What is Adrenal Myelolipoma?
Adrenal myelolipomas are harmless, non-cancerous tumors located in the adrenal glands, made up mostly of fat tissue and bone marrow-like tissue. They make up 6% to 16% of all incidental adrenal gland tumors discovered, being the second most common type of adrenal gland growth after adrenal adenomas.
The number of detected adrenal myelolipomas has increased, largely due to the more widespread use and availability of CT and MRI scans. These scans often find these tumors while looking for unrelated issues. The cause of adrenal myelolipomas is generally attributed to either the abnormal transformation of certain cells in the adrenal gland, or overstimulation by a hormone called ACTH.
Typically, these tumors are small and cause no symptoms. However, they may cause abdominal pain, nausea, or vomiting. When viewed on a CT scan, they show up as clearly defined, low-density masses. Their appearance on the scan varies based on their fat content.
Myelolipomas are very rarely found outside the adrenal glands. When they are, they’re referred to as extra-adrenal myelolipomas, and are usually found in areas like behind the peritoneum (the membrane lining the abdominal cavity), in the chest, or the pelvis. Surgery is considered in cases where the tumor grows significantly in size, is larger than 7cm (due to risk of rupture and bleeding), causes pain or other problems, if there’s suspicion of cancer, or if it’s secreting too many hormones.
What Causes Adrenal Myelolipoma?
The exact cause of a type of tumor called adrenal myelolipomas is unclear, but research links it to stimulus from a hormone called ACTH. Some believe that processes like tissue death or inflammation might lead to transformations in specific cells, which then form adrenal myelolipomas. This idea is supported by the fact that these tumors are more common in older people.
Another possibility is that the formation of myelolipomas is rooted in our developmental stages. Before birth, the adrenal gland can produce blood cells until the bone marrow takes over this role. Even in adults, especially those with chronic anemia or blood disorders like thalassemias, hereditary spherocytosis, or sickle-cell anemia, this function can return. Reports that high doses of hormone-stimulating drugs can cause adrenal myelolipomas also support this theory.
In a historical study, researchers found that injecting certain substances in rats led to the transformation of adrenal cortex into bone marrow-like tissue, or even pure fat. So, it is thought that too much ACTH could cause adrenal myelolipomas. Patients with a condition where ACTH is greatly increased, called congenital adrenal hyperplasia (CAH), may have a 36% chance of developing adrenal myelolipomas. These tumors are generally larger and occur more often on both adrenal glands.
Fascinatingly, adrenal myelolipomas have a unique micro-RNA footprint difference than other adrenal cortex tumors. This may be a critical factor in the formation of myelolipomas and could also serve as a biological marker for evaluation.
These tumors are sometimes linked to conditions like Cushing disease, obesity, high cholesterol, high blood pressure, and diabetes, as these may stimulate the adrenal glands. Some also believe that stress and poor diet may contribute to tumor development.
Risk Factors and Frequency for Adrenal Myelolipoma
Most adrenal incidentalomas – unexpected masses found in the adrenal gland – are neither active nor malignant, with around 80% being nonfunctional and more than 90% being benign. The second most frequent type of adrenal accidentaloma is the adrenal myelolipoma, which makes up 6% to 16% of all these adrenal masses. Despite a higher likelihood of occurrence in patients with CAH, a specific type of adrenal gland disorder, the Endocrine Society doesn’t advise regular screening for adrenal tumors in these patients.
- Adrenal myelolipomas are found in about 0.08 to 0.2% of autopsies.
- Typically, they are diagnosed in individuals aged 40 to 70.
- The median age of diagnosis is approximately 62 years.
- These tumors don’t favor a specific gender, however, some studies indicate a mild tendency towards the right side of the adrenal gland.
Signs and Symptoms of Adrenal Myelolipoma
Adrenal myelolipomas are typically presented through a variety of symptoms. Some common symptoms include:
- Abdominal pain—22.5%
- Flank pain—13.9%
- Abdominal mass—5.2%
- Dyspnea
- Back pain
- Fever
- Unexplained weight loss
- Virilization
These symptoms are more likely in patients with larger lesions (over 6 cm), which may also lead to symptoms of mass effect, such as more pains and shortness of breath depending on position. Adrenal myelolipomas larger than 8 cm have a higher risk of rupture.
There are four distinct patterns of adrenal myelolipomas, including:
- Isolated Adrenal Myelolipoma: This is the most common type where myelolipomas appear in a normal adrenal gland without symptoms and are usually detected during imaging for other reasons.
- Adrenal Myelolipoma with Acute Hemorrhage: This involves retroperitoneal hemorrhage, a common complication in myelolipomas over 4 cm in size or those largely made of fat (over 50%).
- Extra-Adrenal Myelolipoma: This type occurs outside of the adrenal gland, often found in the retroperitoneum and mostly does not include endocrine abnormalities or acute hemorrhage.
- Adrenal Myelolipoma with Associated Adrenal Disease: In these cases, patients with adrenal myelolipoma may also have a separate endocrine disorder like hypercortisolism (Cushing syndrome), congenital adrenal hyperplasia, or primary hyperaldosteronism.
Testing for Adrenal Myelolipoma
Tumors that aren’t producing hormones are often difficult to detect, so doctors use endocrinological tests to rule out the possibility of other adrenal masses including pheochromocytomas. When these tumors show up on the scans, they can look like low-density fat mixed with higher-density myeloid components.
If the tumor is hormonal or ‘functional’, it may cause symptoms which include diabetes, fatigue, muscle weakness, obesity, uncontrollable high blood pressure, significant weight gain, unusual feminization and virilization.
To diagnose adrenal masses such as myelolipomas, doctors can use imaging tests like ultrasound, CT scans, and MRI scans. CT scans are the most effective at spotting the fat within the lesions because the adipose tissue, which can vary in amount in myelolipomas, is easily seen and measured.
If a CT scan shows a mass with a density of less than 10 Hounsfield units (HU) and the endocrinological evaluation comes back negative, then a biopsy might not be needed. However, up to 12% of myelolipomas might be functional. There are other kinds of fat-containing masses that do not function, and these may be lipomas, liposarcomas, and renal angiomyolipomas.
On an ultrasound, myelolipomas often appear as a bright mass with mixed-in darker zones. From there, the echogenicity or the ability to produce an echo, is determined by the different elements within the lesion. The most echogenic parts are where the fat and myeloid tissue mix, but pure fat regions could appear darker. If there’s calcification, it will appear bright with acoustic shadowing. The borders of myelolipomas might be hard to distinguish on an ultrasound because their densities are similar to surrounding fat.
On a CT scan, myelolipomas often look like areas of low-density fat mixed with areas of higher-attenuation tissue that can be more solid. The denser tissue could range from 20 HU to 30 HU, suggesting the presence of both fat and myeloid cells. Even though these can be relatively well-defined, the large amount of fat may make them hard to distinguish from surrounding fat tissue.
On an MRI, the areas of a myelolipoma that are mainly fat may appear very bright on T1-weighted scans and medium to bright on T2-weighted scans. The mixed marrow-like elements produce medium-intensity signals similar to that of the spleen. Techniques that suppress fat are best for showing fat in a myelolipoma on an MRI.
In most situations, the diagnosis of myelolipomas can be accurately made with these different imaging tests, so a biopsy is usually not needed. As an exception, a biopsy might be required when the imaging results are uncertain or when there are signs of adrenal carcinoma or liposarcoma.
Treatment Options for Adrenal Myelolipoma
The treatment plan for adrenal myelolipomas, a type of tumor in the adrenal gland, depends on the size of the tumor, any signs that it might be cancerous, and whether it is causing symptoms. If there’s a reasonable chance it could be cancerous, it should be removed surgically. However, for most adrenal myelolipomas, the treatment leans more towards monitoring rather than immediate surgery.
Small (less than 5cm), non-functional tumors that aren’t causing symptoms are usually monitored through imaging scans. The first follow-up scan is typically conducted six months after diagnosis, followed by yearly scans for one to two years.
Signs that surgery may be needed include an increase in the tumor’s size by 1cm or more on follow-up scans, or if the tumor starts to produce hormones. However, most non-functional adrenal tumors don’t grow considerably or ever start producing hormones.
Surgery is usually recommended when there are signs of active tumor growth, hormonal activity, or potential malignancy. Other factors include symptoms like pain, potential tumor rupture, or active bleeding. Tumors larger than 6 cm also typically need to be removed surgically, as tumors of this size have a 25% chance of being cancerous.
Depending on the size of the tumor, either conventional surgery or minimally invasive endoscopic techniques may be used. Minimally invasive techniques are generally best for smaller tumors, while conventional surgery is preferred for larger ones.
Follow-ups are necessary regardless of the treatment method. For tumors that don’t meet the criteria for surgical removal, monitoring is crucial. This typically includes hormone tests and imaging scans conducted six months after the initial diagnosis and then yearly for one to two years. Some guidelines recommend continuing annual imaging and testing for up to five years.
Recommendations for monitoring adrenal myelolipomas may vary depending on the professional society. For instance, the European Society of Endocrinology and European Network for the Study of Adrenal Tumors suggests no follow-up imaging or testing if the initial examination shows a small tumor with very low density. In contrast, the American College of Radiology supports a risk-stratified approach for further evaluation of small adrenal masses with uncertain characteristics.
What else can Adrenal Myelolipoma be?
Adrenal myelolipomas are usually non-active and do not produce hormones. To confirm this, doctors might perform specific endocrinological tests. Besides myelolipomas, there could be other non-active masses in the adrenal glands:
- Benign adrenal adenoma: This is the most common non-active adrenal mass. It’s usually very structured and even, showing a low response in most medical scans due to the presence of fatty tissue. When such adenomas lack lipid content, they may appear more prominently on scans. Although they usually are non-active, some can produce hormones.
- Adrenal carcinoma: These are large uneven tumors that may often bleed and break down. They have undefined edges and can affect nearby structures such as the inferior vena cava, kidney, and liver. These depict a higher value on scans and show an increased intake in a PET scan.
- Retroperitoneal liposarcoma: This is a slow-growing tumor situated in the retroperitoneal region (behind the abdomen), consisting of fat cells. It can look quite similar to an adrenal myelolipoma on scans.
- Renal angiomyolipoma: These tumors consist of fat, blood vessels, and muscle cells. They are big, irregularly shaped tumors, which can spread through the capsule that surrounds the kidney. These can look similar to adrenal myelolipomas on scans but originate from the kidneys.
All things considered, other masses located in the retroperitoneal region, like retroperitoneal lymphoma, lipoma, or metastatic disease, could be involved in the diagnosis as well.
It’s worth noting that the adrenal glands, given their extensive blood supply, are particularly vulnerable to the spread of disease from other parts of the body.
What to expect with Adrenal Myelolipoma
The general outlook for adrenal myelolipoma, a kind of tumor in the adrenal gland, is pretty positive. One study followed 163 patients for about 7 years and found that only 16% experienced a growth of the tumor that was 1 cm or more. Moreover, those patients who undergo surgery usually fare well, as these tumors do not have the potential to become cancerous. In fact, there are reports of patients surviving long term without any recurrence of the myelolipoma.
Possible Complications When Diagnosed with Adrenal Myelolipoma
The main problems you might experience with adrenal myelolipomas are pain and the effects of having a large mass in the body. It’s uncommon, but bleeding and rupture can happen when the mass grows larger than 8 to 10 cm. If an operation is needed to treat this, there may also be complications linked to the type of surgery, whether it’s open surgery or done using a laparoscope (a small, tube-like camera that’s used to look inside the body).
Preventing Adrenal Myelolipoma
Those diagnosed with adrenal myelolipomas should be informed about this condition. This includes understanding what it is, the variety of treatment options available, potential diagnostic tests, recommended check-ups, and signs that could suggest the need for surgery. It’s comforting to know that adrenal myelolipomas are mostly harmless and can often be managed with careful monitoring and non-invasive treatments.
