In this study, the investigators want to investigate whether online-adapted radiotherapy using a special magnetic resonance imaging (MRI) simulator and computed tomography (CT)-guided radiation therapy, is feasible and offers advantages for the treatment of pelvic lymph node metastases.

MR-guided online adaptive radiotherapy has been implemented recently using MR-linac technology, where MR-imaging is combined with linac technology in hybrid systems. These systems offer MR imaging for online adaptive radiotherapy (RT) within the treatment room and with the patient in treatment position. Despite results regarding technical and clinical feasibility being promising, the high costs of MR-linacs systems and the high demands for staffing resources of MR-linacs are limiting a boarder implementation of this technology. At the Department of Radiation Oncology, University Hospital Zurich, MRI-simulations for RT planning using a dedicated scanner have been performed since 2023. The investigators have developed and implemented an MRI-only RT planning workflow followed by RT delivered at a CBCT-guided, conventional linac. The aim to expand the MRI-only workflow to online adaptive RT. This study will investigate the feasibility of MRI-guided, online adaptive SBRT for pelvic and para-aortic lymph node metastases using a dedicated MR simulator and treatment delivery using conventional CBCT-linac technology.

Inclusion Criteria:

• Intra-pelvic and para-aortic lymph node metastases from a histologically confirmed malignancy;
• Indication for SBRT based on a multidisciplinary tumor board recommendation and / or clinical practice guideline;
• Intra-pelvic and para-aortic lymph node metastases confirmed by imaging and amenable for SBRT

Exclusion Criteria:

• Large body size that would not fit the MRI-simulator bore;

• Contraindications for MRI including but not limited to

  • electronic devices such as pacemakers, defibrillators, deep brain stimulators, cochlear implants that are labeled as MR unsafe. Electronic devices labeled as MR safe or MR conditional are not a contraindication;
  • metallic foreign body in the eye or aneurysm clips in the brain;
  • severe claustrophobia;
  • hip prosthesis that are labeled as MR unsafe;

• More than 5 pelvic lymph node metastasis

• Previous radiation therapy directly overlapping with SBRT in this study and leading to exceeding tolerance of OARs;

• History of Crohn's disease, ulcerative colitis