Importance: There are no large prospective cohort studies validating the previously reported high appendiceal tumor rate in patients with periappendicular abscess.

Objective: To confirm the appendiceal neoplasm rate associated with periappendicular abscess.

Design, setting, and participants: The Peri-Appendicitis Acuta Tumor prospective multicenter cohort study was conducted at 12 hospitals in Finland. All consecutive adult patients diagnosed with acute appendicitis were included, focusing on patients with imaging confirmed periappendicular abscess undergoing interval appendectomy or emergency appendectomy with available histopathology from December 3, 2020, through December 2, 2022. These data were analyzed from May 2023 to September 2024.

Interventions: Recommended treatment was initial antibiotics and, if needed, drainage, followed by interval appendectomy at 3 months.

Main outcomes and measures: The main outcome was appendiceal tumor rate associated with periappendicular abscess. Secondary outcomes included tumor histology and prognostic factors for appendiceal tumor (age, sex, body mass index, duration of symptoms, C-reactive protein level, white blood cell count, computed tomography findings, and tumor markers).

Results: There were 6165 consecutive patients with acute appendicitis. Of these, 396 patients (6.4%) presented with periappendicular abscess, with 9 patients declining to participate and 17 patients without available appendiceal histopathology, leaving 370 patients for the analysis (181 female [48.8%] and 189 male [51.2%]; median age, 58 [range, 18-90] years). Appendiceal tumor rate in patients with periappendicular abscess was 14.3% (53 of 370; 95% CI, 10.8%-17.9%), which was higher compared with uncomplicated acute appendicitis (14.3% vs 1.5%; 48 of 3170; 95% CI, 1.1%-1.9%; P < .001) and complicated acute appendicitis without periappendicular abscess (14.3% vs 2.4%; 63 of 2599; 95% CI, 1.8%-3.0%; P < .001). Patient age (odds ratio, 1.06; 95% CI, 1.04-1.09; P < .001) was the only factor associated with periappendicular abscess tumor prevalence. A cutoff point of age 35 years with high sensitivity was chosen (area under the curve, 0.75; 95% CI, 0.68-0.82) and only 1 patient below this threshold presented with an appendiceal tumor (sensitivity, 98.1%; 95% CI, 94.5-100). Of the 54 tumors on 53 patients, there were 21 low-grade appendiceal mucinous neoplasms, 20 adenocarcinomas, 8 adenomas, and 5 neuroendocrine tumors. The complication rate of emergency appendectomy was higher than after interval appendectomy (31.3% [70 of 224] vs 4.8% [5 of 105], respectively [P < .001]).

Conclusions and relevance: This large prospective cohort study confirmed a high appendiceal tumor rate associated with periappendicular abscess, especially in patients older than 35 years. At the least, all patients older than 35 years should undergo routine interval appendectomy.