by Bibianna Purgina, MD FRCPC
March 9, 2023
Deep fibromatosis or desmoid tumour is a non-cancerous type of tumour that starts in connective tissue. It is considered locally aggressive because it can grow into surrounding tissues and organs. The tumour can also grow back if not completely removed. However, it will not metastasize (spread) to other parts of the body.
Deep fibromatosis develops in a deep location such as inside the abdomen, within a muscle, or around an organ. In contrast, superficial fibromatosis tends to develop just under the skin.
No, deep fibromatosis is not a type of cancer. The tumour, however, can grow into surrounding normal tissues and organs and can grow back if not fully removed.
No. The tumour cells in deep fibromatosis will not metastasize (spread) to other parts of the body. However, the tumour can regrow in the same location if not completely removed.
Other names for deep fibromatosis are desmoid tumour, aggressive fibromatosis, abdominal fibromatosis, extra-abdominal fibromatosis, and intra-abdominal fibromatosis. The name used depends on where in the body the tumour was located.
Deep fibromatosis can occur almost anywhere in the body. However, the most common locations include the extremities (arms and legs), retroperitoneum (the space at the back of the abdomen), abdominal cavity, and chest wall.
Deep fibromatosis is associated with both genetic conditions and physical factors including prior trauma, pregnancy, and surgery. People with Familial Adenomatous Polyposis (APC) syndrome, Gardener syndrome, and familial desmoid syndrome have a much higher risk of developing deep fibromatosis.
Some types of deep fibromatosis are given a special name based on the location in the body where the tumour develops. Types of deep fibromatosis include:
The diagnosis of deep fibromatosis is usually made after a small piece of the tumour is removed in a procedure called a biopsy. The tissue is then sent to a pathologist who examines it under a microscope. Sometimes additional tests such as immunohistochemistry or molecular testing may be performed to confirm the diagnosis.
When viewed under the microscope, the tumour is made up of long thin spindle cells that look like the cells found in normal connective tissue. Most of these spindle cells are specialized fibroblasts and myofibroblasts and they form a mass that grows into the surrounding normal tissues.
Because deep fibromatosis can look like other tumours that develop from connective tissue, it can be difficult for your pathologist to make a definite diagnosis of deep fibromatosis with only the small amount of tissue provided with a biopsy. However, your pathologist may suggest this diagnosis as a possibility to your clinician in the pathology report.
Immunohistochemistry is a test that allows pathologists to see different types of proteins made by the tumour cells. When this test is performed, the tumour cells in deep fibromatosis are often described as positive or reactive for the proteins smooth muscle actin and desmin. In addition, the cells in deep fibromatosis often show abnormal expression of a protein called beta-catenin. This protein is normally found in a part of the cell called the membrane. In deep fibromatosis, the beta-catenin protein does not move normally to the membrane of the cell. Instead, the beta-catenin protein builds up in a part of the cell called the nucleus. Pathologists often describe this as nuclear expression. If the beta-catenin protein is found mostly in the nucleus of the cell, this is considered abnormal and may be associated with a mutation in the genes for either APC or CTNNB1.
Some people inherit particular genes that put them at a much higher risk of developing deep fibromatosis. These people are said to have a syndrome and the most common syndromes associated with deep fibromatosis are Familial Adenomatosis Polyposis Syndrome/Gardner Syndrome and familial desmoid syndrome.
Deep fibromatosis in patients that have Familial Adenomatosis Polyposis Syndrome/Gardner Syndrome is caused by inherited mutations in the APC gene. Most deep fibromatoses that develop in patients without a genetic syndrome have mutations in the CTNNB1 gene (also known as the beta-catenin gene).
Pathologists can test for these genetic changes by performing next-generation sequencing (NGS) on a piece of the tissue from the tumour. This type of testing is can be done on the biopsy specimen or when your tumour has been surgically removed.
Deep fibromatosis is usually a poorly defined tumour that grows into or around neighbouring muscles, bone and blood vessels. Your pathologist will examine samples of the surrounding tissues under the microscope to look for tumour cells. Any surrounding organs or tissues that contain tumour cells will be described in your report.
The border between deep fibromatosis and the surrounding normal tissue is often not easy to see. For this reason, most surgeons will remove the tumour with some normal-looking tissue in order to make sure the entire tumour is removed. The normal tissue removed with the tumour is called a margin.
All margins will be very closely examined under the microscope by your pathologist to determine the margin status. A margin is considered negative when there are no tumour cells at the edge of the cut tissue. A margin is considered positive when there are tumour cells at the edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will recur in the same site after treatment (local recurrence).