Your pathology report for mucinous adenocarcinoma of the appendix

By Jason Wasserman MD PhD FRCPC
November 24, 2025

Mucinous adenocarcinoma of the appendix is a type of cancer that starts in the gland-forming cells that line the appendix. These cancer cells produce large amounts of mucin, a thick, jelly-like material. Unlike low-grade mucinous neoplasms (LAMNs) and high-grade mucinous neoplasms (HAMNs), mucinous adenocarcinoma shows destructive invasion, meaning the tumor grows aggressively into the wall of the appendix and surrounding tissues. This cancer can spread to lymph nodes, nearby organs, or throughout the abdominal cavity.

Because the appendix is narrow and has a thin wall, the tumor may grow silently at first. Once the wall is weakened or ruptures, mucin and cancer cells may spill into the abdominal cavity, leading to a condition called pseudomyxoma peritonei, in which mucin builds up in the abdomen.

Anatomy of the appendix

The appendix is a small pouch attached to the beginning of the large intestine. It contains mucin-producing epithelial cells. Because the appendix is long and narrow, tumors that produce large amounts of mucin can stretch or rupture the wall, allowing mucin and sometimes cancer cells to move into the abdominal cavity.

What are the symptoms of mucinous adenocarcinoma?

Some people develop symptoms similar to appendicitis, including abdominal pain, fever, nausea, or vomiting. Others may experience abdominal bloating, changes in bowel habits, or unexplained weight loss. If mucin spreads beyond the appendix, the abdomen may become enlarged or uncomfortable, or a new hernia may appear. In some cases, the tumor is discovered incidentally during imaging or surgery performed for unrelated reasons.

Who gets mucinous adenocarcinoma?

Mucinous adenocarcinoma can occur in adults at any age but is most commonly seen in people in their 50s and 60s. Men and women are affected equally. There is no clear environmental or lifestyle cause.

What causes mucinous adenocarcinoma?

The cause is not fully understood. Many tumors harbor mutations in growth-regulating genes, such as KRAS, while high-grade tumors may exhibit additional genetic changes characteristic of more aggressive cancers. Unlike LAMN, mucinous adenocarcinoma is less commonly associated with GNAS mutations. These tumors generally do not show features such as mismatch repair deficiency, microsatellite instability, or BRAF mutations.

How is this diagnosis made?

Mucinous adenocarcinoma is usually diagnosed after the appendix is removed during surgery for abdominal pain, suspected appendicitis, or an unexpected finding on imaging. The diagnosis is made by a pathologist, who examines the entire appendix under a microscope.

In mucinous adenocarcinoma, the tumor shows irregular, infiltrative glands that grow destructively through the wall of the appendix. The cancer often produces large pools of mucin, within which are clusters or strips of malignant cells. These invasive glands and mucin deposits replace the normal tissue layers of the appendix. The pathologist may also see complex, crowded glandular structures (called cribriform glands), individual cancer cells spreading into surrounding tissue, and a reaction of scar-like tissue (desmoplasia) around the tumor.

The wall of the appendix may be severely distorted or replaced by mucin and cancer. In some cases, the tumor penetrates the outer surface (serosa) and ruptures, allowing mucin and cancer cells to spill into the abdominal cavity. When this occurs, tumor cells may implant on the surfaces of other abdominal organs, leading to pseudomyxoma peritonei.

A key part of the diagnosis is distinguishing mucinous adenocarcinoma from LAMN and HAMN. LAMN and HAMN may show mucin dissecting through the wall and pushing growth, but they lack the destructive, infiltrative pattern that defines adenocarcinoma. The pathologist also considers other conditions, such as diverticular disease or serrated polyps, which may alter the appendix but do not show this invasive pattern.

Tumor extension (depth of invasion)

Tumor extension describes how deeply the cancer has grown into the wall of the appendix and whether it has spread beyond the appendix into the abdominal cavity or nearby organs. This is a central part of staging and prognosis.

Mucinous adenocarcinoma begins in the inner lining (mucosa) but soon invades the submucosa, a layer containing blood vessels and connective tissue. From there, the tumor commonly grows into the muscularis propria, the muscle layer that helps move material through the appendix. As the cancer progresses, it may reach the subserosa, which lies just beneath the outer covering, and then the serosa, the thin outermost layer of the appendix.

When the tumor penetrates the serosa or when the appendix ruptures, mucin and tumor cells can escape into the abdominal cavity. This spread can be limited to the area around the appendix or can involve many surfaces within the abdomen and pelvis. In advanced cases, the tumor may grow directly into nearby organs such as the cecum, small intestine, or even the abdominal wall.

Extension beyond the appendix into the peritoneal cavity is particularly significant. Mucin-containing tumor cells can coat the surfaces of abdominal organs without forming a single large mass, producing the characteristic picture of pseudomyxoma peritonei. This pattern of spread can cause progressive abdominal distention and can be challenging to treat, which is why careful assessment of tumor extension is so important.

Lymphovascular invasion

Lymphovascular invasion (LVI) occurs when cancer cells are found within or near blood or lymphatic vessels. These vessels act as pathways for cancer cells to travel to other parts of the body, especially lymph nodes or the liver. When the pathologist sees tumor cells in these vessels, it indicates a higher risk that the cancer has already spread or may spread in the future. Lymphovascular invasion is therefore an important adverse prognostic feature and is included in the pathology report.

Perineural invasion

Perineural invasion (PNI) means that cancer cells are found growing around nerves in the tissue surrounding the tumor. Nerves run through the tissues of the appendix and nearby organs like small cables. When tumor cells follow or surround these nerves, they can move deeper into tissues, making them more difficult to remove completely. The presence of perineural invasion is associated with a greater risk of local recurrence and usually signals a more aggressive tumor.

Margins

A margin is the cut edge of the appendix after it is removed. The pathologist examines the margin to determine whether cancer cells are present at the edge. A negative margin means the tumor does not extend to the cut surface, suggesting complete removal. A positive margin means cancer cells reach the cut edge, which may increase the risk of recurrence and may influence the need for additional treatment.

Lymph nodes

Lymph nodes are small, bean-shaped immune organs located throughout the body, including near the appendix. They filter lymph fluid and help fight infection. Many cancers, including mucinous adenocarcinoma, can spread through lymphatic vessels to nearby lymph nodes.

If lymph nodes are removed during surgery, the pathologist examines each one under a microscope for cancer cells. If tumor is found in any lymph node, that node is considered positive. The number of positive lymph nodes relative to the total number examined helps determine the pathologic N stage. Tumors that have spread to lymph nodes are considered more advanced and usually require closer follow-up and often additional treatment.

Pathologic stage (pTNM)

The pathologic stage, often summarized as pTNM, combines three key pieces of information:

• T (tumor): How deeply the tumor has grown through the appendix wall and whether it has reached or breached the serosa or invaded nearby organs.

• N (nodes): Whether cancer has spread to regional lymph nodes and, if so, how many are involved.

• M (metastasis): Whether cancer has spread to distant sites, including the peritoneal cavity, liver, or other organs.

Tumors that are confined to the appendix wall and have not spread to lymph nodes or distant sites have a lower stage and a more favorable prognosis. Tumors that invade through the serosa, spread to lymph nodes, or disseminate within the abdominal cavity are assigned a higher stage and have a more guarded outlook.

Prognosis

The prognosis for mucinous adenocarcinoma of the appendix depends on several important factors. Tumors confined to the appendix and detected early have a more favorable prognosis, especially when completely removed with negative margins. However, cancers that invade deeply into the wall, spread to lymph nodes, or disseminate into the abdominal cavity behave more aggressively.

When mucin-containing cancer cells spread throughout the peritoneum, the disease may progress slowly but can be challenging to control. The extent of disease within the abdomen, the grade of tumor cells present in the peritoneum, and the ability to remove all visible tumor are key predictors of long-term outcome. Many patients with peritoneal involvement benefit from cytoreductive surgery combined with hyperthermic intraperitoneal chemotherapy (HIPEC), which has improved survival in selected cases.

After the diagnosis

After your diagnosis is confirmed, your healthcare team will review your pathology report and any imaging results to determine how far the tumor has spread. If the tumor appears confined to the appendix and has been completely removed, further treatment may not be necessary, but careful follow-up is still important. If there is evidence of spread to lymph nodes or the peritoneal cavity, you may be referred to a surgical oncologist who specializes in treating appendiceal and peritoneal surface tumors.

Treatment options may include additional surgery, cytoreductive surgery with HIPEC, systemic chemotherapy, or a combination of these approaches. Your doctors will design a personalized plan based on your overall health, the stage of the disease, and your treatment goals.

Questions to ask your doctor

• How far had the tumor spread?

• Were tumor cells found in the peritoneum?

• Were any lymph nodes involved?

• Were the surgical margins clear?

• What stage is my cancer?

• Do I need additional treatment such as chemotherapy, HIPEC, or more surgery?

• How often should I be monitored?

• Should I see a specialist in appendiceal or peritoneal surface tumors?