Adenocarcinoma of the appendix arises from pre-existing adenoma. This tumor harbored residual areas of villous adenoma. Delicate finger like structures protrude into and fill the lumen.

The villous adenoma shows the transition to invasion.

The tumor elicits a desmoplastic response as it penetrates through the appendiceal wall.

Limited extracellular mucin is found in this tumor. It appears to have a conventional adenocarcinoma morphology similar to those in the colon. Growth into the fat can be appreciated.

Unlike its neighboring colon, adenocarcinoma is rare in the appendix, with an age-adjusted incidence of 0.12 cases per 1,000,000 people per year (McGory). The SEER study which reviewed the clinicopathologic features of 2514 appendiceal tumors between 1973 and 2001 found that most cancers of the appendix are noncarcinoid, a somewhat surprising finding. Mucinous adenocarcinomas (38%) were the most common, followed by adenocarcinomas (26%), carcinoids (17%), goblet cell carcinoids (15%) and signet ring (4%) (McGory).

Mucinous adenocarcinomas may lead to pseudomyxoma peritonei due to the abundant production and subsequent extrusion of mucus and mucus-producing malignant cells into the peritoneum. Adenocarcinomas (without mucin production) may be seen adjacent to an adenoma (Fletcher).

The vast majority are not diagnosed preoperatively, but present with acute appendicitis or are detected as incidentally during exploration for another surgical disorder.

If detected at exploratomy as an incidental finding, it is suggested to try to determine the tumor size and whether the tumor is carcinoid. If non-carcinoid, then right hemicolectomy is recommended.

Carcinoids have significantly better outcomes than the noncarcinoid group (consisting of mucinous, adenocarcinoma, goblet, and signet histologies). In a study of 196 appendiceal tumors, overall 5-year survival was 83.1% for carcinoid vs. 49.2% for non-carcinoid tumors (Benedix).