Teaching Case

58-year-old female patient. During a routine blood test, an increase of the hepatic enzymes was seen. The patient was asymptomatic and she had no history of hepatic chronic disease. Clinical examination was anodyne. An abdominal ultrasound was carried out.

During the abdominal ultrasound examination, an heterogeneous, well delimited, 6 cm mass, is seen in the left hepatic lobe. It presents as mainly hyperechoic with hypoechoic areas (1).

A dynamic CT is carried out. The lesion is subcapsular, located in the II hepatic segment. In the basal phase it shows macroscopic fat density areas within an hypodense parenchyma (2). During the early arterial phase the tumor shows a strong and heterogeneous enhancement, with areas remaining hypodense (3). In the portal phase the lesion is still slightly hyperdense, returning isodense in the delayed venous phase (4). Knowing the vascular behaviour of the lesion with the CT, the MRI was discarded.

The differential diagnosis included lesions with macroscopic fat (PEComa and hepatocarcinoma). The percutaneous biopsy was dismissed because of its high risk of bleeding, so an hepatic segmentectomy was decided. The anatomopathological study confirmed the tumor as a primary hepatic PEComa.

The perivascular epithelioid cells were proposed in 1992 by Bonetti et al [1]. In 2002, the World Health Organization defined PEComa as unusual mesenchymal tumours composed of histologically and inmunohistochemically distinctive perivascular epithelioid cells [2-4]. Nowadays, this family of tumours includes different entities, among which are classic angiomyolipomas (AML) or lymphangioleiomyomatosis (LAM) [2-4].The PEComas have been reported throughout the body. Primary hepatic tumors are very rare, with less than 40 cases published in the English language.

The hepatic PEComas develop mostly in women, with median age of 50 years and no history of chronic hepatic disease. The etiology is still uncertain and there are several hypotheses based on its histogenesis. Although the association between renal AML and tuberous sclerosis is well known, the relation with the hepatic PEComa is much less frequent and still uncertain [3-5].

The hepatic PEComas are usually asymptomatic, being diagnosed accidentally in physical examinations.
The imaging features depend on the quantity of adipose tissue and the smooth muscle cells that the tumor contains.
In ultrasonography images the lesion can be hypo/iso/hyperechoic with characteristic hypervascularity in color Doppler examination.
In a non-contrast CT the hepatic PEComa is displayed as a well delimited subcapsular mass with heterogeneous low density. During the dynamic evaluation, it shows arterial hyperenhancement, with changeable behaviour in venous phase. In the delayed phase the neoplasm is hypo or isoenhanced. The lipid component is hypodense in all phases.
On MRI the tumour appears hypointense on T1WI and hyperintense on T2WI. The mass presents strong hyperenhancement after gadolinium injection with attenuation during portal phase [2-5].

The final diagnosis of PEComas is given with the anatomopathological examination. The epithelioid cells show a radial pattern around the vessel lumen and accumulate large amounts of lipid. They show positive inmunostaining for melanocytic and smooth muscle markers. Recent studies shows that PEComa mutation is involved in the mTOR pathway.
The hepatic PEComas are typically benign, but there have been reported some malignant cases with metastases at the diagnoses.

There are no guidelines concerning treatment but, because of its uncertain potential, most of the tumours are removed by surgery.
The prognosis after resection is good unless they present 2 of the 7 worrisome features on the histology described by Folpe et al in 2002 [6].

Primary hepatic PEComas are rare but increasingly recognised tumors and should be taken into account when a subcapsular, well delimited, hypervascular mass with lipid component is seen in a healthy liver.