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Facts about Myelolipoma ?
(1) Myelolipomas are rare, benign nonfunctional tumours composed of mature adipose tissue and hematopoietic elements including normoblastic, granulocytic and megakaryocytic series.
(2) Myelolipoma was first described by Gierke in 1905 and the term "myelolipoma" was coined by Oberling in 1929. Extra adrenal myelolipoma was first reported by Krag in 1972.
(3) The hypotheses of the cause of the myelolipoma include -
- Degenerative changes in hyperplastic tumour cells or adenomas of the adrenal glands,
- Metaplasia in primary stem mesenchymal cells of the adrenal cortex and
- Displacement of differentiated bone marrow cells during embryogenesis.
- Some authors suggested metaplasia of reticuloendothelial cells of blood capillaries in the adrenal gland in response to stimuli such as infection, stress, necrosis, or prolonged exposure to adrenocorticotropic hormone. This has become the most widely accepted theory.
(4) Chromosomal translocations (3;21) (q25;p11) detected in myelolipomas and in benign lipomatous neoplasia seen in patients with acute myelogenous leukemia or myelodysplastic syndrome suggest a bone marrow origin of this tumour, and may indicate that myelolipoma is derived from erroneously transferred erythroid cells.
(5) Adrenal myelolipomas are generally hormonally inactive.
Some cases may be associated with overproduction of adrenal hormones.
Occasionally myelolipomas may be accompanied by endocrine disorders such as Cushing’s disorder, Addison’s disease, Conn’s syndrome, phaechromocytoma, adrenal gland cancer or adenoma, diabetes mellitus or even obesity or hypertension.
Some authors have emphasized that myelolipoma may be correlated with prolonged excessive steroid production or genome defects of the endocrine glands responsible for multiple endocrine neoplasia type 1.
(6) The tumour can occur in both men and women between the ages of 50 and 70 years.
While most cases are clinically asymptomatic, approximately 20% of patients can experience abdominal or lumbar pain secondary to hemorrhage, tumour necrosis, or mechanical compression by the tumour.
Also, hematuria and renovascular hypertension were seen.
(7) The adrenal gland is the most common site and more frequently affect the right adrenal gland.
Adrenal myelolipomas constitute 1.9% of adrenal "incidentalomas".
It is usually discovered incidentally on autopsies, surgeries, or imaging studies such as ultrasonography or computed tomography performed for other purposes.
With the recent advancement in diagnostic imaging methods, the discovery of small adrenal incidentalomas has been increasing.
(8) Myelolipomas are also rarely present in extra-adrenal sites, including the pelvis, mediastinum, retroperitoneum, and paravertebral region, as an isolated soft tissue mass.
Extra-adrenal myelolipomas have also been reported in the stomach, liver, lymph nodes, spleen, lungs and skull.
(9) Adrenal myelolipomas are in the majority of cases unilateral and more frequently affect the right adrenal gland. These tumours are often less than 4 cm in diameter.
However, they may attain very large size and also be bilateral. The term giant myelolipoma is preferred when the size exceeds 8 cm.
The size of extra-adrenal myelolipoma has been reported to range from 4cm to 15cm, with a mean diameter of 8.2cm.
(10) Histologically, extra-adrenal myelolipoma should be differentiated from mass forming foci of extra-medullary hematopoiesis.
- Mass forming extra-medullary hemopoiesis is symptomatic ( associated with myeloproliferative disorder, hemolytic anemia or severe skeletal disease ).
- In contrast to extra-adrenal myelipomas that are well encapsulated, extra-medullary hemopoietic 'tumours' lack circumscription and are ill defined.
- Microscopically, extra-medullary hemopoietic 'tumours' have a predominance of hemopoietic elements with erythroid hyperplasia. Fat is not an enlarged component of the process.
Extra-adrenal myelolipomas are composed of a variable proportion of mature adipose tissue and bone marrow cells.
The presence of megakaryocytes are considered to be essential for the diagnosis of extra-adrenal myelolipomas.
These lesions can have a prominent fat component and a more conspicuous lymphocyte population.
Extra-adrenal myelolipoma is distinct from true bone marrow in that no reticular sinusoids or bone spicules are present. However extra-adrenal myelolipomas containing bone spicules have been reported. The bone spicules are thought to be the result of osseous metaplasia.
One should carefully differentiate extra-adrenal myelolipomas from lesions that contain hemopoietic tissue and adipocytes, such as teratomas. Histology of the latter shows tissue elements from all the three germ layers.
(11) Treatments of adrenal myelolipoma vary from case to case and there is no global consensus.
It has been argued that asymptomatic cases with a small lesion ( less than 3–4 cm in diameter) should be followed up for 1 or 2 years and monitored by CT or MRI.
Symptomatic patients with a tumour diameter more than 4–5 cm or a tumour considered to be malignant are indicated for surgical intervention.
Even asymptomatic tumours need to be removed if growth is detected during the follow-up period.
It was established that abdominal or flank pain, large tumor size, atypical radiologic appearance, and/or inferior vena cava compression were operative indications for patients with a presumptive diagnosis of adrenal myelolipoma.
Retroperitoneal hemorrhage caused by spontaneous rupture of adrenal myelolipoma is very rare, and surgical resection is also recommended in those cases.
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