This article was last reviewed and updated on July 14, 2018.
by Bibianna Purgina, MD FRCPC
Intramuscular lipoma is a non-cancerous tumour made up of fat.
The tumour starts and grows inside a muscle.
Some tumours look and feel like a lump under the skin.
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.
Intramuscular lipomas are non-cancerous (benign) tumours made up of fat.
These tumours develop deep inside a muscle and they may feel like a lump. Under the microscope, intramuscular lipomas look nearly identical to normal fat.
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
These tumours are measured in three dimensions but only the largest dimension is typically included in the report. For example, if the tumour measures 5.0 cm by 3.2 cm by 1.1 cm, the report may describe the tumour size as 5.0 cm in greatest dimension.
Tumour size is not reported in a biopsy specimen.
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 (malignant tumours) 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. A normal copy number confirms that the tumour is a non-cancerous (benign) intramuscular lipoma.