Rectal thickening on CT means that the wall of the rectum is thicker than usual. This is a subjective findings for which the radiologist uses his experience, especially when it is mild to moderate. Rectal thickening is also a difficult and overcalled diagnosis because the rectum is not well distended at times.
Perhaps the most common reason for rectal thickening is simply poor distinction of the rectum. The thickening is usually smooth and involving the wall uniformly. The lumen or inside of the rectum is not well seen because of poor distention. The fat around the rectum is usually normal and does not appear hazy. In these cases, a repeat CT exam with rectal contrast or direct visualization with endoscopy are the best options.
Proctitis or inflamed rectum is the next most common cause of rectal thickening on CT. Patients will present with rectal discharge, bleeding, and painful defecation. The causes can be infectious from a sexually transmitted disease, from radiation treatment, ischemic or lack of blood flow, and from fecal impaction. CT will show thickening of the rectum uniformly throughout, wall edema, and haziness of the fat surrounding the rectum. Engorged blood vessels around the rectum are also seen.
In the case of fecal impaction, this will show a rectum that is filled with stool and inflamed. This is referred to as stercoral proctitis or colitis depending on if this extends into the colon. Infectious and radiation proctitis will often not have a specific appearance but there will be an appropriate clinical history.
Rectal cancer will cause thickening of the rectum on CT. The thickening will often be more irregular or involving a segment of the rectum. This can be tough to see on CT because stool can mimic this appearance as can poor distention of the rectum. There may be haziness of the surrounding fat and abnormal lymph nodes. Diagnosis will involve endoscopy and biopsy.
Rectal thickening is therefore a non specific finding. This can be a normal finding because of poor distention of the rectum, inflamed rectum or cancer. Therefore, further testing is needed if this finding is raised on CT. Options can include a repeat CT with rectal contrast or rectal MRI to see if this persists. Direct visualization of the rectum with endoscopy and biopsy is more definitive.