Teaching Case

A 63-years old female referred to the radiology department complaining for vague abdominal pain. Routine abdominal US was normal, except for the incidental finding of a right renal lesion not related to patient’s symptoms. Physical examination and blood tests were unremarkable, as well as urinalysis. Abdominal CT was performed.

Abdominal US incidentally revealed a 2 cm hyperechoic lesion of the lower third of the right kidney located in the renal sinus (Figure 1). Subsequent abdominal CT confirmed an ovoid, solid lesion of the right renal sinus with smooth margins: on unenhanced scans the lesion appeared homogeneous, with similar density to normal renal parenchyma and with an eccentric, small punctate focus of macroscopic fat (Figure 2a). No calcifications were seen. After contrast administration, the lesion was markedly hypervascular with prominent wash-out on venous and equilibrium phase (Figures 2b-d and 3).

MRI was not performed.

The presence of a small focus of macroscopic fat and the absence of calcifications raised the suspicion of renal angiomyolipoma containing minimal fat, as subsequently confirmed by US-guided percutaneous biopsy.

Angiomyolipoma (AML) is one of the most common benign tumors of the kidney, found in approximately 0.3 - 2.1% of the general population [1-3]. It is typically composed by a variable amount of three elements: dysmorphic blood vessels, smooth muscle components and mature adipose tissue [3]. The large majority of renal AMLs contain enough macroscopic fat to be easily identified on CT and MRI, making the diagnosis pretty straightforward. Despite this, approximately 5% of renal AMLs show no or little fat and may mimic renal cell carcinoma (RCC) [1-3]: therefore, preoperative diagnosis is crucial in order to avoid unnecessary biopsies or surgery. Song et al. classified renal AMLs into fat-rich, fat-poor and fat-invisible upon their CT and MRI features [4]. Jinzaki et al. divided sporadic renal AMLs into classic, fat-poor or epithelioid on the basis of clinical, imaging, pathologic and genetic features [3]. The main differential diagnosis of AMLs, especially the fat-poor and fat-invisible subtypes, is with RCC; however, the presence of fat into a RCC is usually accompanied by calcifications, as the pathogenesis is thought to be due to osseous metaplasia leading to marrow fat formation [3; 5-7]. On the other hand, the presence of calcifications is very rare in renal AMLs, so it is generally considered appropriate to diagnose an AML when a fat-containing, non-calcified renal mass is encountered in an adult. Percutaneous biopsy should be only performed when the differential diagnosis between AML and RCC cannot be made on either CT or MR imaging. In our case, even if the presence of a punctate focus of macroscopic fat was in favor of a fat-rich AML, US-guided biopsy was performed in order to exclude a rare case of fat-containing, non-calcified RCC [3; 8; 9].

Teaching points

• Renal AMLs are typically composed by a variable amount of dysmorphic blood vessels, smooth muscle components and mature adipose tissue.
• Approximately 5% of renal AMLs show no or little fat and may mimic renal cell carcinoma (RCC).
• The presence of fat into a RCC is usually accompanied by calcifications, as the pathogenesis is thought to be due to osseous metaplasia leading to marrow fat formation.
• It is generally considered appropriate to diagnose an AML when a fat-containing, non-calcified renal mass is encountered in an adult.

All patient data have been completely anonymised throughout the entire manuscript and related files.