Teaching Case

A 57-year-old male presents with a 48 hour lasting pain in the right lower quadrant (RLQ) and 39.5º fever. Routine laboratory tests were normal. The patient refers previous episodes of fever and RLQ pain during the last 3 months. An examination revealed a non-tender abdomen and a palpable soft mass.

Abdominal x-ray depicted a suspicious radiolucent area at the lower right quadrant. Ultrasound showed an encapsulated and well-defined anechoic mass with internal echogenic debris; a gas bubble within the mass was also depicted. Contrast enhanced CT confirmed the ultrasound findings describing a 13.5 x 6 x 6 cm mass in continuity with the caecum and displaying thickened walls, containing fluid and gas bubbles. A moderate inflammatory reaction of the perivisceral fat was also present. Moreover, the intestinal loop medial to the mucocele exhibited the "small bowel faeces sign" due to delayed intestinal transport. The patient underwent a right hemicolectomy. Histopathological examination revealed a low grade mucinous neoplasm of the appendix.

Appendiceal mucocele is a rare disease with a reported prevalence at appendectomy of 0.2-0.3% [1], and is often considered to be a differential diagnosis [2]. Three histological groups can be distinguished: nonneoplastic mucocele and mucinous cystoadenoma (combined they represent 40 to 50% of total cases) and mucinous cystoadenocarcinoma (glandular and stromal invasion is present and are similar to mucinous colonic tumours). Regarding the epidemiology, they are more frequent in women and usually detected in patiens older than 50 years of age. Presentation can be quite variable, acute appendicitis being the most common manifestation [3]. Approximately 25% are incidentally diagnosed as they remain asymptomatic [4].

Mucoceles pose a surgical management challenge, as many surgeons believe that laparoscopy appendectomy is contraindicated since rupture of the mucocele can lead to free spillage of mucus into the peritoneal cavity and resultant pseudomixoma peritonei, which has a poor prognosis [5]. However, and considering that survival rates strongly vary between the nonneoplastic and neoplastic mucocele (mucinous cystoadenoma) and the malignant mucocele (mucinous cystoadenocarcinoma) groups, a right hemicolectomy may be performed in case a neoplasm is believed to be the underlying cause, since the 10-year survival rate among patients with mucinous cystadenocarcinomas is 65% if treated with hemicolectomy and only 37% for patients who undergo appendectomy alone [6]. Therefore the preoperatively diagnose of appendiceal mucocele is important.

CT typical features of appendiceal mucocele are a cystic round or tubular and well-encapsulated mass [7], which might present wall calcifications that strongly support its diagnosis [8] (although seen in less than 50% of the cases). Although nonspecific, irregular walls and soft-tissue thickening are features most likely to be associated with malignancy [9] and right hemicolectomy is believe to be the primary approach if both features are present. It is important to distinguish wall thickening from wall irregularity as the first did not prove to be helpful in differentiating benign from malignant mucoceles [10]. Free intraperitoneal fluid has not been shown to be useful either, unless ascites is related to pseudomyxoma peritonei. Periappendiceal fat stranding is more frequently found in patients with appendicitis [3].

In conclusion, differentiation of malignant from benign mucoceles poses a diagnostic challenge based solely on CT features. Irregular walls and soft-tissue thickening are, however, the most likely to be associated with malignancy.