Low-grade appendiceal mucinous neoplasm (LAMN) is a slow-growing neoplastic lesion of the appendix lined by mucin-producing epithelium. It is characterized by mucin accumulation within the appendiceal lumen, leading to cystic dilatation (mucocele) of the appendix. It demonstrates epithelial dysplasia without invasive growth into the wall; however, mucin dissemination into the peritoneal cavity can lead to pseudomyxoma peritonei. It is defined as a distinct entity in the WHO 2019 classification and represents the most common subtype of appendiceal mucinous neoplasms.
Age Range
40-70
Peak Age
55
Gender
Female predominant
Prevalence
Rare
LAMN arises from neoplastic proliferation of the mucin-secreting columnar epithelium of the appendiceal mucosa. This neoplastic epithelium produces far more mucin than normal, leading to progressive accumulation within the appendiceal lumen, causing luminal distension and cystic dilatation of the appendix (mucocele formation). Mucin accumulation increases intraluminal pressure, which can lead to wall thinning and eventually calcification; this appears as characteristic mural calcification on imaging. Pathologically, LAMN demonstrates pushing-type invasion, meaning the neoplastic epithelium pushes against the muscularis propria without true stromal invasion; this feature is reflected on imaging as a well-circumscribed, thin-walled cystic lesion. The critical clinical significance is the potential for mucin dissemination through the appendiceal wall into the peritoneal cavity — this leads to a serious complication known as pseudomyxoma peritonei, characterized by widespread peritoneal mucinous implants.
Curvilinear, thin, continuous or interrupted calcification line in the appendiceal wall — considered pathognomonic for LAMN. This finding reflects wall dystrophic calcification under chronic intraluminal mucin pressure and is rarely seen in simple retention mucoceles.
Cystic dilatation of the appendix >15 mm in diameter. The lumen is filled with mucin demonstrating low density (0-20 HU). Connection to the cecum at the appendiceal base may be visualized.
Report Sentence
There is a cystic dilatation of the appendix measuring __ mm, filled with low-density content (__ HU), compatible with mucinous neoplasm.
Curvilinear (eggshell-type) calcification in the appendiceal wall. Appears as a thin, interrupted or continuous calcific line along the wall. Highly specific finding for LAMN.
Report Sentence
Curvilinear (eggshell-type) calcification is seen in the appendiceal wall, highly compatible with low-grade appendiceal mucinous neoplasm (LAMN).
Post-contrast appendiceal wall is thin (<3 mm), smoothly contoured with minimal or mild wall enhancement. Absence of irregular wall thickening or nodular enhancement reduces the likelihood of malignant invasion.
Report Sentence
The wall of the dilated appendix is thin and smoothly contoured without significant enhancement or nodularity.
High signal intensity mucinous content within the appendiceal lumen on T2-weighted sequences. May show slightly lower signal than simple fluid (due to proteinaceous content). Can be homogeneous or heterogeneous depending on mucin composition.
Report Sentence
High signal intensity mucinous content is observed within the appendiceal lumen on T2-weighted sequences.
Mucinous content within the appendiceal lumen demonstrates intermediate-to-low signal intensity on T1-weighted sequences. High-protein mucin may show slightly increased signal due to T1 shortening. T1 signal may be notably increased in the presence of hemorrhagic component.
Report Sentence
The appendiceal lumen is filled with intermediate-to-low signal intensity mucinous content on T1-weighted sequences.
Cystic/anechoic or internally echogenic cystic mass in the right lower quadrant at the appendiceal location on ultrasonography. Appears as anechoic, hypoechoic, or cystic structure with internal echoes depending on mucin content. Does not demonstrate peristalsis (distinguishing feature from bowel loop). Mural calcification may be seen with posterior acoustic shadowing.
Report Sentence
A cystic lesion measuring __ mm is observed in the right lower quadrant at the appendiceal location, which should be evaluated for mucinous neoplasm.
Demonstration of the connection between the cystic lesion and the cecal base confirms appendiceal origin. Continuity between the appendiceal base and cecum is clearly visualized on multiplanar reformations (MPR) in sagittal and coronal planes.
Report Sentence
Continuity of the cystic lesion with the cecal base is confirmed on MPR images, supporting appendiceal origin.
Criteria
Neoplastic mucinous epithelium is confined within the appendiceal wall, not reaching the serosal surface. No peritoneal mucin dissemination.
Distinct Features
Smooth, thin wall on CT; periappendiceal fat planes preserved; no peritoneal implants. Appendectomy is surgically sufficient.
Criteria
Mucin has spread beyond the appendiceal wall into the periappendiceal area. Acellular mucin may be present in the peritoneal cavity.
Distinct Features
Low-density periappendiceal collection on CT, scalloped hepatic surface or omental caking may accompany. Right hemicolectomy + peritoneal mucin assessment may be needed.
Criteria
Mucinous neoplasm demonstrating high-grade cytologic atypia. No invasion is present but epithelial atypia is prominent.
Distinct Features
May be indistinguishable from LAMN on imaging; however, thicker wall, nodular wall components, and periappendiceal infiltration are more common. Pathological diagnosis is required.
Distinguishing Feature
Simple mucocele is generally smaller (<2 cm), mural calcification is rare, and it has a retention (non-neoplastic) origin. LAMN shows mural calcification and larger size.
Distinguishing Feature
PMP is the peritoneal dissemination form of LAMN. PMP shows widespread peritoneal mucinous implants, scalloped liver surface, and omental caking; LAMN is confined to the appendix.
Distinguishing Feature
Appendicolith is an intraluminal calcified fecalith appearing as round/oval dense calcification. LAMN shows mural curvilinear calcification and cystic dilatation.
Distinguishing Feature
Appendiceal endometriosis shows high signal on T1 (blood products), cystic dilatation is typically smaller, and is associated with cyclical symptoms. LAMN shows low-intermediate T1 signal and mural calcification.
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralSurgical resection is recommended upon diagnosis of LAMN — appendectomy may suffice for appendix-confined lesions, while right hemicolectomy and peritoneal assessment may be needed in the presence of extraappendiceal mucin. Biopsy is not indicated as percutaneous intervention may trigger mucin dissemination. Due to the risk of pseudomyxoma peritonei, preoperative peritoneal assessment (CT) and postoperative pathological examination are mandatory in all patients. If HAMN is detected or surgical margins are positive, broader resection should be planned.
LAMN is the most important precursor lesion of pseudomyxoma peritonei. Surgical resection (appendectomy or right hemicolectomy) is the mainstay of treatment. If rupture or peritoneal dissemination is present, cytoreductive surgery + HIPEC (hyperthermic intraperitoneal chemotherapy) may be required. Long-term follow-up is recommended.