Precision Radiotherapy Using MR-linac for Pancreatic Neuroendocrine Tumours in MEN1 Patients

All patients meeting the following criteria will be assessed for in the tumour board: * lesions measuring between 2cm and 3cm. * pNET lesions with a size between 1.0 and 2.0 cm and moderate growth of the lesion (2-4 mm/ year) on sequential follow-up scans. * pNET lesions with a size between 1.0 and 2.0 cm and minimal growth of the lesion (1 mm/ year) reconfirmed on 3 or more sequential follow-up scans. * Patients with in situ remaining 1.0 - 2.0 cm lesions after previous resection of a larger lesion. All patients with such lesion and an indication for surgery are considered eligible for participation in the PRIME study. Exclusion Criteria: * Suspected malignant pNET as per the tumour board assessment, including the criteria: * pNET lesions of more than 3 cm in size * rapid growth of pNET lesions with more than 4mm per year * Symptomatic pNET because of hormone production, with the exception of gastrinomas which are located in the submucosa of the duodenum * concurrent treatment with a somatostatin analog * concurrent treatment with chemotherapy * peptide receptor radionuclide therapy in the past 12 months * history of radiotherapy in the upper abdominal region * MRI contraindications as per usual clinical care, such as claustrophobia and metal or electronic implants not compatible with MRI. * Pregnancy * (Other) metastatic disease * WHO performance score 3-4