Pseudotumour of the liver

Clinical History

Focal fat sparing and infiltration are common physiologic processes in the liver. Knowledge of these is important as they can mimic malignant pathology on imaging. Here we describe a patient with a previous history of malignancy who had focal fat sparing in an atypical location.

Imaging Findings

A 67-year-old male with a history of type 2 diabetes was incidentally found to have a malignant left renal tumour on imaging. The tumour was treated with radical nephrectomy. Histology confirmed a clear cell renal carcinoma. The patient re-presented 17 months later with symptoms of urinary retention. Ultrasound examination of the abdomen demonstrated diffuse fatty infiltration of the liver with a 4x2 cm hypoechoic lesion in segment 2 (figure 1). Triple phase CT of the liver was performed for further evaluation. On the precontrast images the liver was generally of low attenuation in keeping with fat infiltration. Though the lesion in Segment 2 was not better visualised on the precontrast images it was hyper dense on both the arterial and portal phases (figure 2). Imaging appearances were not typical of a malignant lesion as it had a geographic configuration and lacked local mass effect. A radical of the left portal vein was seen to traverse the lesion with no evidence of displacement (figure 3). A probable diagnosis of focal fatty sparing was made but given the atypical location and the previous history of malignancy further imaging was performed to confirm the diagnosis. Although MRI would have been the imaging modality of choice this was unavailable locally during this time period. However, an interval CT scan was performed in 6 months which showed no change in appearance, thus confirming the diagnosis.

Discussion

Fat infiltration of the liver is a common finding on cross sectional imaging. Typical patterns include diffuse infiltration, diffuse infiltration with focal sparing and focal accumulation. Fatty liver may be idiopathic or secondary to a wide spectrum of conditions such as alcohol use, obesity, diabetes mellitus, steroid therapy, and viral hepatitis. The prevalence of fatty liver is estimated to be between 15-95% with it being higher in excess alcohol consumption, hyperlipidaemic states and obesity [1]. Diffuse fatty infiltration of the liver with focal sparing can simulate a metastatic lesion especially if seen in an atypical location. Therefore, this diagnosis must be made with caution in patients with a known or suspected malignancy. However, focal fat sparing of the liver has characteristic features on cross sectional imaging as described below and awareness of these will avoid the need for a biopsy which is the definitive test for the diagnosis of a focal hepatic lesion. Focal fat sparing of the liver usually occurs in segment 4, segment 5, in the gall bladder fossa and porta hepatis [2]. Occasionally focal fat sparing occurs outside these typical locations, with segment 2 estimated to show these changes in 0.9%-6% of patients with diffuse fat infiltration of the liver [3]. The most likely cause for fat sparing in these locations is thought to be due to reduced portal flow leading to reduced accumulation of triglycerides [4]. Focal fat sparing of the liver may be diagnosed on cross sectional imaging, although focal fat sparing can be more of a diagnostic challenge than homogeneous fat infiltration as they can simulate a mass lesion. Imaging features supporting focal fat sparing are absence of mass effect, lack of vessel displacement, typical location, geographic configuration rather than round or oval shape and contrast enhancement similar to that of normal liver. On ultrasound focal sparing in a fatty liver has the appearance of a spherical or ovoid hypoechoic lesion on the background of a diffusely hyperechoic liver [6]. Target like appearances on ultrasound have also been described [7]. Appearances on ultrasound are generally nondiagnostic and further characterisation with CT and/ or MRI is usually required [7]. At both unenhanced and enhanced CT these areas are relatively hyperdense compared with the surrounding liver parenchyma [8]. Chemical shift gradient-echo imaging with in-phase and opposed-phase acquisitions is the most widely used MRI technique for the assessment of fatty liver. On opposed- phase imaging there is signal drop out in the surrounding fat infiltrated parenchyma thereby highlighting focal fat sparing as areas of relative hyperintensity [9]. Unlike metastases these lesions are poorly visualised on both T2 weighted imaging and gadolinium-enhanced sequences. In conclusion, focal fatty sparing of the liver can mimick a metastatic lesion especially if found in an atypical location in a patient with a known malignancy. However, accurate diagnosis with cross sectional imaging is possible preventing unnecessary invasive intervention with biopsy.

Differential Diagnosis List

Final Diagnosis

Atypical focal fat sparing in liver

URL:	https://www.eurorad.org/case/6240
DOI:	10.1594/EURORAD/CASE.6240
ISSN:	1563-4086