Contrast-enhanced ultrasonography of solitary necrotic nodules of the liver

A 64-years-old woman was admitted to our hospital with a diagnosis of suspected cholangiocarcinoma, detected with US, MRI and CT performed in another Institute. The patient had no history of tumours but had a history of psoriatic arthritis, gout and arterial hypertension. After the onset of a non-specific abdominal pain in January 2016, a US exam was performed with the finding of a hypoechoic hepatic nodule of 1.2 cm, showing no specific sonographic characteristics, but worthy of further analysis (Figure 1A).

On the MRI that followed, this lesion was hypointense on T1, isointense on T2, hypovascularized, surrounded by a thin capsule which enhances on post-contrast phases and without diffusion restriction, described as a suspicious nodule compatible with cholangiocarcinoma (Figure 1A - G). The CT performed later documented a hypodense nodule with a thin rim of enhancement on arterial and venous phase, supporting the diagnosis of cholangiocarcinoma with no significant central delayed enhancement due to the small dimensions. A cytological biopsy was suggested. At our Hospital the woman was examined with US and CEUS in April 2016 (Figure 2, 3). Harmonic micro-bubble specific imaging with low acoustic ultrasound intensity (2- to 4-MHz coherent contrast imaging; mechanical index, <0.2; 12-13 frames per second) was performed on a Sequoia 512 6.0 system (Acuson, Siemens media solutions, Mountain View, CA), with a 2.4 mL intravenous bolus injection of sulphur hexafluoride in the form of microbubbles (SonoVue, Bracco, Milan, IT). Observation of the target lesion was continuous with a dynamic study from the unenhanced phase to the late contrast-enhanced phase. During all the phases, the lesion was completely avascular. Two days later, a fine-needle aspiration biopsy was executed and the cytological examination found no neoplastic characteristics, but a necrotic-abscessualized lesion, leading to the diagnosis of a solitary necrotic nodule. Consequently, an annual follow up was programmed at our Institute by MRI (1.5T Magnetom Symphony system, Siemens Healthcare, Erlangen, Germany), showing stability of the lesion. After three years, in March 2019, another CEUS was performed, confirming the previous diagnosis and ending the surveillance.

Solitary Necrotic Nodule (SNN) of the liver is a rare non-malignant mass-forming lesion. Most of the patients are asymptomatic and SNNs are usually first detected incidentally at conventional ultrasonography (US), computed tomography (CT) and magnetic resonance imaging (MRI) [1]. About 62 cases had been reported since 1983 [2].

The first to describe the SNN of the liver were Shepherd and Lee [3], who found four cases of necrotic core surrounded by a dense collar of hyalinised collagen, incorporating elastic fibres. Since then, many hypotheses of different aetiologies were made, such as traumatic origin [3], previous infection remnants [3], parasitic infestation [4], sclerosing haemangioma [1], but there is still no certain pathogenesis.

SNNs have similar imaging findings to those of necrotic metastases or other necrotic hepatic masses [5] and are often misinterpreted [3, 6-10]. For this reason, it is fundamental to make the correct diagnosis without overestimating and overtreating such non-malignant lesions.

Contrast-enhanced ultrasonography (CEUS) has been increasingly used for the study of hepatic lesions [11]; in fact, CEUS allows a real-time continuous evaluation of the lesion enhancement during the different phases and it is particularly useful in demonstrating the complete avascularity of these lesions.

However, the reported experiences of CEUS in SNN of the liver are extremely limited. In fact, to our knowledge, there are only two papers in literature [10, 12-15] describing the use of CEUS in these lesions.

The purpose of this case report is to remember that for lesions too small to be safely characterized with conventional US, CT or MRI and showing no enhancement on CEUS, the rare entity of SNN should be included in the differential diagnosis.