This article will help you read and understand your pathology report for low grade appendiceal mucinous neoplasm (LAMN).
by Stephanie Reid, MD FRCPC, updated December 29, 2020
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 a 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.
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.
The diagnosis of LAMN is usually only made after the entire tumour has been removed and sent to a pathologist for examination under the microscope. For some patients, a LAMN is found when 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.
This is the size of the tumour measured in centimetres. The tumour is usually measured in three dimensions but only the largest dimension is described in your report. For example, if the tumour measures 4.0 cm by 2.0 cm by 1.5 cm, your report will describe the tumour as being 4.0 cm.
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.
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, 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).
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.
By definition, all LAMN are considered a grade 1 tumour.
A margin is any tissue that 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 that 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 negative margin means that no tumour cells or mucin were seen at the cut edge of the tissue. A positive margin means that your pathologist saw tumour cells or mucin at the cut edge of the tissue. A positive margin increases the risk of the tumour coming back in the same site (local recurrence) after surgery and treatment.
Blood moves around the body through long thin tubes called blood vessels. Another type of fluid called lymph which contains waste and immune cells moves around the body through lymphatic channels.
Tumour cells can use blood vessels and lymphatics to travel away from the tumour to other parts of the body. The movement of tumour cells from the tumour to another part of the body is called metastasis.
Before tumour cells can metastasize, they need to enter a blood vessel or lymphatic. This is called lymphovascular invasion. Lymphovascular invasion increases the risk that tumour cells will be found in a lymph node or a distant part of the body such as the lungs. However, it is extremely uncommon for tumour cells from a LAMN to show lymphovascular invasion.
Lymph nodes are small immune organs located throughout the body. Tumour cells can travel from the tumour to a lymph node through lymphatic channels located in and around the tumour (see Lymphovascular invasion above). The movement of tumour cells from the tumour to a lymph node is called metastasis.
Your pathologist will carefully examine each lymph node for cancer cells. Lymph nodes that contain tumour cells are often called positive while those that do not contain any tumour cells are called negative. Most reports include the total number of lymph nodes examined and the number, if any, that contain tumour cells.
Tumour cells found in a lymph node increases the risk that tumour cells will spread to other parts of the body. However, 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.
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.
LAMN is given a nodal stage (pN) between 0 and 2. This is based on the number of lymph nodes that 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.
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 a metastatic stage of M1. This stage is then further divided into stage M1a, M1b, and M1c.
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.