Low grade appendiceal mucinous neoplasm (LAMN)
This article was last reviewed and updated on March 22, 2019
by Stephanie Reid, MD FRCPC
Low grade appendiceal mucinous neoplasm (LAMN) is a tumour that develops only in the appendix.
It starts from the cells that line the inside of the appendix and produces a substance called mucin.
LAMN is different from other tumours in the gastrointestinal tract because of its unique behaviour and prognosis.
The normal appendix
The appendix is a small finger-shaped organ that connects with your large bowel (colon) by a thin opening. The appendix and the large bowel have similar structure. The appendix is lined by the same cells that line the large bowel. These cells produce mucin which helps food move smoothly through your large bowel. The appendix also contains cells that are a part of your immune system and these cells help to form a protective barrier between your body and the contents of the bowel.
What is low-grade appendiceal mucinous neoplasm?
Low-grade appendiceal mucinous neoplasm (LAMN) is a tumour that starts from the mucin producing cells in the appendix. These tumours often produce swelling or enlargement of the appendix as the abnormal mucin producing cells cause the appendix to become filled the mucin. In some cases, the mucin can sometimes be seen leaking into the large bowel from the appendix when a medical procedure called a colonoscopy is performed. The mucin and the abnormal mucin producing cells can also spread outside of the appendix and into nearby organs or the abdominal cavity.
For some patients, a LAMN is found when for the appendix is removed for appendicitis. In other situations, the tumour is discovered incidentally when the patient undergoes an imaging study (CT scan, MRI, or ultrasound) of the abdomen for another reason.
Some LAMN are only discovered after the mucin and tumour cells have spread outside of the appendix into the intra-abdominal space or onto nearby organs. In these situations, the tumour may appear to be coming from another organ such as the ovary and the correct diagnosis may not be made until the appendix is removed and sent for examination by a pathologist.
The diagnosis of LAMN is very difficult to make when only a small sample of tissue (a biopsy) is sent for microscopic examination. However, your pathologist may suggest this diagnosis is mucin or mucin producing cells are seen in the biopsy. The final diagnosis can be made when the appendix is removed and examined.
After the tumour is completely removed, your pathologist will measure it in three dimensions but only the largest dimension is typically included in your report. For example, if the tumour measures 5.0 cm by 3.2 cm by 1.1 cm, the report may describe the tumour size as 5.0 cm in greatest dimension. In situations where the tumour has spread outside of the appendix, it may not be possible for your pathologist to determine the size of the tumour.
The wall of the appendix is made up of four layers of tissue: mucosa, submucosa, muscularis propria, and serosa. All LAMN start in the mucosa which lines the inside of the appendix. Tumour extension describes how far the tumor cells or the mucin they produce have traveled from the mucosa into the other layers of tissue. The movement of tumour cells from the mucosa into other types of tissue is called invasion.
The muscularis propria is a thick muscle that is found in the middle of the wall of the appendix. In order to make the diagnosis of LAMN, your pathologist must see destruction of the normal mucosa and submucosa by tumour cells or the mucin they produce must be touching the muscularis propria. Tumour cells or mucin may also be seen inside the muscularis propria.
The serosa is a thin layer of tissue on the outer surface of the appendix. In between the muscularis propria and the serosa is a layer of fat or tissue called the subserosa or mesoappendix. Once tumour cells or the mucin they produce have broken through the serosa the tumour is in the abdominal cavity and can spread to nearby organs.
Why is this important? Tumour extension is important because tumour cells that invade deeper in the wall are more likely to come back again at the same location (local recurrence), travel to a distant body site (metastasize), or spread throughout the abdominal cavity.
LAMN is different from other tumours in the large bowel in that they are not considered at risk of local recurrence, intraabdominal spread, or metastasis unless they have broken through the serosal surface and involve the intraabdominal space.
When examining a low grade appendiceal mucinous neoplasm, it is important for your pathologist to look for mucin outside of the appendix. If mucin is outside of the appendix, your pathologist will look to see if the mucin contains tumour cells (cellular mucin) or does not contain tumour cells (acellular mucin).
Why is this important? This is important because when there is cellular mucin, the risk of the tumour coming back or spreading to other body sites is higher than mucin that does not contain tumour cells.
Grade is a word that pathologists use when describing how different a tumour looks compared to the normal tissue from the same location. In the appendix, tumours that produce mucin are separated into three different grades.
Grade 1 (G1) – These tumours look very similar to the normal tissue that lines the inside of the appendix. The cells in the tumour produce lots of mucin. All LAMN are considered a grade 1 tumour.
Grade 2 (G2) – The cells in a grade 2 tumour look different than the normal cells that line the inside of the appendix, but they still produce mucin.
Grade 3 (G3) – The cells in a grade 3 tumour look very different from the normal cells that line the inside of the appendix. The tumour will often contain a type of cell that pathologist call signet-ring cells.
A margin is any tissue which has to be cut by a surgeon so that a tumour can be removed from your body. The appendix is connected to the large bowel, and generally has two areas which must be cut to remove it from the body. The end that attaches to the large bowel and is in direct communication with it is the proximal margin. The appendix has an area of fat that contains blood vessels and occasionally lymph nodes. This area must be cut to free the appendix and it is called the mesoappendix margin.
A positive margin means that your pathologist saw tumour cells or mucin at the edge of the cut tissue. A positive margin increases the risk of the tumour coming back in the same site (local recurrence) after surgery and treatment.
Lymphatics and blood vessels are long tubes that allow fluid (lymph and blood, respectively) and cells to travel around the body. When tumour cells enter a lymphatic or blood vessel it is called lymphovascular invasion and is associated with a higher risk that tumour cells will travel (metastasize) to a lymph node or a distant site such as the lungs. It is extremely uncommon for tumour cells from a LAMN to show lymphovascular invasion.
Metastatic disease describes the process where tumour cells escape the main tumour and travel to another part of the body. Lymph nodes are small immune organs located throughout the body.
The presence of tumour cells in a lymph node (also called lymph node metastases) is associated with a higher risk that the tumour cells will be found in other lymph nodes or in a distant organ such as the lungs. For this reason, lymph nodes in the area of the tumour are often removed and sent for pathological examination.
If lymph nodes were removed as part of your surgery, then your report will include the total number of lymph nodes examined and the number that contain tumour cells.
Tumour cells from a LAMN rarely travel to a lymph node.
The pathologic stage for low grade appendiceal mucinous neoplasm (LAMN) is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer.
This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M) to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means more advanced disease and worse prognosis.
Pathologic stage is usually not described in your report after a biopsy has been performed. The pathologic stage will be described when the entire tumour has been removed in an excision or resection specimen and sent for pathologic examination.
Tumour stage (pT) for low grade appendiceal mucinous neoplasm
LAMN is different from other tumours in the gastrointestinal tract because it does not have a T1 or T2 stage. The tumour stage (pT) for LAMN includes Tis, T3, T4a, and T4b.
Tis (LAMN) - Tumour cells or the mucin they produce were only seen touching or invading the muscularis propria.
T3 - Tumour cells or the mucin they produce were seen extending through the muscularis propria into the fat below the serosa (subserosa or mesoappendix).
T4 – Tumour cells or the mucin they produce have broken through the serosa and are in the abdominal cavity. This category is further divided into T4a and T4b. In T4a, the tumour cells have broken through the serosa but are not seen on nearby organs. in T4b tumour cells are found on nearby organs.
Nodal stage (pN) for low grade appendiceal mucinous neoplasm
LAMN is given a nodal stage (pN) between 0 and 2. This is based on the number of lymph nodes which contain tumour cells. If there are no lymph nodes in the surgical specimen, an nodal stage cannot be determined, and it is listed as pNX.
Lymph node involvement is extremely rare in low grade appendiceal mucinous neoplasms.
Metastasis stage (pM) for low grade appendiceal mucinous neoplasm
If tumour cells from a LAMN have spread throughout the abdominal space or into other organs away from the appendix it is considered to be metastatic and is given an metastatic stage of M1. This stage is then further divided into stage M1a, M1b, and M1c.
M1a – This stage is given when there is mucin without tumour cells (acellular mucin) found in the abdominal cavity.
M1b – This stage is given when there are tumor cells found in the abdominal cavity or on nearby organs.
M1c – This stage is given if tumour cells are found outside the abdominal cavity.
The metastatic stage can only be determined if other organs, tissues, or mucin that was within the abdominal cavity are submitted to your pathologist for examination.