This prospective phase II study evaluates the efficacy and safety of MRI-guided stereotactic body radiotherapy (MRIgSBRT) in patients with oligometastatic peritoneal carcinomatosis from gastrointestinal or gynecological tumors. Eligible patients with up to 5 MRI-visible peritoneal lesions and PCI ≤20 will be treated with MRIgSBRT using a hybrid MRI-linear accelerator with online adaptive planning and real-time tumor tracking.

The primary endpoint is radiologic response rate according to RECIST 1.1 criteria. Secondary endpoints include overall survival, progression-free survival, local control, chemotherapy-free survival, and treatment-related toxicity. The study aims to assess MRIgSBRT as a safe and non-invasive treatment option for selected patients with limited peritoneal metastatic disease.

Peritoneal carcinomatosis (PC) represents a frequent metastatic pattern in gastrointestinal and gynecological malignancies and is associated with poor prognosis and limited therapeutic options. Although cytoreductive surgery (CRS) combined with hyperthermic intraperitoneal chemotherapy (HIPEC) may improve outcomes in selected patients, many patients are not candidates for surgery because of recurrent disease, comorbidities, or poor performance status.

MRI-guided stereotactic body radiotherapy (MRIgSBRT) allows highly conformal radiation delivery with online adaptive planning and real-time tumor tracking, potentially improving treatment precision and sparing surrounding organs at risk in anatomically complex abdominal sites.

This prospective single-center phase II study will evaluate MRIgSBRT in patients with oligometastatic peritoneal carcinomatosis with up to 5 MRI-visible lesions and PCI ≤20. Treatment will be delivered using a hybrid MRI-linear accelerator with adaptive planning according to daily anatomical variations.

The primary endpoint is radiologic response rate according to RECIST 1.1 criteria. Secondary endpoints include overall survival, progression-free survival, local control, chemotherapy-free survival, and treatment-related toxicity assessed according to CTCAE v5.0.

Inclusion Criteria:

• Age ≥18 years
• ECOG performance status 0-2
• Life expectancy ≥6 months
• Up to 5 MRI-visible peritoneal nodules
• PCI ≤20
• No prior radiotherapy in the same abdominal region within 6 months
• Signed informed consent
• History of prior CRS and/or HIPEC

Exclusion Criteria:

• Diffuse peritoneal carcinomatosis
• Psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule (those conditions should be discussed with the patient before registration in the trial)
• Significant comorbidities (such as: severe cardiac or coagulative disease, moderate or severe
• restrictive/obsiructive lung deficit, severe cognitive impairment, moderate and severe renal and hepatic impairment.
• Pregnancy or breastfeeding
• MRI contraindications (e.g., pacemaker, severe claustrophobia)