Myelolipoma: Understanding Your Pathology Report

Section Editor: Jason Wasserman MD PhD FRCPC
May 29, 2026

Myelolipoma is a benign (non-cancerous) tumor made up of two normal tissues mixed together: mature adipose tissue (fat) and bone marrow-type tissue that produces blood cells. The word myelolipoma comes from “myelo” (referring to bone marrow) and “lipoma” (a benign fat tumor). The combination of these two tissues within a single mass defines the diagnosis.

Myelolipoma is most commonly found in the adrenal gland, where it is one of the more common benign adrenal tumors. There are two adrenal glands in the body, one on top of each kidney. Less commonly, myelolipoma can also occur outside the adrenal gland, in locations such as the area in front of the sacrum (presacral region), behind the abdominal organs (retroperitoneum), in the chest (mediastinum), or in the liver. The article below focuses on the much more common adrenal myelolipoma.

Most myelolipomas are discovered by accident on imaging tests performed for other reasons, do not produce hormones, and do not cause symptoms. This article will help you understand the findings in your pathology report, what each term means, and why those findings matter for your care.

What causes a myelolipoma?

The exact cause of myelolipoma is not known. Most are sporadic, meaning they appear without a known trigger and are not caused by anything the person did or was exposed to. Several theories have been proposed:

Adrenal cortical stem cell change — The leading theory is that cells of the adrenal cortex (the outer part of the adrenal gland) can change into fat- and blood-forming cells under certain conditions, leading to a myelolipoma over time.
Long-standing hormone stimulation — Myelolipomas have been described in people with conditions that cause prolonged adrenal stimulation, including untreated congenital adrenal hyperplasia (a group of inherited disorders that affect adrenal hormone production). In these cases, the chronic stimulation of the gland is thought to drive the development of myelolipomatous change.
Other suggested associations — Obesity, chronic stress, and various endocrine disorders have been linked to myelolipoma in some reports, but the evidence is limited and most patients with myelolipoma do not have any of these conditions.

Myelolipoma is not an inherited condition in the usual sense. It does not run in families, and genetic testing is not part of the standard workup.

What are the symptoms of myelolipoma?

Most myelolipomas do not cause symptoms and are discovered incidentally during abdominal imaging performed for an unrelated reason. When symptoms do occur, they are almost always due to the tumor pressing on nearby structures, not to hormone production. Possible symptoms include:

Abdominal pain, flank pain, or back pain.
A feeling of fullness or discomfort in the upper abdomen.
A lump that can sometimes be felt in larger tumors.
Nausea or reduced appetite when the tumor presses on the stomach.

The most important complication of a large myelolipoma is internal bleeding (hemorrhage) into the tumor, which can cause sudden, severe abdominal or back pain. The risk of bleeding increases with tumor size, particularly for tumors larger than about 6 to 7 centimeters.

Myelolipoma does not produce hormones. However, it can occur in the same gland as a separate hormone-producing tumor (such as an adrenal cortical adenoma or, rarely, a pheochromocytoma), and in that case the symptoms arise from the other tumor, not from the myelolipoma itself.

How is the diagnosis made?

The diagnosis usually begins when an adrenal mass is seen on an imaging test, most often a CT or MRI scan of the abdomen. Myelolipoma has a distinctive appearance on imaging because of its high fat content. On CT, the tumor contains tissue that appears as fat (negative Hounsfield density), which is unusual for other adrenal tumors. On MRI, the tumor shows the same signal as fat elsewhere in the body. These features are often characteristic enough that the diagnosis can be made with confidence on imaging alone, without the need for surgery or biopsy.

When the diagnosis is uncertain, when the tumor is large or symptomatic, or when there is concern for bleeding, the tumor may be surgically removed and sent to a pathologist for examination under the microscope. In a smaller number of cases, a needle biopsy is performed when imaging cannot distinguish a myelolipoma from another type of tumor. Under the microscope, the diagnosis is straightforward in most cases because the combination of mature fat and bone marrow-type tissue is characteristic.

Microscopic features that may be described in your report

Under the microscope, a myelolipoma consists of two tissues normally found in the body, mixed together in a single mass. The pathology report may describe one or more of the following features:

Mature adipose tissue (mature fat cells) — Large, clear cells with a thin border, each filled with a single droplet of fat. These cells make up most of the tumor in most cases and look identical to the fat found elsewhere in the body.
Trilineage hematopoiesis — A mix of the three main types of bone marrow cells: precursors of red blood cells (erythroid cells), precursors of white blood cells (myeloid cells), and large platelet-producing cells called megakaryocytes. “Trilineage” simply means all three lineages are represented.
Megakaryocytes — Large bone marrow cells with multiple-lobed nuclei. They produce platelets and are a normal part of bone marrow. Their presence in an adrenal tumor is one of the key features that confirms the diagnosis of myelolipoma.
A rim of normal adrenal cortex — A thin layer of normal adrenal tissue may be seen around the edge of the tumor, confirming that the mass arose within the adrenal gland.
Hemorrhage — Areas of bleeding within the tumor, more common in larger tumors. The report may describe red blood cells outside their normal vessels, blood clot, or a brown-yellow iron-containing pigment called hemosiderin, which forms after older bleeding has been broken down.
Cystic change — Larger tumors may contain open spaces called cysts, often the result of previous bleeding or tissue breakdown.
Calcification — Areas of calcium deposit can sometimes be seen, particularly in older tumors that have undergone bleeding or cyst formation.

The proportion of fat and bone marrow tissue varies widely from one myelolipoma to another. Some tumors are made almost entirely of fat with only small islands of bone marrow tissue; others have a higher proportion of bone marrow elements. The diagnosis depends on the presence of both tissues, not on a specific ratio.

Mixed tumors

Myelolipomatous change can occasionally occur within another adrenal tumor, such as an adrenal cortical adenoma or, rarely, an adrenal cortical carcinoma or pheochromocytoma. In these mixed lesions, the pathologist describes both components, and the clinical significance of the diagnosis depends on the other tumor type. A myelolipoma occurring alongside or within another tumor does not change the prognosis of the other tumor in most cases, but it is important to recognize and describe so that the report accurately reflects everything that was found.

What is the prognosis?

Myelolipoma is a benign tumor with an excellent prognosis. It does not spread to other parts of the body, and a myelolipoma that has been completely removed does not return. The main long-term issue, when the tumor has not been removed, is the risk of bleeding, which is small for tumors under about 4 centimeters but increases as the tumor enlarges.

What happens after this diagnosis?

The treatment plan depends on the size of the tumor, the presence of symptoms, the certainty of the imaging diagnosis, and the patient’s other medical conditions. The treatment team typically considers:

Observation (no surgery) — Small, asymptomatic myelolipomas with typical imaging features are usually followed without surgery. Repeat imaging is performed at intervals decided by the treatment team to confirm that the tumor is stable.
Surgical removal — Generally considered for tumors that are large (often more than 6 to 7 centimeters), that are causing symptoms, that are bleeding, or whose imaging features are not clearly typical of myelolipoma. Surgery removes the affected adrenal gland together with the tumor in one piece.
Workup for hormone production by surrounding gland — When a myelolipoma is found alongside an apparently normal adrenal gland on imaging, no additional hormone testing is needed. If imaging suggests a separate nodule in the same or opposite gland, blood and urine tests for adrenal hormones (cortisol, aldosterone, sex hormones, and metanephrines) may be performed to look for a second tumor.
Evaluation for congenital adrenal hyperplasia — In selected patients, particularly those with very large or bilateral myelolipomas, blood tests for congenital adrenal hyperplasia may be considered to look for a treatable underlying cause.
No long-term cancer surveillance — Because myelolipoma is benign, no cancer-related follow-up is needed once the diagnosis is confirmed and any concerning features have been addressed.

Questions to ask your doctor

Is my myelolipoma in the adrenal gland or in another location?
How large is the tumor, and is it likely to grow further?
Was the diagnosis confirmed on imaging alone, or is surgery or biopsy needed?
Are there any signs of bleeding within the tumor?
Should I have surgery, or can the tumor be safely watched with follow-up imaging?
If the tumor is being watched, how often should follow-up imaging be done?
Is there another adrenal tumor present, and does it produce hormones?
Should I be evaluated for congenital adrenal hyperplasia or any other underlying condition?
What symptoms should make me seek urgent medical attention (for example, signs of bleeding)?
If surgery is performed, will the entire adrenal gland be removed, and what does that mean for hormone replacement?