Dose-escalated, Hypofractionated, Definitive Proton Radiotherapy for Patients With Inoperable Soft Tissue Sarcoma.

Inclusion Criteria: 1. ≥ 18 years of age at the time of informed consent. 2. Histological diagnosis of soft tissue sarcoma including gastrointestinal stromal tumor (GIST). 3. Measurable disease according to RECIST v1.1. 4. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2 5. For patients with metastatic disease a life-expectancy greater than 2 years should be expected. 6. Before patient registration, written informed consent must be given according to national and local regulations. 7. Ability to fill in patient questionnaires and comply with study procedures, including travelling to Bergen or Oslo for Proton Beam radiotherapy. Exclusion Criteria: 1. Patients with a prior or concurrent malignant disease whose natural history or treatment have the potential to interfere with the safety or efficacy assessment of this clinical trial are not eligible. Patients with a history of breast cancer, requiring continued hormonal treatment (e.g. anti-estrogen or an aromatase inhibitor) may be included. Patients with a history of prostate cancer, requiring continued support with luteinizing hormone releasing hormone (LHRH) agonists, with or without androgens, may be included. 2. Previous radiotherapy to the tumor site. 3. Patients with pacemakers and/or implanted defibrillators. 4. Patients not able to give an informed consent or comply with study regulations as deemed by study investigator. 5. Administration of systemic cancer therapy (i.e. chemotherapy, targeted therapy or immune therapy) within 14 days prior to the first fraction of radiotherapy.