Low-grade appendiceal mucinous neoplasms (LAMN), previously known as appendiceal mucinous cystadenomas, are rare mucinous tumours of the appendix showing low-grade cytologic atypia, cf. high-grade appendiceal mucinous neoplasms.
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Terminology
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 behaviour has been proposed. The previously known term "appendiceal mucinous cystadenomas" is no longer recommended, being replaced by “low-grade appendiceal mucinous neoplasm” 7,8.
Epidemiology
They may be histologically present in around 0.3% appendiceal resection specimens 3.
Clinical presentation
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 tumour has perforated and complicated with pseudomyxoma peritonei.
Pathology
It is characterised 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 tumours 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 tumoural cells outside the appendix
Radiographic features
CT
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
Treatment and prognosis
Staging and management of these are different from those of colorectal carcinoma, please refer to the appendiceal mucinous neoplasms TNM staging for further details.