High-Grade Appendiceal Mucinous Neoplasm (HAMN): Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC
August 28, 2026

A high-grade appendiceal mucinous neoplasm, usually shortened to HAMN, is a tumor that starts in the inner lining of the appendix. It is made of cells that produce mucin, a thick jelly-like substance. The words “high grade” describe how the cells look under the microscope, with enlarged, irregular nuclei and more cells caught in the act of dividing.

HAMN sits between two other appendix diagnoses. A low-grade appendiceal mucinous neoplasm (LAMN) grows the same way, but its cells look only mildly abnormal. A mucinous adenocarcinoma invades and destroys the surrounding tissue. HAMN has the growth pattern of the first and the cell appearance of the second.

HAMN is uncommon, and it was only recognized as a separate diagnosis in 2016. Because it is a newer category than the others, some questions about how it behaves are still being worked out.

Is a high-grade appendiceal mucinous neoplasm cancer?

Not exactly, and the answer sits closer to yes than it does for a LAMN. A HAMN does not invade and destroy tissue the way a cancer does. It advances with a broad, rounded, pushing front and spreads mainly by leaking mucin.

The concern with HAMN is what the cells look like rather than how the tumor grows. These abnormal cells carry a higher chance of eventually behaving like cancer, and a higher chance that any material reaching the abdomen will contain cells capable of growing. For that reason, HAMN is managed more cautiously than a LAMN, and staged using the system built for appendiceal cancers.

If the pathologist sees destructive invasion, with tumor glands burrowing into the wall and the surrounding tissue reacting with dense scarring, the diagnosis is no longer HAMN. It becomes mucinous adenocarcinoma.

What causes a high-grade appendiceal mucinous neoplasm?

The cause is not known. HAMN happens by chance rather than being inherited, and no environmental or lifestyle cause has been identified. It is usually diagnosed in middle age or later, and affects men and women at similar rates.

The genetic changes in HAMN are largely the same as those in LAMN. Nearly all carry a mutation in KRAS, and roughly half carry one in GNAS. That shared pattern is one reason researchers think the two begin the same way.

What separates HAMN at the genetic level is the addition of further changes. Mutations in TP53, a gene that normally halts damaged cells from dividing, are found in a substantial minority of HAMN and are essentially absent from LAMN. Acquiring a change like this is thought to be what pushes a tumor from low grade to high grade.

HAMN does not show mismatch repair deficiency, and it does not carry the BRAF changes seen in some colon cancers. None of this testing is routine after an appendectomy, and none of it currently changes treatment.

What are the symptoms of a high-grade appendiceal mucinous neoplasm?

Many people have no symptoms at all, and the tumor is found unexpectedly during surgery or on a scan done for another reason. When symptoms do occur they often resemble appendicitis, with pain in the lower right abdomen, nausea, vomiting, or fever.

If mucin has collected in the abdomen, the picture is different and develops more slowly. People may notice their abdomen swelling over months, a new hernia at the navel or groin, or a feeling of pressure and fullness. Some people can feel a mass. Scans often show an enlarged, fluid-filled appendix, sometimes with flecks of calcium in the wall.

How is the diagnosis made?

A HAMN is almost always diagnosed after the appendix has been removed, usually in an operation done for suspected appendicitis or for an abnormal scan. A pathologist makes the diagnosis by examining the tissue under a microscope.

Because the findings that matter can be confined to one small area, the entire appendix is normally examined rather than a sample of it. This is especially important for HAMN, since high-grade cells may be present in only part of a tumor that otherwise looks low grade.

Under the microscope, mucin-producing cells replace the normal lining. The defining feature of HAMN is how those cells look. The nuclei are enlarged, dark, and irregular; they lose their orderly arrangement along the base of the gland, and dividing cells are easy to find. Areas of dead cells, called necrosis, may also be present. Everything else about the tumor, including the pushing growth pattern and the stretched, scarred, sometimes calcified wall, is shared with LAMN.

The pathologist also confirms that the tumor is not invading destructively, which would make it an adenocarcinoma, and checks whether mucin has reached or passed through the wall.

Tumor extension into the wall of the appendix

Your report will describe how far the tumor, or the mucin it produces, has reached through the wall of the appendix. This determines the T category of the stage.

HAMN is staged using the system built for appendiceal adenocarcinoma, so the full range of T categories applies. This differs from LAMN, which uses a special category called Tis(LAMN) that does not apply here. If you have read about LAMN staging, expect your report to look different.

T1. The tumor reaches the submucosa, the supporting layer just beneath the lining.
T2. The tumor reaches the muscularis propria, the wall’s thick muscle layer.
T3. The tumor or mucin has passed through the muscle into the fatty tissue beneath the outer surface.
T4a. The tumor or mucin has reached the serosa, the appendix’s outer surface. This is the point at which material can escape into the abdomen.
T4b. The tumor has grown directly into a neighboring organ or structure.

Which staging system HAMN should use is genuinely debated. Current staging manuals apply the adenocarcinoma system because HAMN was assumed to carry a higher risk of recurrence than LAMN. Some studies since have found outcomes closer to LAMN, particularly when nothing has spread beyond the appendix, and have argued for the LAMN system instead. The question is unresolved, and it is reasonable to ask your doctor how much weight your T category should carry.

Mucin outside the appendix

When mucin escapes the appendix, the most important question is whether tumor cells came with it. Your report will address this directly.

Acellular mucin. Mucin is present outside the appendix, but no tumor cells are found within it. Without cells, the material generally cannot grow. This is staged M1a.
Mucin-containing tumor cells. Living tumor cells are present in the mucin outside the appendix and can settle on surfaces in the abdomen. This is staged M1b.

Reporting mucin as acellular requires examining a great deal of tissue, because cells can be sparse and flattened at the edge of a mucin pool. This is part of why these specimens take longer to process than a routine appendix.

The distinction matters more for HAMN than for LAMN. When cells are present in escaped mucin from a HAMN, they are more often high grade, and high-grade peritoneal disease behaves differently from low-grade disease. A separate category, M1c, is used for spread outside the abdominal cavity, which is uncommon.

Margins

The margin is the cut edge of the tissue removed at surgery. For an appendectomy, the margin that matters is the base of the appendix, where it was separated from the large intestine.

Negative margin. No tumor cells and no mucin reach the cut edge. The tumor appears to have been removed completely.
Positive margin. Tumor cells or mucin extend to the cut edge, so tissue may remain at the junction with the bowel.

A positive margin at the base is one of the few findings that may lead to further surgery. The remaining tissue sits on the wall of the large intestine, rather than inside a structure that has already been taken out.

Lymph nodes

Lymph nodes are small immune organs that filter fluid from the tissues, and many cancers spread through them. HAMN generally does not. Like LAMN, it spreads by leaking mucin onto surfaces in the abdomen rather than through lymphatic channels, so node involvement is rare.

Nodes are examined only when they are included in a larger operation. Most simple appendectomy reports will not mention them.

When tumor is found in a lymph node, it prompts the pathologist to re-examine the appendix closely. Node involvement suggests an invasive adenocarcinoma may be present somewhere that the first sections did not capture.

Pathologic stage (pTNM)

The stage combines the findings above into a single summary. The letter p means the stage is based on tissue examined under a microscope rather than on imaging.

Stage I. T1 or T2, with nothing outside the appendix.
Stage IIA. T3, with nothing outside the appendix.
Stage IIB. T4a, with nothing outside the appendix.
Stage IIC. T4b, with nothing outside the appendix.
Stage III. Tumor found in nearby lymph nodes. This is rare for HAMN.
Stage IVA. Acellular mucin in the abdomen, or mucin-containing low-grade tumor cells.
Stage IVB. Mucin in the abdomen containing higher-grade tumor cells.
Stage IVC. Spread beyond the abdominal lining.

Stage IV normally signals advanced cancer with a poor outlook, and that expectation does not transfer directly to this diagnosis. Stage IVA may mean jelly-like material in the abdomen with few or no living cells in it. The staging system places it at stage IV because the material is outside the organ, not because the outlook matches other stage IV disease.

Pseudomyxoma peritonei

Pseudomyxoma peritonei, usually shortened to PMP, is the name for mucin accumulating inside the abdominal cavity. It builds up slowly, over years rather than weeks, gathering under the diaphragm, in the pelvis, and across the omentum.

PMP is graded by how the cells within it look, and that grade matters more than the amount of mucin present. Deposits with no cells carry the best outlook, followed by deposits with low-grade cells. Deposits with high-grade cells behave more like a conventional cancer.

HAMN is more likely than LAMN to produce PMP, and more likely to produce the high-grade form when it does. That difference is the main practical reason clinicians separate the two diagnoses.

Treatment for PMP is concentrated in specialized centers. It usually involves cytoreductive surgery, an operation to remove all visible disease from the abdomen. Heated chemotherapy is often delivered directly into the abdomen during the same operation, a technique known as HIPEC. Whether this approach is appropriate depends on how much disease is present, where it sits, the grade of the cells, and your general health.

Prognosis

The outlook for HAMN is generally good, and better than the word “high grade” suggests to most people. What determines it is whether anything escaped the appendix, and if so, what the cells in that material look like.

Confined to the appendix. The outlook is excellent and close to that of a LAMN. Removing the appendix is usually the only treatment needed.
Acellular mucin outside the appendix. The outlook remains very good, with recurrence uncommon.
Low-grade cells outside the appendix. Reported five-year survival is roughly 70 to 80% after treatment at a specialist center.
High-grade cells outside the appendix. Reported five-year survival falls to around 55%, and the disease behaves more like a conventional cancer.

Across all people with a HAMN, reported five-year survival is around 90%, compared with roughly 93% for LAMN. The larger difference is in recurrence, with about 81% remaining free of disease at five years for HAMN versus about 90% for LAMN.

These figures come from small groups of patients, because HAMN is uncommon and was defined only recently. They cannot predict what will happen to any one person, and results at high-volume specialist centers are generally better than published numbers suggest.

What happens after this diagnosis?

What comes next depends mostly on what your report showed about spread beyond the appendix. When the tumor was confined to the appendix, and the margin was clear, removing the appendix is generally the whole treatment. A larger operation to remove part of the colon is not usually recommended, which surprises many people. That larger operation is standard for appendiceal adenocarcinoma, a different diagnosis.

Follow-up is usually closer than for a LAMN, given the higher chance of peritoneal disease. Most people are followed with periodic imaging and sometimes blood tests for tumor markers, since PMP can appear years after the original surgery. No single schedule is agreed upon, and practice varies between centers.

When mucin or tumor cells are found outside the appendix, referral to a surgical oncologist who specializes in peritoneal disease is usual. Your pathology report is one of several factors the team weighs, and what happens after your pathology report depends on the wider process.

Questions to ask your doctor

Was my tumor confined to the appendix, or did something reach the outer surface?
Was any mucin found outside the appendix?
If mucin was found outside, did it contain tumor cells, and what grade were they?
What was my T category, and what was my overall stage?
Was the margin at the base of the appendix clear?
Was my whole appendix examined under the microscope?
Does my report mean I need any further surgery?
How different is my outlook from a low-grade tumor of the appendix?
Does the fact that my stage is IV mean the same thing it would for other cancers?
Do I need follow-up imaging, and how often and for how long?
Should I be referred to a center that specializes in pseudomyxoma peritonei?
Would cytoreductive surgery or HIPEC be considered in my case?
What symptoms should prompt me to call you between visits?