What is the most common site for radiation enteritis in the large bowel?

Radiation Enteritis of the Large Bowel Most Commonly Affects the Rectum

The rectum is the most common site of radiation enteritis in the large bowel due to its fixed position and proximity to pelvic organs that frequently receive radiation therapy. 1

Anatomical Distribution of Radiation Enteritis

• The rectum is the most commonly affected segment of the large bowel in radiation enteritis due to its fixed anatomical position in the pelvis and proximity to organs commonly treated with radiation therapy 1, 2
• The ileum is the most frequently affected segment of the small bowel (71% of cases), while the rectum is the most commonly affected segment of the large bowel (28% of cases) 3
• Radiation-induced proctitis is a common complication of pelvic radiation therapy, occurring due to the rectal proximity to pelvic organs and its fixed position 1

Pathophysiology of Radiation Enteritis

• Radiation damage occurs through direct energy absorption and free radical release from radiation interacting with cellular water 1, 2
• Damage to stem cells within intestinal crypts leads to reduced mucosal integrity and flattening of intestinal villi 1, 4
• Radiation induces arteriole endarteritis, submucosal connective tissue fibrosis, and neoangiogenesis followed by telangiectasias in the rectal wall 1
• Modification of intestinal microflora and deterioration of enzymatic activities contribute to radiation enteritis 1, 4
• The fixed position of the rectum makes it particularly vulnerable to cumulative radiation exposure compared to more mobile segments of the bowel 1, 2

Clinical Presentation

• Bleeding is the most common symptom of radiation proctitis 1
• Other symptoms include strictures, perforation, fistula formation, and rectal obstruction 1, 5
• Loss of distensibility due to rectal wall fibrosis results in tenesmus and defecation difficulties 1
• Acute radiation proctitis occurs almost immediately after starting radiation therapy and lasts up to 3 months 1
• Chronic radiation proctitis may begin during the acute phase but symptoms typically become apparent 8-12 months after completing radiation therapy 1, 6

Risk Factors for Radiation Enteritis

• Large irradiated volume, radiation dose (>45 Gy or above 70 Gy), and older radiation techniques (3D-CRT vs. IMRT) increase risk 1
• Patient-related risk factors include low BMI, diabetes mellitus, vascular disease, arterial hypertension, atherosclerosis, inflammatory bowel disease, collagen disease, and HIV infection 1, 2
• Previous intestinal surgery may predispose to radiation enteritis due to anatomical changes leading to increased bowel exposure to radiation field 1
• Concomitant chemotherapy, particularly with fluoropyrimidines, increases the risk and severity of radiation enteritis 1

Prevention and Management

• Intensity-modulated radiation therapy (IMRT) is associated with significantly less rectal toxicity than 3D conformal radiation therapy (3D-CRT) 1
• Image-guided radiation therapy (IGRT) reduces the risks of target miss and/or organs at risk overdose during radiation therapy delivery 1
• Probiotics during treatment can significantly reduce acute toxicity 1
• Treatment of acute radiation enteritis includes probiotics to restore intestinal microbiota, loperamide for symptom control, and dietary counseling 1, 2
• Surgical management is indicated for complications (abscess, perforation, fistula) or resistance to medical treatment; intestinal resection is preferred over internal bypass 5

Clinical Implications

• About 90% of patients who receive pelvic radiation therapy may develop permanent changes in bowel habits 1
• Quality of life is affected by gastrointestinal symptoms in 50% of patients (moderate to severe effect in 20-40% of cases) 1
• Regular follow-up evaluation is essential to assess late toxicity for early intervention by a specialist multidisciplinary team 1
• The main risk with iterative resections is short bowel syndrome and the potential need for definitive nutritional assistance 5