Retroperitoneal Extrarenal Angiomyolipomas: An Evidence-Based Approach to a Rare Clinical Entity

Extrarenal angiomyolipomas (ERAMLs) are rare tumors that present as incidentalomas upon imaging for other conditions. Retroperitoneal ERAMLs present a unique challenge from a diagnostic and treatment standpoint as they can mimic other benign and malignant retroperitoneal tumors. We present a case of a 39-year-old female with a 19.3 cm × 13.5 cm × 10.7 cm left extrarenal retroperitoneal mass. Histopathologic examination and HMB-45 staining revealed the mass to be a retroperitoneal ERAML. Our case report provides a comprehensive literature review and an evidence-based algorithm for taking care of patients with ERAMLs.


Introduction
Extrarenal angiomyolipomas (ERAMLs) represent a very rare subset of tumors that often present as incidentalomas upon imaging for other conditions. Lesions located in the retroperitoneum present a unique challenge from a diagnostic and treatment standpoint as they can mimic other malignant retroperitoneal lesions. We present a case of a 39-year-old female with a 19.3 cm × 13.5 cm × 10.7 cm left extrarenal retroperitoneal mass detected on a follow-up chest computerized tomography, obtained after prolonged treatment for pneumonia. Histopathologic examination and HMB-45 staining demonstrated the mass to be an extrarenal angiomyolipoma. A comprehensive literature review of ERAMLs as well as evidence-based care algorithm is provided.
Angiomyolipomas (AMLs) are common primary renal parenchymal tumors which comprise about 1% of all renal masses. In contrast, extrarenal angiomyolipomas (ERAMLs) represent extremely rare tumors with less than 60 reported cases since they were first described by Friis and Hjortrup in 1982 [1]. ERAMLs typically present as incidentalomas identified on imaging for other conditions. Typically, these tumors have prominent vascular pedicles and may present symptomatically with abdominal pain and hemorrhagic shock [2]. This paper discusses the case of a 39-year-old female who presented asymptomatically with a 19.3 cm × 13.5 cm × 10.7 cm left extrarenal retroperitoneal mass detected on a follow-up chest computerized tomography (CT) after prolonged treatment for pneumonia. Given the location and appearance of the mass, differential diagnosis included a retroperitoneal liposarcoma, leiomyosarcoma, lipoma, angiomyolipoma, adrenal adenocarcinoma, renal cell carcinoma, or leiomyoma with fatty change.

Case Report
A 39-year-old female with a past medical and surgical history significant for hypertension, gastroesophageal reflux disease, and a cesarean section presented with dysfunctional uterine bleeding (DUB) in November, 2010. Transvaginal sonographic evaluation was performed and failed to reveal any gynecologic pathology. A dilatation and curettage were performed and she was started on oral contraceptives without resolution of her DUB. Two months later, she 2 Case Reports in Nephrology  Given the location of the lesion, the differential diagnosis included a retroperitoneal liposarcoma, leiomyosarcoma, lipoma, angiomyolipoma, adrenal adenocarcinoma, renal cell carcinoma, or leiomyoma with fatty change. The retroperitoneal mass was resected en bloc with the left kidney through a midline incision. A total abdominal hysterectomy (TAH) was simultaneously performed to treat her persistent DUB. The patient recovered uneventfully and was discharged home on postoperative day 6.

Pathology
The en bloc gross specimen included the left kidney and weighed 1693 gm. The kidney measured 11.5 cm × 4.5 cm × 3 cm, and the mass located near the superior renal pole measured 23 cm × 14 cm × 9 cm (Figure 3). Serial sections of the mass and the kidney revealed it to be fully circumscribed and separate from the renal parenchyma. The mass was homogeneously yellow without stigmata of necrosis or hemorrhage. A second well-circumscribed, intrarenal mass, measuring 2 cm × 1.8 cm × 1 cm, was also identified within the inferior midportion of the renal cortex.
Both lesions had similar gross and microscopic features, with predominant adipose tissue and smaller areas of smooth muscle with epitheloid features and characteristic abnormal vessels. The larger lesion was distinct and separate from the renal parenchyma. HMB-45 staining performed on the larger lesion was positive, which is characteristic for an angiomyolipoma ( Figure 4). Of note, the uterus and cervix had no microscopic abnormalities.

Discussion
Angiomyolipomas (AMLs) are rare complex mesenchymal neoplasms typically arising within the kidney and are composed of mature adipose tissue, smooth muscle cells, and thick-walled blood vessels [22]. Renal AMLs account for 1% of renal lesions, occurring more commonly in women [2] with an overall incidence in the general population of 0.07-0.3% [25]. Renal AMLs are sometimes referred to as hamartomas (a benign tumor-like growth composed of typical cells and tissues found in the area of the body where it occurs, but growing in a nonorganized fashion) or choristomas (a mass of normal tissue found in an ectopic location). Renal AMLs are generally felt to be more like a choristomas than hamartomas since kidneys do not normally contain smooth muscle or adipose cells [27,28]. The presence of perivascular epithelioid cells (PEC) is often used to characterize angiomyolipomas since these cells show immunoreactivity for muscle markers (epithelial membrane antigen, keratin, vimentin, desmin, and actin) and HMB-45 [29]. Positive immunoreactivity for HMB-45, a monoclonal antibody raised against a melanoma-associated antigen, is characteristic of AMLs and can be used to differentiate AMLs from other similar appearing lesions such as liposarcomas, lipomas, leiomyosarcomas or, leiomyomas [20,25].
Computerized tomography (CT) and computerized tomographic angiography (CTA) are the most commonly used imaging modalities to investigate AMLs. Wang et al. [28] analyzed the radiologic abdominal CTs characteristics of retroperitoneal extrarenal AMLs (ERAMLs) in an effort to distinguish them from liposarcomas. These authors noted that retroperitoneal ERAMLs typically display aneurysmal dilatation of the intratumor vessels, intratumoral linear vascularity, bridging veins, beak sign, hematomas, and discrete intrarenal/extrarenal fatty tumors, yet none of these are pathogneumonic. Magnetic resonance imaging (MRI), may also be used in conjunction with CT imaging and is particularly useful in delineating the anatomical relationship between ERAMLs, the kidney, and its vasculature, especially when dealing with perinephric and retroperitoneal AMLs. Brain CTs are recommended for patients with renal AMLs since 30-40% of these patients may also have features of tuberous sclerosis (TS) and similarly 80% of patients with TS will develop renal AML [30][31][32]. The brain CT of these patients typically demonstrates characteristic periventricular subependymal nodules with calcifications [2].  Surgery, and less often tumor embolization, is the primary treatment for ERAMLs. Surgical excision is indicated for symptomatic, complex appearing, radiologically enlarging, or large ERAMLs, which also have a higher tendency to bleed. In patients who present symptomatically with retroperitoneal hemorrhage, selective arterial embolization has been used effectively to control hemorrhagic lesions in hemodynamically unstable patients, resulting in tumor involution and subsequently allowing for elective resection or clinical observation [21,23,24]. Surgical resection is always advocated to differentiate suspected ERAMLs from other retroperitoneal lesions. Definitive diagnosis also dictates the length and type of appropriate followup since ERAMLs, unlike malignant retroperitoneal sarcomas or renal/adrenal carcinomas, may require long-term surveillance.
Distinct from renal AMLs, ERAMLs are extremely rare tumors with less than 60 reported cases worldwide in the literature. Friis and Hjortrup reported the first ERAML (1982) [1] involving a 22-year-old female presenting with abdominal pain and weight gain who was found on exploratory laparotomy to have an 11 kg retroperitoneal AML. Ditonno et al. [16] have reported the largest series of ERAMLs, involving 40 cases. In their report, the liver was the most common extrarenal location (N = 18), followed by the uterus (N = 7), retroperitoneum (N = 4), and head and vagina (N = 2 each) as well as one each involving the penis, nasal cavity, hard palate, abdominal wall, fallopian tube, spermatic cord, and colon. Other reported uncommon sites include the mediastinum, [33,34] duodenum, appendix, stomach, and adrenal glands [9].
Retroperitoneal ERAMLs present a unique diagnostic challenge since they must be distinguished from other retroperitoneal masses including retroperitoneal sarcomas, atypical lipomas, adrenal adenocarcinomas, leiomyomas with fatty change, and renal cell carcinomas. Although the majority of ERAMLs are benign, 2 cases of metastatic and recurrent ERAMLs have been reported. Gupta et al. [26] described a case of a 29-year-old male with a history of tuberous sclerosis and a retroperitoneal AML which metastasized to the liver and mediastinum 19 years after initial diagnosis and resection. The second case involved an 80year-old female who developed metastasis to liver and bone one year following surgical resection of a retroperitoneal AML. Although malignant transformation is difficult to predict, high mitotic activity within the primary tumor was a common factor in both metastatic cases. Additionally, certain ERAMLs variants, most notably the epitheloid variants, are thought to be the most aggressive, suggesting a higher likelihood of metastatic transformation and distant spread [26]. Rare cases of AML malignant transformation with lymph node involvement have been documented in the literature [18,[29][30][31][32]; however, all cases involved patients with renal AMLs and tuberous sclerosis.
To date, only 16 cases of retroperitoneal ERAMLs (including our case) have been reported, making the retroperitoneum the second most common extrarenal location of AMLs (Table 2). Among patients with retroperitoneal ERAMLs, the average age was 45 years (ranging from 22 to 80 yrs) with a male: female ratio of 1 to 5.3. Sixtynine percent of patients with retroperitoneal ERAMLs presented symptomatically with nonspecific abdominal pain, 13% presented with incidentalomas, and another 13% with abdominal fullness. The most common imaging modality used to identify the ERAMLs was a CT scan (94% of cases). Retroperitoneal ERAMLs differed widely with respect to size, ranging from 6 cm 3 to 7980 cm 3 and weighing between <1 kg and 11 kgs. The majority of cases (69%) were managed surgically via en bloc radical nephrectomies and in 4 cases a renal-sparing resection was performed. One case was managed with embolization without resection.
56% of patients had follow-up evaluation ranging from 2 to 60 months after surgical resection. Outside the context of tuberous sclerosis, the only reported recurrence to date happened after a radical en bloc nephrectomy with distal metastasis to liver and bone 12 months postoperatively [26]. All other patients have remained disease-free and asymptomatic at last follow-up and no recurrence has been documented after a renal sparing nephrectomy or embolization. To date, the longest documented followup duration is 5 years, with that patient being asymptomatic and without a recurrence. As the only recurrence was documented 12 months after an en bloc radical nephrectomy, these lesions should be followed closely with CT imaging during the first year after resection, with continued yearly followup for 5 years or dictated by symptoms.

Conclusion
Extrarenal angiomyolipomas are rare and occur most commonly in the liver; however, the retroperitoneum is the second most common location. Lesions in the retroperitoneum present a unique diagnostic challenge since they can mimic other retroperitoneal benign and malignant tumors, which must be differentiated. CT scans and MRIs are the diagnostic imaging modalities of choice and are useful in delineating the anatomical relationship of these lesions to the kidney and its vasculature. Hemodynamically stable patients should undergo surgical resection, while unstable patients may benefit from emergent tumor embolization and a subsequently staged surgical resection. Once the pathology specimen is obtained, immunoreactivity to an HMB-45 stain is a useful tool to differentiate ERAMLs from other retroperitoneal tumors. ERAMLs should be analyzed from mitotic index and the presence of epitheloid variant as  these characteristics may be associated with distal metastasis and disease recurrence. To date, only one case of disease recurrence and distal metastasis has been documented in a patient with tuberous sclerosis. This occurred in an 80year-old female one year after a radical en bloc nephrectomy.
Since the longest documented follow-up duration for an ERAML is 5 years, it is recommended that patients undergo serial CT imaging for the first year and be followed for 5 years based on symptoms. The ERAML reported in this paper measured 23 cm × 14 cm × 9 cm (2898 cm 3 ), weighed 1.7 kgs (Table 1), and did not have increased mitotic index or epitheloid variance. Follow-up CT scans every 4 months revealed no recurrence and patient has remained disease-free 16 months postoperatively.