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

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.

Study Overview

• Status: Recruiting
• Conditions: Pelvic Tumor; Abdominal Tumor
• Intervention / Treatment: Radiation: SBRT with adaptive fractionation; Radiation: Standard of Care (SOC)

• Study Type: Interventional
• Enrollment (Estimated): 100
• Phase: Not Applicable

Contacts and Locations

• Study Contact: Name: Jan Unkelbach, Prof. Dr. rer. nat.; Phone Number: +41 44 255 85 49; Email: jan.unkelbach@usz.ch
• Study Contact Backup: Name: Lena Kretzschmar, Dr. med.; Email: lena.kretzschmar@usz.ch

Study Locations

• Switzerland
  • Canton of Zurich
    • Zurich, Canton of Zurich, Switzerland, 8091
      • Recruiting
      • University Hospital Zürich

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

Description

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

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Non-Randomized
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: RT with AF; Patients with PTV/OAR overlap will receive AF, with dose adapted daily based on anatomy.	Radiation: SBRT with adaptive fractionation; The fraction dose to the PTV will be adapted based on the volume overlap of PTV and the dose-limiting OAR(s) (bowel / duodenum / stomach) on the planning MRI scan of the day ("geometry-of-the-day"). The treatment plan is then approved by the treating physician and independently verified according to department-internal standard. MR-guided fraction dose adaptation will be repeated at each of the 5 fractions, until the prescribed total dose is reached.
Active Comparator: RT without AF; Patients without PTV/OAR overlap will receive standard fractionated treatment.	Radiation: Standard of Care (SOC); Patients that qualify for inclusion in the study but for whom AF is no viable treatment due to lacking PTV/OAR overlap in the planning scans. These patients are treated according to clinical standard of care (SoC), i.e. without dose adaptation, and function as a control group.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Proportion of patients with successful daily dose adaptation	Proportion of patients treated with daily dose-adapted treatment, who received at least one fraction of non-uniformly fractionated treatment with expectation of a benefit in PTV-coverage of ≥ 1ccGy	from enrollment to end of treatment at 1-2 weeks

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Acute treatment toxicities	Acute treatment toxicities for all included patients measured with Common Terminology Criteria for Adverse Events	during treatment (1-2 weeks) and until 90 days after
Interfractional variation in PTV/OAR overlap volume	interfractional variation in PTV/OAR overlap volume, in cc	during treatment (1-2 weeks)
Interfractional variation in GTV/OAR distance	interfractional variation in GTV/OAR distance, in mm	during treatment (1-2 weeks)
Dosimetric difference in PTV-coverage between algorithm-based calculation and the reference plan	PTV coverage quantified as the area above the dose-volume-histogram curve measured in cc x Gy	after end of treatment (2 weeks after last fraction)

Collaborators and Investigators

• Sponsor: University of Zurich
• Investigators: Principal Investigator: Reinhardt Krcek, Dr. med., University of Zurich

Study record dates

Study Major Dates

• Study Start (Actual): June 16, 2026
• Primary Completion (Estimated): December 1, 2028
• Study Completion (Estimated): December 1, 2028

Study Registration Dates

• First Submitted: April 30, 2026
• First Submitted That Met QC Criteria: June 29, 2026
• First Posted (Actual): July 7, 2026

Study Record Updates

• Last Update Posted (Actual): July 7, 2026
• Last Update Submitted That Met QC Criteria: June 29, 2026
• Last Verified: June 1, 2026

More Information

Terms related to this study

• Keywords: stereotactic body radiotherapy; SBRT; adaptive radiotherapy; MR-guided radiotherapy
• Additional Relevant MeSH Terms: Neoplasms by Site; Neoplasms; Pelvic Neoplasms; Abdominal Neoplasms; Health Services Administration; Health Care Quality, Access, and Evaluation; Investigative Techniques; Therapeutics; Surgical Procedures, Operative; Quality of Health Care; Quality Indicators, Health Care; Radiotherapy; Stereotaxic Techniques; Neurosurgical Procedures; Standard of Care; Radiosurgery
• Other Study ID Numbers: RAO-25-019; 2026-00067 (Other Identifier: BASEC)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No