What is a myelolipoma?

Myelolipoma is a non-cancerous tumour that is typically found in the adrenal gland. Although non-cancerous, the tumour can grow to be very large which can cause symptoms associated with compression of nearby organs. Myelolipoma is the second most common type of adrenal gland tumour.

The adrenal gland

The adrenal glands are small organs that sit just on top of your kidneys at the back of your abdomen. The adrenal glands are responsible for producing hormones (chemical signals released into the bloodstream) to help regulate functions in the rest of the body.

The adrenal glands are made up of two parts, an outer cortex, and an inner medulla. The cells in the outer cortex are called adrenal cortical cells and they produce hormones to help control things like the amount of water and salt in the body and the body’s response to stress.

The cells in the inner medulla are called neuroendocrine cells and they produce hormones to control the body’s sympathetic (“fight or flight”) response. The hormones made by these cells include epinephrine (adrenaline) and norepinephrine (noradrenaline). When these hormones are released into the blood, they cause your heart rate and blood pressure to increase.

What causes a myelolipoma?

There is no known cause for most patients who develop a myelolipoma.

How do pathologists make this diagnosis?

The diagnosis of myelolipoma can be made after a small sample of tissue is removed in a procedure called a biopsy or after the entire tumour is removed. The tissue is then sent to a pathologist for examination.

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 a myelolipoma 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.


by Jason Wasserman MD PhD FRCPC (updated August 1, 2021)
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