Your pathology report for low grade appendiceal mucinous neoplasm (LAMN)

by Stephanie Reid, MD FRCPC
November 24, 2025

A low-grade mucinous neoplasm of the appendix (LAMN) is a tumor made up of mucin-producing cells that grow along the inside lining of the appendix. These cells look only mildly abnormal under the microscope but can make large amounts of mucin, a jelly-like substance. As mucin builds up, the appendix may swell, and in some cases, mucin can leak into the abdomen if the appendix wall becomes weakened or ruptures.

LAMN is considered “low grade” because the cells do not look highly aggressive. However, it is still an important diagnosis because the mucin and tumor cells can sometimes spread beyond the appendix and cause long-term problems in the abdomen. LAMN differs from high-grade mucinous neoplasm (HAMN) and mucinous adenocarcinoma, which are more aggressive.

Anatomy of the appendix

The appendix is a thin, finger-shaped pouch located at the beginning of the large intestine. Its inner lining contains glandular cells that produce mucin. Because the appendix is narrow and enclosed, mucinous tumors can cause the organ to stretch, thicken, or rupture. This unique structure explains how mucin from LAMN may occasionally spill into the abdominal cavity.

What are the symptoms of a low-grade mucinous neoplasm?

Many people with LAMN do not have symptoms, and the tumor is discovered unexpectedly during a scan or surgery for another reason. When symptoms do occur, they often resemble appendicitis and may include abdominal pain (especially in the lower right side), nausea, vomiting, or fever.

If the appendix ruptures or mucin begins to collect in the abdomen, people may notice abdominal swelling, a new umbilical hernia, or a feeling of pressure or fullness. Some patients develop a palpable mass. Imaging studies such as CT or ultrasound often show a fluid-filled or enlarged appendix, sometimes with small calcium deposits along the wall.

Who gets low-grade mucinous neoplasms?

LAMNs most commonly affect adults in their 50s and 60s, but they can occur at any age. Men and women are equally affected. The exact cause is unknown, and most cases are not inherited.

What causes a low-grade mucinous neoplasm?

Most LAMNs have changes in genes such as KRAS and GNAS, which help regulate cell growth and mucin production. These changes likely contribute to the high mucin levels observed in these tumors. Unlike colorectal cancers, LAMNs rarely harbor mutations in APC, TP53, or SMAD4 and do not exhibit microsatellite instability or mismatch repair deficiency. These molecular differences help explain why LAMN behaves differently from typical colon cancer.

How is this diagnosis made?

LAMN is almost always diagnosed after the appendix is removed. Most patients undergo surgery because of pain, suspected appendicitis, or an unexpected finding on imaging. The diagnosis is made by a pathologist examining the entire appendix under a microscope.

In LAMN, the normal lining of the appendix is replaced by mucin-producing epithelium. The tumor cells often contain large mucin-filled vacuoles that push the nucleus to the side, giving the lining a “hyper-mucinous” appearance. The cells may grow in thin, finger-like projections (filiform or villous), in scalloped or undulating patterns, or as a thin, flattened layer. Even though the cells look abnormal, the changes are low-grade, meaning the nuclei are only mildly enlarged and irregular.

As the tumor produces more mucin, the appendix may become stretched, and its wall may show fibrosis, hyalinization, or calcification. The mucin can track through the wall and reach the outer surface. If the wall breaks, mucin may spill into the abdominal cavity. If tumor cells accompany the mucin, this can lead to pseudomyxoma peritonei, a condition where mucin slowly accumulates in the abdomen.

LAMN typically grows in a pushing pattern, meaning the lining expands outward in smooth, rounded contours rather than deeply invading or destroying tissue. If the tumor shows destructive invasion, the diagnosis changes to mucinous adenocarcinoma, a more aggressive cancer. If the tumor shows architectural patterns of LAMN but with high-grade nuclear features—such as very abnormal nuclei, frequent mitoses, or necrosis—the diagnosis may be upgraded to high-grade mucinous neoplasm (HAMN).

LAMN must also be distinguished from conditions such as diverticular disease or serrated polyps, which may cause mucin leakage but do not exhibit the architectural and wall changes typical of LAMN.

Tumor extension (depth of spread)

LAMN begins in the inner lining of the appendix. As mucin builds up and the lining expands, the tumor can extend into the deeper layers of the wall or even beyond the appendix. The degree of spread is used to determine the pathologic stage, which plays a significant role in prognosis and treatment planning.

Tumors that are confined to the submucosa and muscularis propria—the inner layers of the appendix—are staged as pTis, meaning “carcinoma in situ.” These tumors behave in an indolent manner and have an excellent prognosis.

If the tumor or mucin reaches the subserosa, the layer just beneath the outer surface, the cancer is staged as pT3. This represents deeper involvement but remains confined within the appendiceal wall.

When mucin or tumor cells reach or perforate the serosa, the outermost layer of the appendix, the tumor is staged as pT4a. At this stage, mucin may escape into the abdominal cavity.

If mucin or tumor cells are found outside the appendix, the tumor is classified as metastatic:

• pM1a: mucin outside the appendix without tumor cells

• pM1b: mucin-containing tumor cells outside the appendix

Distinguishing whether mucin contains tumor cells is important because it influences prognosis and follow-up.

Margins

A margin is the edge of the tissue removed during surgery. After examining the appendix, the pathologist determines whether any LAMN cells or mucin reach the cut edge. A negative margin means the tumor was removed entirely, while a positive margin means tumor cells or mucin were present at the edge. A positive margin may increase the risk of recurrence, especially if the tumor has spread beyond the appendix.

Lymph nodes

Lymph nodes are small immune organs that filter lymph fluid and help the body fight infections. In many cancers, tumor cells can travel to lymph nodes. However, LAMN typically spreads via mucin leakage rather than through lymphatic channels, so lymph node involvement is rare.

Lymph nodes may be removed if they are taken as part of a larger surgical procedure. If lymph nodes are present, the pathologist examines them to confirm whether any tumor cells are present.

Pathologic stage (pTNM)

Your pathology report assigns a stage based on how far the tumor has grown and whether mucin or tumor cells have spread outside the appendix.

• pTis: Tumor confined to the inner layers of the appendix

• pT3: Tumor reaches the subserosa

• pT4a: Tumor perforates the serosa

• pM1a: Acellular mucin outside the appendix

• pM1b: Mucin-containing tumor cells outside the appendix

Prognosis

The outlook for LAMN depends almost entirely on how far the tumor has spread. When the tumor is wholly confined to the appendix, the prognosis is excellent, and surgery is usually curative. If mucin spreads to the abdominal cavity, the prognosis becomes more variable and depends on:

• Whether the mucin contains tumor cells.

• The grade of the tumor cells in the peritoneum.

• The amount and distribution of mucin.

• Whether surgeons can remove all visible tumor.

Many patients with peritoneal disease benefit from a combination of cytoreductive surgery and hyperthermic intraperitoneal chemotherapy (HIPEC), which has improved long-term survival in selected cases.

After the diagnosis

Your doctor will review your pathology report with you to determine whether further treatment or monitoring is needed. If the tumor was limited to the appendix and removed completely, no additional therapy may be required.

If mucin or tumor cells were found outside the appendix, you may be referred to a surgical oncologist with expertise in treating pseudomyxoma peritonei. Additional imaging, blood tests, or follow-up surgery may be recommended. Treatment decisions depend on the stage, your overall health, and the best way to control or remove remaining disease.

Questions to ask your doctor

• Was my tumor confined to the appendix, or did it spread outside?

• Did the mucin contain tumor cells?

• Were the surgical margins clear?

• What stage is my tumor?

• Do I need additional imaging or follow-up?

• Should I be referred to a specialist in pseudomyxoma peritonei?

• Would cytoreductive surgery or HIPEC be appropriate in my case?

• How often should I be monitored after treatment?