Daily Adjustment of the Radiation Dose in MRI-guided Stereotactic Radiotherapy for Tumors in the Abdominal and Pelvic Regions

Adaptive Fractionation in Online MR-guided SBRT for Abdominal and Pelvic Target Volumes

SBRT delivers high-dose radiation to precisely defined target volumes and is an effective locally ablative treatment option for both primary tumours and oligometastases in various settings, including lesions in the abdomen or pelvis. However, SBRT in these regions faces considerable challenges such as inter-fractional organ motion and anatomical deformation, which can compromise treatment precision.

MR-guided SBRT offers a solution, providing superior soft-tissue contrast and enabling real-time, online treatment plan adaptation over the course of SBRT. This approach reduces the risk of geographical miss and overdose to adjacent, vulnerable OARs. Currently, MR-guided SBRT uses a fixed dose per fraction, only adjusting target volume and OAR contours for anatomical changes when performing plan adaptation. However, overlaps between planning target volumes (PTVs) and dose-limiting OARs regularly require compromises in PTV coverage. To address this, AF aims to also adjust dose per fraction based on daily PTV-OAR overlap. With this approach, more dose is applied on treatment days with less PTV-OAR overlap while less dose is applied on days with more PTV-OAR overlap. This trial assesses the feasibility of AF in MR-guided SBRT for abdominal and pelvic tumours that show PTV-overlap with one or more dose-limiting OARs, ultimately aiming to enhance PTV dose coverage without increasing toxicity.

Inclusion Criteria:

• Karnofsky performance status ≥70
• Abdominal or pelvic malignancy (primary or secondary) with previous histological confirmation of the primary tumour, amenable to SBRT in 5 fractions (excluding SBRT to primary tumour in patients with prostate cancer) Note: patients with primary prostate cancer are eligible, but SBRT to the primary tumour in the prostate cannot be performed within the study
• Total prescribed SBRT dose > 30 Gy (constraint for dose-limiting OAR) over 5 treatment fractions
• Distance of the GTV ≤ 1 cm from at least one dose-limiting OAR (bowel / duodenum / stomach) in the available diagnostic images
• Patients able to tolerate long (approximately 60 minutes) treatment time according to clinical evaluation of the treating radiation oncologist
• Patients willing and able to comply with scheduled visits, treatment, and other trial procedures

Exclusion Criteria:

• Previous radiation therapy directly overlapping with planned SBRT (type I re-irradiation)
• More than one PTV irradiated in the same plan
• Contraindications for MRI, e.g., patients with metal fragments or implanted devices that are not MR compatible such as certain types of pacemakers and aneurysm clips or severe claustrophobia
• SBRT not indicated or not possible
• Inability to follow the procedures of the trial, e.g. due to language problems of the participant