in the brain from extracranial solid tumors or patients with primary brain tumors who will be treated with fractionated radiotherapy.