Low-grade appendiceal mucinous neoplasm

Last revised by Mohammad Taghi Niknejad on 16 Mar 2024

Low-grade appendiceal mucinous neoplasms (LAMN), previously known as appendiceal mucinous cystadenomas, are rare mucinous tumors of the appendix showing low-grade cytologic atypia, cf. high-grade appendiceal mucinous neoplasms.

Considerable controversy still exists on mucinous neoplasms of the appendix pathologic classification and nomenclature 9. According to a panel of specialist review in 2016, new nomenclature and classification for the appendiceal mucinous neoplasms based on their histologic type and biologic behavior has been proposed. The previously known term "appendiceal mucinous cystadenomas" is no longer recommended, being replaced by “low-grade appendiceal mucinous neoplasm” 7,8.

They may be histologically present in around 0.3% appendiceal resection specimens 3

A commonly described clinical presentation is right iliac fossa pain, similar to that of acute appendicitis. Around 25% of patients can be asymptomatic 2

Symptoms are more likely to be present if a tumor has perforated and complicated with pseudomyxoma peritonei

It is characterized by villous adenomatous changes of the appendiceal epithelium associated with marked distension of the appendiceal lumen with mucin. This can typically result in an appendiceal mucocele.

Histologically these tumors show low-grade cytologic atypia associated with at least one of the following 7,8

  • loss of the muscularis mucosae 

  • fibrosis of the submucosa

  • rupture of the appendix

  • mucin and/or tumoral cells outside the appendix

Differentiation between low-grade and high-grade mucinous neoplasms of the appendix can be difficult by imaging. Size, density and wall thickness of these mucoceles can be similar between the two groups. Furthermore, calcification, internal septations, periappendiceal fat stranding and intraperitoneal free fluid is nonspecific.

The following three features are more suggestive of malignancy 6:

  • soft tissue thickening (internal soft tissue attenuation nodularity of mucoceles)

  • wall irregularity

  • pseudomyxoma peritonei

Staging and management of these are different from those of colorectal carcinoma, please refer to the appendiceal mucinous neoplasms TNM staging for further details. ​

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