Intramuscular lipoma

by Bibianna Purgina MD FRCPC
June 6, 2023

What is an intramuscular lipoma?

Intramuscular lipomas are non-cancerous tumours made up of fat. These tumours develop deep inside a muscle and they may feel like a lump. Often, intramuscular lipomas are not well separated from the surrounding normal muscle which can make it challenging for your surgeon to remove the tumour completely.

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 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, intramuscular lipomas look nearly identical to normal fat.

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

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 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.

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. A normal copy number confirms that the tumour is a benign (non-cancerous) intramuscular lipoma.