Primary adenocarcinoma of the appendix is uncommon [1]. On gross examination this tumor may be presented as mucocele. Although there are many pathology reports of primary cystadenocarcinoma of the appendix, only a limited number of cases have appeared in the radiology or surgical literature [2–6]. Preoperative diagnosis is difficult but important for surgical management. We present a case of primary cystadenocarcinoma of the appendix occurring concurrently with adenocarcinoma of the colon.

Mucocele of the appendix is a descriptive term for mucinous distension of the appendiceal lumen, regardless of the underlying pathology. The incidence of mucocele in appendectomy specimens is about 0.25% [7]. The male to female ratio is 1:4 with a mean age of 55 years at diagnosis [7]. The most common symptom is right lower abdominal pain mimicking appendicitis, although mucocele can also be asymptomatic, as in our patient, found incidentally on imaging [2, 6]. In our case the underlying pathology proved to be a mucinous cystadenocarcinoma of the appendix. Primary adenocarcinoma of the appendix is uncommon, occurring in 0.08% of all appendectomies [1]. Mucinous cystadenocarcinoma is usually a well-differentiated neoplasm, and the gross morphology of this tumor is that of a mucocele [2]. Although there are many pathology reports of primary mucinous cystadenocarcinoma, only a few cases have been reported in the radiology literature [2–5]. Preoperative diagnosis, although important for proper surgical management, is difficult due to the absence of specific imaging findings. The presence of a cystic mass in the expected area of the appendix with enhancing wall and nodularity on CT examination suggests the possibility of mucinous cystadenocarcinoma [2–4, 8]. Wall thickness has not been proven to be a reliable differential point between neoplastic and non-neoplastic causes of mucocele of the appendix [3], thus the presence of relatively thin wall in our case can not exclude malignancy. Curvilinear nodular or punctuate wall calcifications, as in our case, have been reported in both malignant and benign mucoceles [2, 3].

In our case, the two very small nodules present close to the wall of the mucocele were consistent with lymph nodes. This finding along with minimal stranding in the surrounding fat may be suggestive of a malignant lesion and should be carefully evaluated as an aid in the differential diagnosis. The cystadenocarcinoma of the appendix was associated with an adenocarcinoma of the ascending colon and an adenomatous polyp with malignant transformation of the sigmoid colon. There have been reports of other tumors associated with adenomatous change in the appendix, including the gastrointestinal tract, ovary, breast and kidney [6]. However, the most common association was with adenocarcinoma of the colon. Indeed, 21.4% of a series of appendiceal adenomas were associated with colon neoplasia [6]. A common underlying cause is suggested, since the histological features of appendiceal adenocarcinomas are identical to those of adenocarcinomas elsewhere in the large bowel [6]. Considering that the appendix is a part of the large intestine, adenomatous polyps, papillary adenomas and adenocarcinomas in the appendix are simply an expression of colonic neoplasms in this particular anatomic location. Thus, low incidence of mucoceles is attributed to the relatively small size of the appendix in comparison to the rest of the large bowel [6].

Mucoceles are often an incidental finding on imaging examination of the abdomen. Small lymph nodes or stranding in the surrounding fat on CT may arise the possibility of a malignant underlying lesion. In these cases a thorough search for other colonic neoplasms is essential.