Case Description

The patient is a 61-year-old male who had a CT scan performed for flank pain that revealed a 2.5 cm heterogenous lesion in the right medial perinephric space inferior to the right renal hilum. Past medical history was significant for atrial fibrillation and hypercholesterolemia, and serum creatinine was 1.2.

Repeat CT scan with contrast 6 months later demonstrated a 3.1- × 2.6-cm axial mass just inferior to the right renal artery with soft tissue intermixed with fat and equivocal enhancement along the anterior aspect of the lesion (Fig. 1). Two additional smaller lesions (<1.5 cm) with similar features were noted at the renal hilum and lower pole of the kidney. The patient was counseled on the differential diagnosis of tumors of the retroperitoneum to include liposarcoma, extra-renal angiomyolipoma, hematologic malignancy, secondary metastases or other rare primary soft tissue lesions. He was advised to undergo interventional radiology (IR) guided percutaneous biopsy.

Figure 1. CT scan 3.1- × 2.6-cm axial mass just inferior to the right renal artery with soft tissue intermixed with fat and equivocal enhancement.

Percutaneous 18-gauge core biopsy was performed showing mature adipose tissue infiltrated by delicate collagenous stroma and scattered inflammatory cells. Stromal cells with multilobulated nuclei were observed (Fig. 2). Stains for Melan-A and smooth muscle actin were negative, not supportive of an angiomyolipoma diagnosis, and fluorescence in situ hybridization analysis was negative for MDM2 and CDK4 gene amplification, not supportive of a well differentiated liposarcoma.

Figure 2. Mature adipose tissue and collagenous stroma without significant nuclear hyperchromasia or atypia.

Figure 3. Intraoperative view of soft-tissue packet between right renal vein, inferior vena cava and right ureter.

Figure 4. Gross specimen demonstrating encapsulated 3.9- × 3- × 2.5-cm gray-tan soft tissue nodule.

Due to the remaining possibility of malignancy, the patient was advised to undergo surgical excision of the dominant lesion. He underwent a robotic-assisted laparoscopic excisional removal of the 3 cm mass which was abutting the renal hilum. Intraoperatively, the renal hilum, inferior vena cava, and ureter were dissected and all of the intervening tissue in this anatomic triangle was removed en-bloc (Fig. 3). The mass was moderately adherent to the right renal artery but was carefully dissected away with preservation of the vessels and right kidney. The postoperative course was unremarkable.

Pathological review of the mass showed a 3.9- × 3- × 2.5-cm gray-tan soft tissue nodule (Fig. 4). The lesion was hypocellular with alternating regions of myxoid and collagenous stroma, bland myofibroblasts and benign adipose tissue. Lymphoid follicles, plasma cells and mast cells were seen within the lesion. There was no cytologic atypia or hyperchromasia and no lipoblasts were observed (Fig. 5). Immunohistochemical stains for Melan-A and smooth muscle actin were negative to rule out angiomyolipoma. Final pathological diagnosis was defined as a perinephric myxoid tumor pseudotumor of fat.

Discussion

This case highlights the first minimally invasive resection of a perinephric myxoid pseudotumor of fat and highlights several important clinical points.

First, soft tissue tumors of the retroperitoneum present a diagnostic dilemma. There is a broad differential diagnosis including liposarcoma, leiomyosarcoma, neurofibroma and hematologic entities. These tumors should be managed in multidisciplinary fashion depending on size, location and invasion of surrounding retroperitoneal structures. Primary management includes cross-sectional imaging, IR-guided biopsy and excisional removal. In this case, the patient demonstrated interval growth and enhancement at 6-month reimaging, and IR biopsy excluded metastasis or a hematologic malignancy that would be managed with systemic therapy, thus excision was advised.

Figure 5. A, at low magnification the lesion shows mature adipose tissue embedded in a loose, hypocellular myxoid and collagenous stroma. There are distinctive arborizing blood vessels throughout the lesion. B, on high magnification the cells were without atypia or hyperchromasia.

Second, there are several considerations for patients planned for surgical excision. There are demonstrated benefits of the robotic approach to removal of these lesions, including smaller incisions, earlier convalescence and finer regional dissection. Both size and location impact patient positioning and port placement. In this case, we utilized a standard position and port placement for robotic partial nephrectomy as the mass was in close proximity to the renal hilum. Another decision relates to the extent of en-bloc resection of nearby organs. The traditional approach to known retroperitoneal sarcoma advises the removal of solid organs abutting these lesions. When pathology is unclear, we believe that feasible attempts should be made to spare adjacent organs. In this case, we dissected the tumor away from the inferior vena cava, renal hilum and ureter with minimal difficulty, allowing for maximal renal preservation.

Third, perinephric myxoid pseudotumor of fat is a relatively new pathological entity defined by the presence of mature adipose tissue, myxoid stroma, collagenous septa and inflammatory cells with or without arborizing vessels. Initial theories suggest that nonneoplastic renal diseases such as end-stage renal disease, diabetes or pyelonephritis can induce changes within the perinephric fat, leading to abnormal cellular growth.^1–3 When presented with a fat-containing lesion in the retroperitoneum, the main differential diagnosis is a well-differentiated or de-differentiated liposarcoma. Histological features of liposarcoma include adipocytes with nuclear enlargement and hyperchromasia, fibrous septa with atypical fibroblasts, multivacuolated lipoblasts and/or a frank sarcomatous component. None of the features of liposarcoma should be present in a perinephric myxoid pseudotumor of fat,⁴ and negative fluorescence in situ hybridization analysis for MDM2 and CDK4 can provide further clinical evidence to rule out this malignancy.

Several other fat-containing lesions may be considered based on the histological features of perinephric myxoid pseudotumor of fat. Although the presence of fat, myxoid stroma and arborizing vessels may be reminiscent of myxoid liposarcoma, this sarcoma most often occurs on the extremities, contains lipoblasts and ovoid tumor cells, and has a characteristic translocation involving DDIT on chromosome 12. Angiomyolipoma can present as a perinephric fat-containing tumor, but this lesion contains spindled to epithelioid eosinophilic tumor cells and large vessels, and expresses melanocytic markers such as Melan-A or HMB-45 by immunohistochemistry. Finally, the presence of inflammatory cells may raise the possibility of an IgG4-related disease such as mesenteric fibrosis, but storiform fibrosis and obliterative phlebitis are not features of perinephric myxoid pseudotumor of fat.

At this time, there are 14 previously reported cases in the literature.⁵ There have been no reports of progression to malignancy, thus imaging surveillance after resection seems to be the most appropriate method of followup. Management of nonneoplastic renal diseases may aid in reducing recurrence risk; however, this stems from suspected pathogenesis of this disease.