Lipoma



What is a lipoma?

A lipoma is a non-cancerous tumour made up of fat cells. Since we have fat everywhere in our bodies, lipomas can occur just about anywhere. But most lipomas occur just under our skin where they feel like a soft lump.

Fat

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 serves as an energy reserve for the body and keeps our internal organs warm. Fat is made up of large cells called adipocytes that look clear when viewed through a microscope.

How do pathologists make this diagnosis?

The first diagnosis of a lipoma 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. Under the microscope, lipomas look nearly identical to normal fat.

Most lipomas are 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.

Types of lipomas
  • Intramuscular lipoma – An intramuscular lipoma is a type of lipoma that develops inside a muscle. Often, intramuscular lipomas do not have a capsule and can be more challenging for your surgeon to remove.
  • AngiolipomaAngiolipomas have lots of small blood vessels and some of these vessels contain small blood clots.  Symptoms of an angiolipoma include pain in the area of the tumour.
  • Spindle cell/pleomorphic lipoma – These two types of lipomas commonly occur around the upper back and neck but can occur anywhere. Both spindle cell lipoma and pleomorphic lipoma are non-cancerous.

What to look for in your report after the tumour has been removed

Tumour size

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. Not all reports for lipoma will include the tumour size while others will only include the size in the gross description section of the report.

MDM2

MDM2 is a gene that allows a cell to divide in order to create new cells. Normal cells and those in lipomas have two copies of the MDM2 gene. In contrast, some cancers that look like lipomas have more than two copies of the MDM2 gene.

Two types of cancer that can look like a 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. A normal copy number confirms that the tumour is a non-cancerous lipoma.

Your pathologist may order a test to see how many copies of MDM2 are present in your tumour. A test called fluorescence in situ hybridization (FISH) is commonly used to count the number of MDM2 genes in a cell. This test is typically only ordered for tumours that are greater than 10 cm in size, found in deep soft tissue (such as inside a muscle or in the abdomen), or where the fat cells start to look abnormal under the microscope. Pathologists use the term atypical or atypia to describe abnormal-looking cells.

by Bibianna Purgina, MD FRCPC (updated August 19, 2021)
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