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A 54-year-old woman presented with a large retroperitoneal tumor between the right lobe of her liver and kidney. MRI revealed a right adrenal mass which was excised. Received was a lobulated mass weighing 1630 g and measuring 18.0 x 18.0 x 10.0 cm. The external surface was smooth and encapsulated. Upon sectioning, a rim of normal-appearing adrenal gland was present. The cut surface was red-brown with patchy yellow areas, some of which had a gelatinous consistency.
Archive Case and Diagnosis:
This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2008, Case 40, and is myelolipoma.
Criteria for Diagnosis and Comments:
The correct diagnosis is adrenal myelolipoma which was first described by Gierke in 1905. This is a rare benign lesion accounting for less than 1% of adrenal lesions. The mean age at presentation is 56 years. Patients with adrenal myelolipoma are usually asymptomatic, though larger lesions may cause abdominal pain. Rarely, the lesion may rupture (usually >10 cm), presenting as an “acute abdomen” with hemorrhage, warranting immediate surgical intervention.
Histologically, the lesion is composed of mature adipose tissue intermixed with benign hematopoietic elements. The cellularity in myelolipomas is variable and ranges from scant to very cellular within the same lesion. Unlike normal bone marrow, there is no relation between cellularity in myelolipomas and the patient’s age. These lesions typically show normal trilineage hematopoiesis with early myeloid precursors rarely present. Most tumors have numerous megakaryocytes, exceeding the numbers seen in the normal bone marrow. Occasionally, some lesions may have increased numbers of plasma cells, eosinophils, or lymphocytes.
In a recent study by Bishop et al., the authors found a strikingly different stromal composition in myelolipomas from that of normal bone marrow. Reticulin fibrosis was present in nearly all 19 cases analyzed. The majority of cases also showed increased expression of nerve growth factor receptor, resulting in an increase in adventitial reticulum cells, a stromal precursor cell population. Interestingly, well-developed capillary-venous sinuses are rarely seen in adrenal myelolipoma, which are essential for release of hematopoietic cells into the circulation. Taken together, this may explain why myelolipoma derived cells do not appear to contribute to circulating blood elements. Using X-chromosome inactivation, Bishop et al. also demonstrated that the majority of adrenal myelolipoma cells were monoclonal, supporting the neoplastic nature of these lesions.
The most important differential diagnosis is extramedullary myeloid tumor (granulocytic sarcoma). The lack of increased blasts or significant dysplasia of other cell lineages readily distinguishes adrenal myelolipoma from extramedullary myeloid tumor (granulocytic sarcoma). Other differential diagnostic considerations of myelolipoma include lipoma, liposarcoma, extramedullary hematopoiesis, angiomyolipoma, chronic lymphocytic leukemia/small lymphocytic lymphoma, neuroblastoma, and primitive neuroectodermal tumor. Lipoma of adrenal glands is rare and does not contain hematopoietic elements. Liposarcoma of the retroperitoneum may extend into adrenal glands. The lack of lipoblasts distinguishes myelolipoma from liposarcoma. Extramedullary hematopoiesis may also be seen in adrenal glands, but the lesion is usually small and do not present as tumor-like lesion. Angiomyolipoma is characterized by the presence of vascular, smooth muscle and adipose tissue elements. Chronic lymphocytic leukemia/small lymphocytic lymphoma involving adrenal glands will not have normal adipose component. The typical morphologic appearance of adrenal myelolipoma allows for easy distinction from neuroblastoma, and primitive neuroectodermal tumor. In difficult cases, immunohistochemical staining may be helpful. Myelolipoma lacks neuroendocrine differentiation. Neuroblastoma may be associated with N-myc amplification, which is lacking in myelolipoma.