Ⅰ. INTRODUCTION

Myelolipoma is a benign tumor-like lesion consists of mature adipose tissue and hematopoietic elements (1). Mostly, this tumor has been found around adrenal gland (2). There also have been many cases of extra-adrenal area (3). However, only few cases of intraosseous myelolipoma have been documented in the English-language literature till date (4, 5). Thus, the etiology of intraosseous myelolipoma remains uncertain and it is difficult to diagnose this tumor (6). This case report includes a patient with intraosseous myelolipoma of maxilla and mandible and literature review about histopathologic and clinical features of intraosseous myelolipoma.

Ⅱ. CASE REPORT

A 61 year-old male patient was referred to our clinic because of swelling on the left side of the mandible. He complained of slow-growing, painless mass on his left side of the jaw. The enlargement on the left side of the mandible was distinguishable with the unaided eye. The panoramic radiograph showed multilocular and radiolucent lesions on both sides of maxilla and mandible (Fig. 1). The computed tomography (CT) showed rough trabeculation inside of the lesions. The left maxillary sinus was severely compressed by the lesions. Cortical bone thinning on both maxilla and mandible was observed (Fig. 2). The lesion was cold on radionuclide bone scan (Fig. 3). For the definite diagnosis, the incisional bone biopsy was conducted on the left side of the mandible. The histopathologic finding was composed of mature adipose tissue and hematopoietic elements such as erythroid and myeloid. The neoplasm was diagnosed as myelolipoma (Fig. 4). Since the lesions were painless and the range was severely wide, our clinic decided to conserve the lesions.

Ⅲ. DISCUSSION

Myelolipoma is a relatively rare and benign tumor composed of adipocytes and hematopoietic elements (7). myelolipoma was documented by Geirke in 1905 for the first time and the term “myelolipoma” was first used by Obenling in 1929 (8, 9). Mostly, this lesion has been found around adrenal gland (10). There also have been more than 40 cases of extra-adrenal area such as retroperitoneum, liver, other mediastinum, stomach, etc. (4, 11). However, only few cases of intraosseous myelolipoma have been documented in the English-language literature till date (4, 5).

The etiology of myelolipoma remains uncertain but some theories have been proposed including development from embryonic mesenchymal cells in the adrenal glands, development from seeded bone marrow emboli, and metaplasia of reticuloendothelial cells as a result of chronic stress (6, 12-14). It is hypothesized that the event starts with a metaplastic change in the reticuloendothelial cells of the blood capillaries due to infection or stimulation (15, 16). Bishop et al. found that the most of myelolipoma had non-random X-chromosome inactivation. This fact suggests the clonal origin of myelolipoma (17). Another theory suggests that myelolipoma has its origin from the interaction of two different types of progenitor cells. First, adipocytes developed from mesenchymal stem cells took place in the vessel wall of stromal fat of adrenal cortex under certain stimulation. when the adipocytes are differentiating and maturing, they become inflammatory and stimulate the adrenal cortex to secrete factors or substances that assemble floating hematopoietic progenitors to get settled and differentiate (18). Another major group of hypotheses relates to hormonal events. Some patients with untreated congenital adrenal hyperplasia having very high serum adrenocorticotropin (ACTH) levels develop myelolipoma. S.C. Sieber et al. presented a case of ectopic ACTH overproduction due to a lung carcinoma associated with myelolipoma (19). However, H. Hagiwara et al. could not confirm the overexpression of ACTH to be a major causative factor of myelolipoma (20). It might be possible that both altered mesenchymal stem cell functioning and hormonal events act together in the pathogenesis of myelolipoma (7).

Rao P et al. suggested a classification for the myelolipoma and subdivide the tumor in 4 patterns. The most common type of myelolipoma is the isolated adrenal lesion and the other types are myelolipoma with acute hemorrhage, myelolipoma with other pathologic conditions, and extraadrenal myelolipoma. The extra-adrenal myelolipoma type accounts for about 15% of all types of myelolipoma. Extraadrenal myelolipoma was usually found in either perirenal or presacral area. Typically, there was no endocrine disorder or acute hemorrhage in extra-adrenal myelolipoma (2, 3, 21, 22).

There are several radiographic features of myelolipoma. The imaging appearances of extra-adrenal myelolipoma are not different from those of adrenal myelolipoma. On CT images, myelolipoma is well capsulated, hypodense, and heterogenous mass. The typical fat density is helpful in diagnosing the mass like myelolipoma (16). Boraschi et al. explained three different morphologies of myelolipoma on MRI images. (23)

It is tough to distinguish myelolipoma from other fatcontaining tumors such as extramedullary hematopoiesis (EMH) only with radiologic findings. Myelolipoma is composed of mature fat cells and hematopoietic cells just as EMH. EMH is another adipocyte containing, mass forming lesion with very similar histologic features of myelolipoma (24). In the general clinical practice, the differential diagnosis between EMH and myelolipoma is based on clinical symptoms: EMH usually occurs in hematological disorders, while myelolipoma is a tumor that is accompanied by a healthy bone marrow (25). Myelolipomas are well-circumscribed lesions and composed of fat cells and normal marrow hematopoietic elements. On the other hand, EMH is not circumscribed, and fat cells are not the obligatory component (26).

Due to the lack of formal guidelines regarding the management of myelolipoma, the clinicians should make decisions on a case by case basis. The majority of myelolipoma is asymptomatic, found incidentally, and not causing any trouble to the patient. If the diagnosis of myelolipoma is clear based on imaging, and there are no myelolipoma-related symptoms, no treatment is necessary. Surgical resection is the only treatment option if needed (27, 28).

The patient in our report had myelolipomas confined to bones. Multiple tumors took place in both maxilla and mandible of the patient. As far as I know, intraosseous myeliolipoma is extremely rare and there has been a single case report till date. However, that case report contains no pre-operative images, and the English translation of the report doesn’t offer the basis of the diagnosis (29). Only few patients with intraosseous myelolipoma of other bones such as femur or rib were reported. There were a few typical features of intraosseous myelolipomas (4, 30).

The patient of our report showed unique clinical characteristic; there were multiple tumors on maxilla and mandible. The broad range of the lesion is also distinguishing. Except for our patient, other cases presented a single and unilocular intraosseous myelolipoma. There should be more case reports about intraosseous myelolipoma and more studies to figure out the physiopathologic characteristics of intraosseous myelolipoma.