Clinical History
A 75-year-old female patient without significant past medical history was admitted to emergency department complaining of left lower abdominal pain. Urinalysis and laboratory assays were within normal limits for her age. On further questioning, she denied symptoms related to the urogenital organ system.
Imaging Findings
Multidetector CT, requested to investigate suspected acute diverticulitis, disclosed sigmoid diverticular disease without acute inflammatory changes, and occupation of the left perirenal space by a large retroperitoneal expansile lesion with predominant fat density, central vessels, and minimal contrast enhancement without solid tissue. Adipose nature of the mass with unrestricted diffusion was confirmed by MRI including fat-suppression techniques.
On both multiplanar CT and MRI images, focal indentations of the renal parenchyma were identifiable, indicating renal origin of the perinephric mass. Furthermore, enlarged vessels originated from the renal pole to reach the central adipose mass. Findings allowed confirming exophytic renal angiomyolipoma, excluding a well-differentiated retroperitoneal liposarcoma.
Surgery was indicated considering the haemorrhagic risk. Complete laparotomic resection of the suprarenal mass was performed, with detachment of the infiltrated upper pole and afferent vessels (corresponding to imaging findings) and kidney preservation. Pathology reported yellowish mass with morphologic and immunochemistry features consistent with angiomyolipoma.
Discussion
Mesenchymal renal neoplasms in adults encompass a wide spectrum of lesions with characteristic histology, variable biologic behaviour and imaging findings [1]. Among them, angiomyolipoma (AML) represents the most common benign mesenchymal lesion of the kidney, with a prevalence approaching 1-3% in the general population, particularly frequent in middle-aged women and in tuberous sclerosis patients. Composed of variable proportions of mature adipose tissue, dysmorphic blood vessels, and smooth muscle, AML is histologically a hamartoma, which usually may range in size from a few millimetres to very large, well-circumscribed expansile masses, often with exophytic growth. When bulky, AML may extensively replace the perinephric space, and surgical removal or embolisation is proposed to prevent the risk of spontaneous, life-threatening haemorrhage (25% of cases) [1, 2].
Furthermore, large exophytic renal AMLs are not easily distinguished from well-differentiated liposarcomas because of their similar imaging, macroscopic pathology and histological features. Liposarcoma (LPS) represents the most common primary retroperitoneal malignancy, commonly involves the peri- or pararenal spaces, grows slowly but requires surgical removal with frequent need for nephrectomy and common postoperative recurrences [3, 4].
AMLs and LPSs occur mostly in middle-aged people and remain asymptomatic until they become large. At imaging, both entities appear as well-circumscribed masses with variable proportion of macroscopic fat, non-fat attenuation areas corresponding to vascular structure, fibrosis and muscle, and mild contrast enhancement. Predominant intralesional fat is diagnosed on the basis of negative CT attenuation values and hyperintense MR signal with suppression by frequency-selective fat saturation. Since size overlaps between the two different lesions, our aim is to describe and exemplify the two useful cross-sectional imaging findings that allow their confident differentiation [2, 3].
Firstly, since exophytic AML originates from the kidney, it is associated with an identifiable apex within the renal parenchyma or focal defect at its origin, a finding that has been termed the ‘angular interface sign’. Conversely, LPS arises in the retroperitoneum, then grows to displace, surround or compress the kidney, but does not invade the renal parenchyma, therefore the interface of the lesion with the kidney is smooth [3, 4].
Secondly, radiologists should consider that AML commonly shows enlarged vessels on enhanced CT or MRI studies, whereas well-differentiated LPSs are relatively avascular [3, 4].
Differential Diagnosis List
Large, exophytic renal angiomyolipoma.
Retroperitoneal liposarcoma
Renal cell carcinoma
Lymphoma
Final Diagnosis
Large, exophytic renal angiomyolipoma.
