This article was last reviewed and updated on July 14, 2018.
by Bibianna Purgina, MD FRCPC
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.
What is a lipoma?
Lipomas are non-cancerous (benign) tumours made up of fat. 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.
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
Some lipomas can occur in deeper locations, such as inside a muscle. These lipomas arising inside muscle are called intramuscular lipomas. Often, intramuscular lipomas do not have a capsule and can more challenging for your surgeon to remove.
Under the microscope, lipomas look nearly identical to normal fat. Some types of lipomas have lots of small blood vessels and some of these vessels contain small blood clots. This type of lipoma is called an angiolipoma and before it was removed, may have caused you some discomfort.
Other less common types of lipomas include spindle cell lipoma and pleomorphic lipoma. These two types of lipomas most commonly occur around the upper back and neck but can occur anywhere. Both spindle cell lipoma and pleomorphic lipoma are non-cancerous. Some pathologists may describe non-cancerous (benign) lipomas with increased amounts of connective tissue as fibrolipomas.
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.
This is the size of the tumour measured in centimeters. 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 lipomas have two copies of the MDM2 gene. In contrast, some canc 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.
Your pathologist may order a test to see how many copies of MDM2 are present in your tumour. A test called florescence in situ hybridization (FISH) is commonly used to count the number of MDM2 genes in a cell.
Why is this important? A normal copy number confirms that the tumour is a non-cancerous (benign) lipoma.