Endoscopic Spacer-Assisted Stereotactic Body Radiation Therapy for Abdominal Oligometastases: A Novel Technique to Reduce Gastrointestinal Toxicity

Spacing agents are used in prostate cancer radiotherapy to reduce rectal dose, but their application in abdominal SBRT remains limited, particularly via endoscopic delivery.We report the case of a 74-year-old woman with recurrent cervical squamous cell carcinoma and oligometastatic disease involving para-aortic and retroperitoneal lymph nodes.The para-aortic lesion was closely abutting bowel, posing a challenge for safe delivery of stereotactic body radiation therapy (SBRT).A polyethylene glycol-based hydrogel spacer was successfully injected endoscopically, creating separation between the lymph node and adjacent bowel.The patient subsequently underwent a five-fraction, 36.25 Gy MR-guided SBRT with adaptive planning and intrafraction cine tracking.An in silico dosimetric analysis comparing plans with and without the spacer demonstrated improved PTV coverage and superior sparing of duodenal dose constraints.Treatment was well tolerated without acute toxicity, and follow-up imaging demonstrated stable disease and treatment at one-year post-treatment.To our knowledge, this is the first reported case of endoscopic hydrogel spacer placement to facilitate safe SBRT for abdominal oligometastases.This technique may expand the therapeutic window for patients with anatomically challenging targets near radiosensitive structures.