Myelolipoma Of Kidney: A Rare Extra‑Adrenal Tumor in An Unusual Site
May 24, 2022
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Abstract
Myelolipoma of the kidney is a very rare benign disease that is composed of mature adipose tissue admixed with mature hematopoietic elements in varying proportions, The most common location of myelolipoma is in the adrenal glands, but extra-adrenal myelolipomas in other locations such as the retroperitoneum, presacral region, pelvis, and mediastinum have also been reported. In this case report, we present a case of huge extra-adrenal myelolipoma in renal parenchyma which is a rare site for its origin. We report a case of extra-adrenal myelolipoma occurring in the left kidney of a 46-year-old Indian man. We describe the radiological and clinicopathologic features of this unusual tumor with a review of the literature. This case is noteworthy because the tumor was very enlarged and its site was unusual. It is generally impossible to distinguish extra-adrenal myelolipoma from other retroperitoneal tumors by radiological imaging given the rarity of the tumor more cases need to be reported to elucidate its biological behavior.
Keywords: Extra-adrenal, kidney, myelolipoma
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Introduction
Myelolipoma of the kidney is a very rare benign disease that may resemble other renal lesions majority of clinicians may be unaware of its entity. It is composed of mature adipose tissue admixed with mature hematopoietic elements in varying proportions. The most common site of involvement in the adrenal gland. Its incidence in extra-adrenal sites is 0.4% at autopsy. Extra-adrenal myelolipomas have been reported in the retroperitoneum, pelvis, thorax, and mediastinum.[2] Worldwide myelolipoma in the kidney or around the kidney has been reported in <10 cases. Here, we present a case of very large renal myelolipoma along with its radiological and clinicopathological discussion.
Case Report
A 46-year-old Indian man, presented in our hospital with left flank pain and left loin swelling 1 year back. Physical examination revealed a well-defined lump in the left hypochondrium and lumbar region which was firm inconsistency. [Figure 1] Ultrasonography (US) of his abdomen showed a huge mass of mixed echogenicity in the left renal fossa. Computed tomography(CT)of his abdomen and pelvis a large heterogeneous lesion of predominantly fat density 15.2 cm×20.2 cm×26 cm arising from the left kidney occupying nearly the entire left lumbar and iliac fossa region showing discrete enhancing soft-tissue density areas (<20 Hounsfield units) within. Multiple radio-dense calculi were noted in the renal pelvis and upper, mid, and lower calyces of the left kidney the largest calculus in the renal pelvis measures 26.5mm×12.5mm×22mm.[Figure2]His adrenal glands were reported normal and no lymphadenopathy was detected. Laparotomy through chevron incision was performed. Intraoperatively, the lesion was well-circumscribed, surgical planes with surrounding structures were maintained and the mass was removed.
On gross examination, the left kidney along with the mass removed from the patient was irregular in shape and weighed 4.2 kg. The cut surface showed a very small area of residual renal parenchyma with a staghorn calculus. The pelvic calyceal system is lined with yellowish-white flakes and a large tumor composed of fatty areas with foci of hemorrhage.
Microscopic examination of the specimen showed changes of chronic pyelonephritis with focal xanthomatous changes and a tumor composed of adipocytes. The intervening septa show hematopoietic tissue with cells of all three hematopoietic lineages (myeloid, erythroid, and megakaryocytes). No adrenal rest was found. Based on these findings, renal myelolipoma was diagnosed. The postoperative course of the patient was uneventful. At follow-up of 1 year, he was asymptomatic. [Figure 3]
Discussion
Myelolipoma is a benign lesion that comprises adipose tissue and hematopoietic elements and commonly has been reported in the adrenal gland in comparison to other organs.3] Myelolipoma was first described in 1905 by Gierke since then myelolipoma has been reported sporadically in various organs.4 The pathogenesis of myelolipoma is not exactly unknown. One theory postulated that it is hamartoma or groups of choristomatous hematopoietic stem cells that transmigrated into the other sites during intrauterine life, while another theory found clonal cytogenetic abnormality, which suggested it was a renal tumor origin,[5-7] Patients with either adrenal or extra-adrenal myelolipoma are typically asymptomatic and are diagnosed incidentally based upon radiological imaging for a different condition based on ultrasound scan, CT scan, or magnetic resonance imaging scan.
No definitive radiological criteria exist for diagnosing extra-adrenal myelolipoma however the characteristics of adrenal myelolipoma on imaging can be exploited to identify these structures which are present in a variety of locations.8]extra-adrenal myelolipoma features on imaging can mimic several malignancies such as retroperitoneal liposarcoma, a renal or adrenal myelolipoma, renal angiomyolipoma, or a retroperitoneal teratoma.
Preoperatively, an attempt for definite diagnosis can be made with a fine-needle biopsy under the US or CT guidance which would then reveal a fat tissue lesion admixed with a variable amount of hematopoietic tissue and no malignant cells. Differential diagnosis after a biopsy can extramedullary hematopoiesis and extra-adrenal myelolipoma, which can be distinguished from the latter by the presence of splenomegaly or other organomegaly, chronic anemia, and marked hyperplasia of the bone marrow.9 In our case, after evaluating the CT scan which contained low density (<20 HU)soft-tissue lesion arising from the kidney angiomyolipoma, myelolipoma of the kidney, and liposarcoma were kept as differential diagnoses. Preoperative biopsy was not done as the patient was planned for a left nephrectomy in view of the large mass effect causing pain to the patient.
Grossly, extra-adrenal myelolipoma is a solitary circumscribed mass ranging in size from a few centimeters to 27 cm.i0It is a well-circumscribed, nonencapsulated, solid mass in the kidney. The cut surface of myelolipoma would tend to reveal areas of soft yellow fatty tissue together with admixed with irregular areas of brownish friable tissue." i] Microscopically, myelolipoma is a mixture
of adipose tissue and normal hematopoietic cells with all the three hematopoietic cell lineages (granulocytic, erythroid, as well as megakaryocytic). Kidney myelolipoma does not tend to have a connection to the medullary cavity of any bone.
Some of the differential diagnoses of myelolipoma of the kidney include angiomyolipoma, sarcoma including liposarcoma, extramedullary hematopoietic tumors, and renal cell carcinoma. Angiomyolipoma represents one of the most common benign renal lesions.12] Microscopically, typical angiomyolipoma is a triphasic tumor that carries varying amounts of dysplastic blood vessels, spindle and epithelioid smooth muscle cells, and mature adipose elements.
Fat-predominant angiomyolipoma mimics other lesions such as liposarcoma and myelolipoma. Liposarcoma involves middle-aged adults with a peak incidence in the 6h decade, with no sex predominance. Retroperitoneal liposarcoma is often asymptomatic until the tumor has become big in size. Very few cases of liposarcoma located in renal parenchyma have been reported.13On gross examination it consists usually of a large, well-circumscribed, lobulated mass with variable colors from yellow to white depending on the proportion of adipocytes, fibrous, and/or myxoid areas. Microscopically, it is composed either entirely or in part of a mature adipocytes proliferation showing a significantly varying number of lip blasts. In our case, the tumor contained, in addition to mature adipose tissue, nests of hematopoietic elements were seen.
Extra medullary hematopoietic tumors are characterized by anemia, frequent hepatosplenomegaly, and abnormal peripheral blood smears. extra-adrenal myelolipoma is well-circumscribed but extramedullary hematopoietic tumors lack circumscription and are ill-defined. Microscopically, extramedullary hematopoietic tumors have a predominance of hematopoietic elements, with erythroid hyperplasia. Fat is not an enlarged component of the processI4]
If myelolipoma is diagnosed is established hand, then subsequent management depends upon its size and the symptomatic status of the patient. Small asymptomatic lesions may be managed expectantly. Radiological follow-up is recommended to look for growth and hemorrhage. In symptomatic patients, who possess an enlarging tumor mass, as in our patient, surgical intervention is warranted. The long-term prognosis is excellent.
Conclusion
Our case was different because the location of giant extra-adrenal myelolipoma in renal parenchyma is extremely rare. It is generally impossible to distinguish extra-adrenal myelolipoma from other retroperitoneal tumors by radiological imaging. Hence, the final diagnosis should depend on the histopathological features of the surgically removed specimen. Acknowledging the rarity of tumors, more cases need to be reported for elucidation of the biological behavior of the disease.
other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
