by Jason Wasserman MD PhD FRCPC
February 12, 2024

Myelolipoma is a non-cancerous tumour typically found in or near the adrenal gland. It is the second most common type of adrenal gland tumour. Most myelolipomas are incidental, which means they are only discovered after tissue is removed and examined under the microscope for another reason.

What are the symptoms of myelolipoma?

Most myelolipomas do not cause any symptoms and are discovered incidentally when abdominal imaging is performed for another reason. However, some tumours can cause symptoms such as abdominal pain or distension and large tumours may cause symptoms associated with compression of nearby organs.

What causes a myelolipoma?

The exact cause of myelolipomas is not fully understood, but doctors believe they might develop in response to stress or injury in the affected area. One theory suggests that these tumours could come from cells present from birth that have the potential to turn into fat and blood-forming tissue. Another theory suggests that they might result from changes in normal adrenal gland cells, possibly influenced by hormonal imbalances or metabolic factors. Although some people have suggested that hormonal or genetic factors play a role, there’s no definitive evidence linking them directly to the development of myelolipomas.

Microscopic features of this tumour

When examined under the microscope, myelolipoma is made up of two types of cells: adipocytes (fat cells) and cells normally found in the bone marrow. The relative proportion of these two types of cells varies between tumours.  Pathologists use the term “mature adipocytes” to describe the normal-appearing fat cells typically found throughout the tumour. The bone marrow cells normally found in this tumour include megakaryocytes and immature white blood cells. Pathologists use the term “trilineage hematopoiesis” to describe this combination of normal bone marrow elements. A rim of the normal adrenal gland may be seen surrounding or adjacent to the tumour. Because large tumours may bleed, your pathology report may describe the presence of hemorrhage, red blood cells, or hemosiderin within the tumour. Large open spaces called cysts may also be seen, especially in large tumours.