by Bibianna Purgina, MD FRCPC
August 18, 2022
Well-differentiated liposarcoma is a type of cancer made up of adipose tissue (fat). It is a slow-growing tumour that can start almost anywhere in the body. Another name for this tumour is an atypical lipomatous tumour.
Well-differentiated liposarcoma and atypical lipomatous tumour (ALT) are two names given to the same tumour. Doctors use the term ALT when describing tumours that start in a superficial part of the body such as the arm or leg and where the tumour can be fully removed by surgery. The term well-differentiated liposarcoma is used for tumours starting in deeper locations such as the back of the abdomen and for those that cannot be fully removed by surgery alone.
The diagnosis of a well-differentiated liposarcoma 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, well-differentiated liposarcoma can look like normal fat. However, unlike normal fat, well-differentiated liposarcoma contains abnormal-looking cells fat cells, known as lipoblasts.
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, well-differentiated liposarcomas 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 well-differentiated liposarcoma.
Pathologists divide well-differentiated liposarcoma into three grades based on a system created by the French Federation of Cancer Centers Sarcoma Group (FNCLCC). This system uses three microscopic features to determine the tumour grade: differentiation, mitotic count, and necrosis. These features are explained in more detail below. The grade can only be determined after a sample of the tumour has been examined under the microscope.
Points (from 0 to 3) are assigned for each of the microscopic features (0 to 3) and the total number of points determines the final grade of the tumour. According to this system, well-differentiated liposarcomas may be either low or high-grade tumours. The tumour grade is important because high-grade tumours (grades 2 and 3) are more aggressive and are associated with a worse prognosis.
Points associated with each grade:
Microscopic features used to determine the grade:
Some well-differentiated liposarcomas will change over time so that some of the cells no longer look like normal fat. This process is called dedifferentiation. Finding dedifferentiation in a well-differentiated liposarcoma changes the diagnosis to dedifferentiated liposarcoma. This change is important because dedifferentiated liposarcoma is a more aggressive cancer that is more likely to grow back after surgery and spread to other parts of the body.
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 (pT).
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.
The pathologic stage for well-differentiated liposarcoma 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 a more advanced disease and a worse prognosis.
The tumour stage for well-differentiated liposarcoma 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.
Well-differentiated liposarcoma is given a nodal stage of 0 or 1 based on the presence of tumour cells in a lymph node. If no tumour cells are seen in any of the lymph nodes examined, the nodal stage is N0. If tumour cells are seen in any of the lymph nodes examined, the nodal stage becomes N1.
Well-differentiated liposarcoma is given a metastatic stage of 0 or 1 based on the presence of tumour 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.