Radiation enteritis is radiation damage to the small bowel after pelvic or abdominal radiotherapy. Acute form (during or immediately after treatment) is characterized by mucosal damage and inflammation; symptoms include diarrhea, abdominal pain, and nausea. Chronic form (months-years later) is characterized by obliterative vasculitis and fibrosis; stricture, fistula, obstruction, and malabsorption may develop. Terminal ileum and pelvic small bowel loops are most frequently affected because pelvic organs (cervix, rectum, prostate, bladder) are the most common radiotherapy sites. On CT, wall thickening, increased mucosal enhancement (target sign), mesenteric vascular changes, stricture, and fistula are observed. Risk significantly increases with cumulative radiation dose >45 Gy. Concurrent chemotherapy increases risk. Treatment is conservative (nutritional support, anti-inflammatory) or surgical in complicated cases.
Age Range
30-80
Peak Age
60
Gender
Equal
Prevalence
Uncommon
Radiation enteritis results from the direct cytotoxic effect of ionizing radiation on small bowel tissue. In the acute phase, radiation targets rapidly dividing crypt cells — DNA double-strand breaks and apoptosis cause mucosal damage. Disruption of villus structures reduces absorptive surface and leads to diarrhea/malabsorption. Inflammatory cytokine release (TNF-alpha, IL-1, IL-6) causes mucosal edema and increased vascular permeability. Wall thickening and target sign on CT reflect mucosal/submucosal edema and hyperemia — the inner mucosal layer enhances while edematous submucosa remains low-attenuation and outer muscularis/serosa enhances. In the chronic phase, obliterative vasculitis develops — small vessel endothelium is damaged, intimal proliferation and thrombosis narrow the lumen, ischemic damage leads to fibrosis. This vascular damage remains limited to the radiation field and creates a sharp boundary between affected and normal segments (field-limited pattern). Fibrosis causes wall thickening, stricture, and mesenteric shortening/retraction.
Target sign (three-layered stratification) in bowel wall consistent with radiation field boundaries + sharp transition to normal segments = radiation enteritis. Normal bowel segments outside the field strengthen the diagnosis.
In the portal venous phase, the classic finding of radiation enteritis is the target sign — three-layered wall stratification. Inner mucosal layer enhances (hyperemic mucosa), middle submucosal layer remains low-attenuation (submucosal edema), outer muscularis/serosal layer enhances. This three-layered appearance is called the 'target' or 'double halo' sign. Wall thickening is generally symmetric and regular. Involvement is limited to the radiation field — sharp boundary is observed between affected and normal bowel segments.
Report Sentence
Segmental wall thickening consistent with the radiation field is seen in pelvic small bowel loops with a target sign (enhancing mucosa + low-attenuation submucosa + enhancing serosa).
The pathognomonic feature of radiation enteritis is wall thickening consistent with radiation field boundaries. Sharp transition is observed between affected bowel segments and normal segments. After pelvic radiotherapy, terminal ileum, sigmoid colon, and pelvic small bowel loops are typically affected. Wall thickening is regular, symmetric, and does not form focal mass. This 'field-limited' pattern is an important clue for distinguishing radiation enteritis from Crohn disease and lymphoma.
Report Sentence
Segmental wall thickening consistent with radiation field boundaries is seen in the terminal ileum and pelvic small bowel loops with sharp transition to normal segments; consistent with radiation enteritis.
In chronic radiation enteritis, bowel stricture, mesenteric retraction, and bowel loop matting are observed. Luminal narrowing with proximal dilatation is seen at stricture segments. Mesentery appears thickened, retracted, and fibrotic — fat infiltration around mesenteric vessels is observed. Bowel loops appear matted (adhered) and normal intervals are lost. Fistula tracts may form abnormal communications with adjacent bowel loops or bladder/vagina.
Report Sentence
Chronic stricture and mesenteric retraction are seen at the terminal ileum level with matted bowel loops; proximal dilatation is present — consistent with chronic radiation enteritis.
On T2-weighted MR images, radiation enteritis shows hyperintense signal with wall thickening and submucosal edema in affected bowel segments. The high signal on T2 reflects increased free water content. MR enterography provides higher soft tissue contrast than CT and shows mucosal/submucosal changes in more detail. Restricted diffusion may be seen in areas of active inflammation on DWI.
Report Sentence
On T2-weighted MR images, segmental wall thickening and submucosal hyperintensity are seen in pelvic small bowel loops; consistent with active radiation enteritis.
Mesenteric vascular changes are observed in radiation enteritis: mesenteric vessel engorgement, mesenteric fat haziness, and 'comb sign' (dilated vasa recta). These findings indicate active inflammation. In the chronic period, mesenteric fibrosis, vascular occlusion, and collateral development may be seen. Mesenteric fat infiltration reflects edema of surrounding tissue.
Report Sentence
Vascular engorgement, fat infiltration, and dilated vasa recta (comb sign) are seen in the mesentery of affected segments, supporting an active inflammatory process.
Fistula formation is an important complication of chronic radiation enteritis. Entero-enteric fistulae (between bowel loops), entero-vesical fistulae (bowel-bladder), and entero-vaginal fistulae (bowel-vagina) may be seen. On CT, the fistula tract appears as a thin channel containing air and/or contrast. Intraluminal gas in the bladder (pneumaturia) is an indirect sign of entero-vesical fistula. Inflammatory changes and fluid collection around the fistula may accompany.
Report Sentence
A fistula tract extending between bowel loops/to bladder/vagina is seen in the radiation field with surrounding inflammatory changes; consistent with chronic radiation enteritis complication.
Criteria
Develops during or within 6 weeks of completing radiotherapy. Predominantly mucosal damage and inflammation.
Distinct Features
Wall thickening, target sign, mesenteric hyperemia on CT. Usually transient and resolves with conservative treatment.
Criteria
Develops months-years after radiotherapy (usually 6 months-30 years). Predominantly obliterative vasculitis and fibrosis.
Distinct Features
Stricture, mesenteric fibrosis, matting, fistula on CT. Progressive and irreversible. Surgery may be needed.
Criteria
Chronic radiation enteritis with mechanical bowel obstruction, fistula formation, or perforation.
Distinct Features
May require emergency surgical intervention. Transition point, intraluminal gas in bladder, free air on CT. Mortality 10-20%.
Distinguishing Feature
Crohn disease shows skip lesions (discontinuous involvement), radiation enteritis shows continuous involvement limited to radiation field. Fistula and stricture also seen in Crohn but distribution differs.
Distinguishing Feature
Lymphoma focal or multifocal mass/wall thickening, not limited to radiation field; radiation enteritis diffuse, field-limited, no mass formation.
Distinguishing Feature
Ischemic enteritis shows vascular territorial distribution (SMA/IMA), radiation enteritis is radiation field limited. Mesenteric vascular pathology (thrombosis/stenosis) accompanies ischemia.
Distinguishing Feature
Metastasis forms nodular/polypoid masses, radiation enteritis diffuse wall thickening. Bull's-eye sign, intussusception in metastasis; target sign, field-limited pattern in radiation enteritis.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
6-monthAcute radiation enteritis usually resolves with conservative treatment (diet modification, antidiarrheals, probiotics). Chronic radiation enteritis may require nutritional support, anti-inflammatory drugs, and surgery for obstruction/fistula. Patients who received radiation require regular follow-up as the chronic form may develop years later. Surgical mortality is high (10-20%) — wound healing is impaired in irradiated tissues.
Radiation enteritis can be acute (during treatment) or chronic (months-years later). Chronic radiation enteritis presents with stricture, fistula, and malabsorption. Treatment is symptomatic; severe cases may require surgery.