HDR Endorectal Brachytherapy With 3D-Printed Guidance and Local Shielding for Rectal Cancer Organ Preservation

Inclusion Criteria: 1. Age 18-80 years. 2. Histologically confirmed rectal adenocarcinoma. 3. Clinical stage cT2-4, any N, M0. 4. Patients who are elderly or frail, medically unsuitable for radical surgery, refuse radical surgery, or are considered by a multidisciplinary team (MDT) to have disease for which safe R0 resection is technically difficult. 5. Considered by the MDT and investigators to be suitable for a definitive non-operative treatment strategy. 6. ECOG performance status 0-2; patients with ECOG performance status 3 may be eligible if considered by the MDT to be able to tolerate the study treatment. 7. Estimated life expectancy of at least 6 months. 8. Considered suitable for pelvic external beam radiotherapy and HDR endorectal brachytherapy. 9. Tumor location, endorectal access, and local anatomy suitable for HDR brachytherapy using the individualized 3D-printed guide template. 10. Adequate hematologic, hepatic, and renal function for the planned treatment. 11. Ability to understand the study requirements and provide written informed consent. Exclusion Criteria: 1. Distant metastatic disease or disease considered unsuitable for definitive local non-operative treatment. 2. Prior definitive pelvic radiotherapy. 3. Intestinal perforation, complete bowel obstruction, uncontrolled active bleeding, or severe active infection. 4. Anatomical or clinical conditions precluding HDR endorectal brachytherapy or use of the individualized guide template/local shielding platform. 5. Inflammatory bowel disease. 6. Severe uncontrolled cardiopulmonary disease, significant bone marrow suppression, or severe hepatic or renal dysfunction. 7. Pregnancy or breastfeeding. 8. Concurrent participation in another clinical trial that may interfere with evaluation of the study treatment or endpoints. 9. Any other condition considered by the investigator to make the patient unsuitable for participation in the study.