Appendiceal Mucinous Neoplasms
Appendiceal mucinous neoplasm comprises a homogeneous group of tumors that pursue different clinical courses depending on cellularity, cytologic atypia (i.e. grade) and stage. The advances in our understanding of their biological behaviors in relation to the morphological features call for a uniform and consistent designation of this group of tumor for adequate therapy. Practically, these tumors can be put into following three categories.
Mucinous cystadenoma is defined as mucinous neoplasia confined to the mucosa with intact muscularis mucosae. Mucinous neoplasia confined to the appendiceal mucosa is cured by excision, regardless of the degree of dysplasia. Therefore, this group of mucinous tumor should be designated as “Mucinous cystadenoma” with or without high-grade dysplasia. High-grade mucinous tumors limited to the appendiceal mucosa are also conceptually termed non-invasive mucinous cystadenocarcinomas. It is important, however, to examine the specimen entirely for possible mural invasion, perforation or serosal mucin. Mucinous neoplasms with acellular mucin in the appendiceal wall or surrounding the appendix are associated with an excellent prognosis, and these cases should be managed similarly to appendiceal adenomas, provided they are completely evaluated to exclude the possibility of extra-appendiceal epithelium. The pathology report of mucinous cystadenoma should also include the presence or absence of mural acellular mucin, margin status and grade of dysplasia.
Mucinous adenocarcinoma includes mucinous tumor with mural invasion by the neoplastic cells and peritoneal disease with neoplastic epithelium by AJCC and WHO. By this definition, appendiceal mucinous tumors associated with neoplastic epithelium beyond the muscularis mucosae are considered to be invasive adenocarcinomas, so pathologists should adopt this nomenclature to achieve uniform reporting. These tumors are further classified as low- or high-grade based on mucin cellularity and the degree of cytologic atypia of the tumor cells. Neoplastic epithelium in the peritoneal cavity is associated with recurrence and death in nearly half of patients, even if it shows low-grade cytologic features. However, low-grade lesions are associated with a significantly better prognosis than tumors with overtly malignant cytology. Peritoneal disease characterized by low-grade epithelium is considered to be Stage IVA, whereas high-grade extra-appendiceal epithelium is designated Stage IVB. Pathology report should include the grade and stage, since tumors in the peritoneum that show high-grade cytology may need systemic chemotherapy in addition to surgery.
Appendiceal mucinous neoplasm of uncertain malignant potential
This group includes cases difficult to classify because of the presence of broad tumor base and mural mucin pools, or mucin limited to the periappendiceal region. The uncertainty largely arises from the absence of unequivocal invasion or cellular mucin pool. Thus, careful and thorough examination of the entire appendix is essential. If, after such examination, the uncertainty remains, the tumor should be reported as “mucinous neoplasm of uncertain malignant potential”. Once the presence of mural or extra-appendiceal epithelium is identified, the tumor must be considered as mucinous adenocarcinoma.
In practice, ruptured appendiceal mucoceles are also a main differential diagnosis. However, such lesions are not neoplastic. The wall of a mucocele shows attenuated mucosa lined by flattened and atrophic epithelium, which can often be identified by adequate sampling.
Appendiceal mucinous tumors are frequently unexpected finding. The challenge arises from uncertainty of its topographic relationship with the appendiceal wall and from limited sampling. Such a lesion with mural mucin must be entirely examined before a definitive diagnosis is rendered. Acellular mucin limited to the periappendiceal region including ruptured mucoceles and adenomas is rarely associated with disease progression, when the absence of epithelium is confirmed by histological examination of the entire appendix. However, any amount of epithelium in the peritoneal cavity has potential to disseminate throughout the peritoneum and result in the death in nearly half of the patient, even if it is limited to the periappendiceal region at the time of diagnosis or shows low-grade cytologic atypia. The more widely accepted approach endorsed by the AJCC and WHO designates all such peritoneal disease as carcinoma.
Reference / Suggested Readings
J. Misdraji: Appendiceal Mucinous Neoplasms: Controversial Issues. Archives of Pathology & Laboratory Medicine. 134(6):864-870, 2010
R. K. Yantiss et al: Prognostic significance of localized extra-appendiceal mucin deposition in appendiceal mucinous neoplasms. Am J Surg Pathol 33 (2):248–255, 2009.
R. F. Bradley et al.: Pseudomyxoma peritonei of appendiceal origin: a clinicopathologic analysis of 101 patients uniformly treated at a single institution, with literature review. Am J Surg Pathol. 30 (5):551–559, 2006.
R. K. Pai & T. A. Longacre.: Appendiceal mucinous tumors and pseudomyxoma peritonei: histologic features, diagnostic problems, and proposed classification. Adv Anat Pathol. 12 (6):291–311, 2005.
J. Misdraji et al.: Appendiceal mucinous neoplasms: a clinicopathologic analysis of 107 cases. Am J Surg Pathol. 27 (8): 1089-1103, 2004.
N. J. Carr et al.: Epithelial noncarcinoid tumors and tumor-like lesions of the appendix: a clinicopathologic study of 184 patients with a multivariate analysis of prognostic factors. Cancer 75 (3):757–768, 1995.
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