Giant perirenal liposarcoma mimicking renal angiomyolipoma

A 44-year-old otherwise healthy woman presented with rapid painless growth of a right-sided abdominal mass.

The abdominopelvic computed tomography scan (CT-scan) with contrast showed a 14 cm well-circumscribed heterogenic fat-containing exophytic mass surrounded by a contrast-enhanced brim – initially interpreted as a peripheral hematoma. The mass was located at the posterolateral face of the right kidney with a minimal cortical defect and arterial supply from the lower kidney pole. After the initial CT scan, a three-phased CT scan (non-enhanced, arterial & portal phase) was performed. It showed no ongoing bleeding and modest but non-significant enhancement of the previously described contrast-enhanced brim. The initial interpretation of a peripheral hematoma did not change following the secondary CT scan. Following multidisciplinary team conference, the patient underwent two-step treatment with coiling followed by surgical resection two weeks later. The preoperative impression being a hemorrhagic angiomyolipoma. Gross examination showed a yellowish fatty tumour. The tumour was surrounded by thick fibrous tissue – most likely coinciding with the contrast-enhanced brim, which was interpreted as a hematoma on CT. Microscopically, the tumour was a well-differentiated liposarcoma. No invasion of the renal parenchyma was observed. Neither was there any proliferation of vascular or muscular tissue to suggest a tumour originating from the fatty tissue of an angiomyolipoma. The patient was referred to a sarcoma centre postoperatively.

When looking at fat-containing tumours of the retroperitoneum, it can be challenging for radiologists to differentiate a perirenal liposarcoma from an exophytic renal angiomyolipoma by CT. However, some differences in CT findings have been demonstrated: the most reliable difference is the tumour's site of origin. Renal angiomyolipomas originate from the renal parenchyma, whereas perirenal liposarcomas originate from the adipose tissue in the retroperitoneum [1–4]. In most patients with renal angiomyolipomas, a renal parenchymal defect is identified on CT, whereas cortical defects rarely are present in liposarcomas [1–4]. Because of this, a careful evaluation of the renal parenchyma in search of a cortical defect or the classic “beak sign” could be helpful. A parenchymal defect, as well as the presence of a tumoral vessel extending into the renal cortex, could represent the tumour's site of origin, favouring an angiomyolipoma [1–4]. Intratumoral calcifications and heterogeneously enhancing nodules are indicators of liposarcoma [1]. Furthermore, encapsulation, as well as severe anterior displacement of the ipsilateral kidney, are more prevalent for liposarcomas. Angiomyolipomas originating from the posterior part of the renal parenchyma can also be the cause of anterior displacement, however not as severely [1]. Haemorrhage is a known complication of angiomyolipomas as they are composed of dysmorphic blood vessels, in addition to adipose tissue and smooth muscle [2,6]. As liposarcomas have not been associated with haemorrhage [1,3], intratumoral and/or perirenal haemorrhage points to an angiomyolipoma.

Retroperitoneal liposarcomas are usually malignant from inception, can develop a rapid growth rate and tend to reach great size before producing symptoms and/or a diagnosis is made. Because of this, complete surgical resection can be challenging, and as they are associated with a high rate of recurrence, they often require long-term follow-up after resection [5]. Contrasting this, angiomyolipomas are typically benign, grow slowly, and only a very rare subtype has been found to have a malignant potential. Conservative management with radiological follow-ups is thus possible with asymptomatic angiomyolipomas. However, as the risk of haemorrhage is higher in angiomyolipomas with a size ≥4 cm and intralesional aneurysms >5 mm, embolization and/or surgical resection might be indicated [2,6,7].

Differentiating between perirenal liposarcoma and renal angiomyolipoma radiologically can be difficult. However, since management and prognosis differ, achieving the accurate diagnosis can have significant impact on patient treatment. In case of larger heterogenic fatty tumours without evident origin in the kidney, one must consider the possibility of a rare liposarcoma.