An atypical lipomatous tumour (ALT) is a type of cancer made up of fat. It is part of a group of cancers called sarcomas. Atypical lipomatous tumours can start anywhere in the body but the most common location for this tumour is the abdomen. Another name for this tumour is well-differentiated liposarcoma.
The human body is made up of many different types of tissue. Fat is a specialized type of tissue that is found throughout the body. It is made up of large cells called adipocytes that look clear when viewed through a microscope. Fat acts as a storage site for energy and provides warmth for our internal organs. The medical term for fat is adipose tissue.
Some atypical lipomatous tumours will change over time so that some of the cells no longer look like normal fat. This process is called dedifferentiation and these cancers are then called dedifferentiated liposarcoma. When compared to an atypical lipomatous tumour, dedifferentiated liposarcoma is associated with a worse prognosis because it is more likely to grow back after surgery and spread to other parts of the body. The movement of cancer cells to another part of the body is called metastasis.
The diagnosis of an atypical lipomatous tumour is usually after a small sample of the tumour is removed in a procedure called a biopsy. The biopsy tissue is then sent to a pathologist who examines it under a microscope. The diagnosis can also be made after the entire tumour is removed as an excision or resection specimen.
When examined under the microscope, an atypical lipomatous tumour can look like normal fat. However, unlike normal fat, atypical lipomatous tumours contain abnormal-looking cells fat cells, known as lipoblasts. Additional tests such as fluorescence in situ hybridization (FISH) may also be performed to confirm the diagnosis (see MDM2 section below).
Pathologists use the word grade to describe how different the cancer cells in an atypical lipomatous tumour look and behave compared to normal fat cells. The grade can only be determined after a sample of the tumour has been examined under the microscope.
Atypical lipomatous tumour is given a grade based on an internationally recognized system created by the French Federation of Cancer Centers Sarcoma Group (FNCLCC). Your pathologist will determine the French Federation of Cancer Centers Sarcoma Group grade of the tumour by looking for three microscopic features:
Your pathologist will give each feature a certain number of points (from 0 to 3) and the total number of points determines the final grade of the tumour. According to this system, atypical lipomatous tumours may be either low or high-grade tumours. High-grade tumours (grades 2 and 3) are associated with a worse prognosis.
Each cell in your body contains a set of instructions that tell the cell how to behave. These instructions are written in a language called DNA and the instructions are stored on 46 chromosomes in each cell. Because the instructions are very long, they are broken up into sections called genes and each gene tells the cell how to produce a piece of the machine called a protein.
MDM2 is a gene that promotes cell division (the creation of new cells). Normal cells and those in non-cancerous tumours have two copies of the MDM2 gene. In contrast, atypical lipomatous tumours have more than two copies of the MDM2 gene.
A test called fluorescence in situ hybridization (FISH) is commonly used to count the number of MDM2 genes in a cell. An increased number of genes (more than two) is called amplification and supports the diagnosis of atypical lipomatous tumour.
The tumour is measured 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 the greatest dimension. Tumour size is important because tumours less than 5 cm are less likely to spread to other parts of the body and are associated with a better prognosis. Tumour size is also used to determine the pathologic tumour stage (see Pathologic stage below).
Nerves are like long wires made up of groups of cells called neurons. Nerves send information (such as temperature, pressure, and pain) between your brain and your body. Perineural invasion means that cancer cells were seen attached to a nerve.
Cancer cells that have attached to a nerve can use the nerve to travel into tissue outside of the original tumour. This increases the risk that the tumour will come back in the same area of the body (recurrence) after treatment. It is very rare for the tumour cells in an atypical lipomatous tumour to show perineural invasion.
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. Cancer cells can use blood vessels and lymphatics to travel away from the tumour to other parts of the body. The movement of cancer cells from the tumour to another part of the body is called metastasis.
Before cancer cells can metastasize, they need to enter a blood vessel or lymphatic. This is called lymphovascular invasion. Lymphovascular invasion is important because it increases the risk that cancer cells will be found in a lymph node or a distant part of the body such as the lungs. It is very rare for the tumour cells in an atypical lipomatous tumour to show lymphovascular invasion.
A margin is any tissue that was cut by the surgeon to remove the tumour from your body. Depending on the type of surgery you have had, the margins can include bones, muscles, blood vessels, and nerves that were cut to remove the tumour from your body. Margins will only be described in your report after the entire tumour has been removed.
A negative margin means that no tumour cells were seen at any of the cut edges of tissue. A margin is called positive when there are tumour cells at the very 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.
Lymph nodes are small immune organs located throughout the body. Cancer 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 cancer cells from the tumour to a lymph node is called metastasis.
Many cancers can spread to the lymph nodes, but the atypical lipomatous tumour does this very rarely. If lymph nodes were part of the surgery to remove your tumour, your pathologist will assess them under the microscope and report whether they are involved by tumour.
The pathologic stage for atypical lipomatous tumour 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 a worse prognosis.
The tumour stage for atypical lipomatous tumour varies based on the body part involved. For example, a 5-centimetre tumour that starts in the neck will be given a different tumour stage than a tumour that starts deep in the back of the abdomen (the retroperitoneum). However, in most body sites, the tumour stage includes the tumour size and whether the tumour has grown into surrounding body parts.
Nodal stage (pN) for atypical lipomatous tumour
Atypical lipomatous tumour is given a nodal stage of 0 or 1 based on the presence of cancer cells in a lymph node. If no cancer cells are seen in any of the lymph nodes examined, the nodal stage is N0. If cancer cells are seen in any of the lymph nodes examined, the nodal stage becomes N1.
Atypical lipomatous tumour is given a metastatic stage of 0 or 1 based on the presence of cancer cells at a distant site in the body (for example the lungs). The metastatic stage can only be assigned if tissue from a distant site is submitted for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined and is listed as MX.