Background and objective: Colorectal cancer (CRC) can mimic acute diverticulitis and can thus be misdiagnosed. Therefore, colonic evaluation is recommended after an episode of acute diverticulitis. The aim of this study was to analyze the risk of CRC after computed tomography (CT) verified uncomplicated and complicated acute diverticulitis in short-term and, particularly, long-term follow-up to ensure the feasibility of the primary CT imaging in separating patients with uncomplicated and complicated acute diverticulitis.

Methods: A retrospective cohort study was conducted in patients with CT-verified acute diverticulitis in 2003-2012. Data on CT findings and colonic evaluations were analyzed. The patients were divided into those with uncomplicated and complicated acute diverticulitis. Patient charts were reviewed 9-18 years after the initial acute diverticulitis episode.

Results: The study population consisted of 270 patients. According to CT scans, 170 (63%) patients had uncomplicated acute diverticulitis and 100 (37%) had complicated acute diverticulitis. Further colonic evaluation was made in 146 (54%) patients. In the whole study population, CRC was found in 7 (2.6%) patients, but CRC was associated with acute diverticulitis in only 4 (1.5%) patients. The short-term risk for CRC was 0.6% (1/170) in uncomplicated acute diverticulitis and 3.0% (3/100) in complicated acute diverticulitis. No additional CRC was found in patients with complicated acute diverticulitis during the long-term follow-up and three cases of CRC found after uncomplicated acute diverticulitis had no observable association with previous diverticulitis.

Conclusions: In short-term follow-up, the risk of underlying CRC is very low in CT-verified uncomplicated acute diverticulitis but increased in complicated acute diverticulitis. Long-term follow-up revealed no additional CRCs associated with previous acute diverticulitis, indicating that the short-term results remain consistent also in the long run. These long-term results confirm that colonoscopy should be reserved for patients with complicated acute diverticulitis or with persisting or alarming symptoms.