Appendix
Other tumors
Mucinous neoplasms (LAMN and HAMN)


Topic Completed: 1 October 2017

Revised: 22 August 2019

Copyright: (c) 2002-2019, PathologyOutlines.com, Inc.

PubMed Search: Mucinous neoplasms appendix[title] free full text[sb]

Michael Feely, D.O.
Raul S. Gonzalez, M.D.
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Cite this page: Feely M, Gonzalez RS. Mucinous neoplasms (LAMN and HAMN). PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/appendixmucinousneoplasm.html. Accessed November 19th, 2019.
Definition / general
  • Evolving nomenclature with considerable controversy, although recent consensus terminology has been established (Am J Surg Pathol 2016;40:14)
  • Variable clinical consequences depending on the location of neoplastic epithelium and associated mucin
  • By definition, must lack infiltrative invasion, which would be termed mucinous adenocarcinoma
Essential features
  • Low grade appendiceal mucinous neoplasm (LAMN) is a low grade noninvasive epithelial proliferation that can cause pseudomyxoma peritonei if the appendix ruptures
  • Similar, rare lesions with high grade nuclear dysplasia are termed high grade appendiceal mucinous neoplasm (HAMN)
Terminology
  • Low grade appendiceal mucinous neoplasm (LAMN): lesion arising in appendix with low grade epithelial features in the absence of infiltrative growth
  • High grade appendiceal mucinous neoplasm (HAMN): lesion arising in appendix with high grade epithelial features in the absence of infiltrative growth
  • Pseudomyxoma peritonei: strictly clinical term for apparent mucinous ascites or peritoneal mucin deposition
  • Mucocele: strictly clinical term for dilated, mucin filled appendix
  • Cystadenoma: outdated diagnostic term that should no longer be used
Epidemiology
Clinical features
  • Typically occurs in patients during their sixth decade of life, although age range is broad; more common in women (Am J Surg Pathol 2009;33:1425)
  • Most patients with disease restricted to the appendix present with acute appendicitis-like symptoms, while those with disseminated disease may present with abdominal or ovarian masses or pseudomyxoma peritonei
Radiology description
  • Appendix with a diameter of more than 15 mm, a soft tissue mass or wall thickening may raise the possibility of a mucinous neoplasm (Cancer Imaging 2013;13:14)
Prognostic factors
  • Mucinous lesions confined to appendix largely considered cured by resection
  • Lesions with extra-appendiceal acellular mucin considered to be low risk for recurrence or progression, occurring in about 4% of cases (Am J Surg Pathol 2009;33:248)
  • If extra-appendiceal mucin contains neoplastic epithelium, patient is at high risk for recurrence or dissemination, which occurs in 33 - 75% of these cases (Am J Surg Pathol 2009;33:248, Am J Surg Pathol 2009;33:1425)
Treatment
  • Simple appendectomy considered sufficient for lesions limited to appendix
  • Close surveillance for patients with localized periappendiceal disease following initial surgery
  • Disseminated peritoneal disease may be treated with hyperthermic intraperitoneal chemotherapy (HIPEC) following cytoreductive surgery
Gross description
  • Typically, appendix appears dilated with luminal mucin, although diameter may appear unremarkable
  • Serosa appears smooth when appendiceal wall is intact
  • Adhesions or extra-appendiceal mucin are concerning for underlying rupture
Gross images

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Mucocele: dilated appendix

Microscopic (histologic) description
  • Villous or occasionally flat proliferation of mucinous epithelial cells originating from appendiceal lumen
  • Lesional cells typically demonstrate abundant apical mucin with elongated nuclei and low grade nuclear atypia (LAMN); however, nuclei may appear compressed or rarely high grade (HAMN)
  • Often associated with atrophy of underlying lymphoid tissue, crypt loss and effacement of muscularis mucosae
  • Broad dissection of mucin, epithelium or both may occur with potential involvement of extra-appendiceal surface, an important finding affecting prognosis
  • Extra-appendiceal mucin incites a serosal reaction and may contain neovascularization, assisting in differentiation from benign transfer of mucin during gross examination
Microscopic (histologic) images

Contributed by Michael Feely, D.O.

LAMN in cross section

Villous architecture in LAMN

Low grade epithelium in LAMN

LAMN with extra-appendiceal mucin

Extra-appendiceal mucin with serosal reaction

Cellular extra-appendiceal mucin



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Intestinal type epithelial lining

Negative stains
Molecular / cytogenetics description
  • Frequently harbor KRAS mutations and loss of chromosome 5q has been reported (Am J Pathol 1999;154:1849)
  • GNAS alterations have been reported to occur in 50% of cases (Br J Cancer 2013;108:951)
  • Microsatellite instability and BRAF mutations have not been demonstrated
Differential diagnosis
Board review question #1
Which finding associated with a mucinous neoplasm has the greatest risk of progression?

  1. Acellular mucin outside the right lower quadrant of abdomen
  2. Acellular mucin restricted to right lower quadrant of abdomen
  3. Extra-appendiceal mucin containing low grade epithelium
  4. High grade epithelium confined to appendix
  5. Low grade epithelium confined to appendix
Board review answer #1
C. Extra-appendiceal mucin containing low grade epithelium.

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