Pleomorphic lipoma

What is a pleomorphic lipoma?

A pleomorphic lipoma is a non-cancerous tumour made up of fat and spindle cells. Pleomorphic lipomas commonly grow around the upper back and neck but can start almost anywhere in the body. Most grow just under our skin where they feel like a soft lump. Another name for this tumour is spindle cell lipoma.

What is fat?

The human body is made up of many different types of tissue. Fat is a special type of tissue that is found throughout the body. Fat is made up of large cells called adipocytes that look clear when viewed through a microscope.

How do pathologists make this diagnosis?

The diagnosis may be made after a small sample of tissue is removed in a procedure called a biopsy. Surgery can then be performed to remove the entire tumour. The tumour is usually surrounded by a thin layer of tissue called a capsule, which keeps the tumour separated from the surrounding tissues. The capsule also makes these tumours relatively easy for your surgeon to remove.

When examined under the microscope, the tumour is made up of a combination of adipocytes (fat cells) and spindle cells. The adipocytes in a pleomorphic lipoma look almost identical to normal adipocytes found in fat throughout the body. The adipocytes are usually surrounded by long, thin spindle cells. Pathologists describe these spindle cells as ‘pleomorphic’ because they range in shape and size. Some tumours are made up almost entirely of spindle cells with very few adipocytes. The stroma or connective tissue holding the tumour together often looks very pink under the microscope.

Your pathologist may perform a special test called immunohistochemistry to confirm the diagnosis. When this test is performed, the adipocytes will be positive or reactive for a protein called S100 while the spindle cells will be positive or reactive for a protein called CD34.

spindle cell lipoma

Tumour size

This is the size of the tumour measured in centimetres. Tumour size will only be described in your report after the entire tumour has been removed. 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.


MDM2 is a gene that promotes cell division (the creation of new cells). Normal cells and those in pleomorphic lipomas have two copies of the MDM2 gene. In contrast, some cancers that look like pleomorphic lipomas have more than two copies of the MDM2 gene.

Two types of cancer that can look like pleomorphic lipoma but have more than two copies of the MDM2 gene are well-differentiated liposarcoma and dedifferentiated liposarcoma. An increased number of genes (more than two) is called amplification.

Your pathologist may order a test to see how many copies of MDM2 are present in the tumour. A test called fluorescence in situ hybridization (FISH) is commonly used to count the number of MDM2 genes in a cell. A normal copy number confirms that the tumour is a non-cancerous pleomorphic lipoma.

by Bibianna Purgina, MD FRCPC (updated September 16, 2021)
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