Subcutaneous epidermoid cyst mimicking hypervascular lesion on ultrasound

Clinical History

A 52-year-old Caucasian woman reported a firm non-tender mass on the anterior side of the right proximal tibia, at the level of the tuberositas tibiae. The lump had been present for an uncertain amount of time and slowly increased in size. There was no relevant medical history, no trauma.

Imaging Findings

Ultrasound shows a subcutaneous, well-circumscribed lobulated heterogeneous, mildly hyperechoic lesion of 32x29x10 mm (Fig. 1a). Colour Doppler shows internal signals (Fig. 1b), interpreted as hypervascularity.

Conventional radiograph (Fig. 2) shows a soft tissue swelling without matrix formation.
On MRI (Fig. 3) the signal intensity of the lesion is slightly hyperintense relative to muscle on T1W-imaging. On T2-Dixon the lesion is hyperintense. After i.v.-gadolinium injection, a “ring-like” enhancement pattern on T1W-imaging and dynamic MR imaging with subtraction was shown (pseudo-enhancement), no internal enhancement. Based on MRI, the lesion was diagnosed as a cyst, most likely an epidermoid cyst. This was surgically proven.

Thus, MRI showed an avascular lesion, posing the suspicion of an artefact on ultrasound. Close inspection of the ultrasound images reveals only focal coloured areas superficial to the level of the focal zone, based on a twinkling artefact. Also the posterior acoustic enhancement is appreciated, consistent with fluid content.

Discussion

The twinkling artefact appears as rapidly alternating colour Doppler signals behind irregular or rough reflective surfaces, imitating turbulent flow. [1] It is most frequently encountered in calcified lesions. The intensity of the twinkling artefact is affected by several ultrasound machine settings such as gain settings and focal depth. [2] For example, the twinkling artefact is enhanced when the focal zone is placed at the same level or below the rough reflective surface, also visible in Fig. 1b.

Epidermoid cysts are common slow-growing subcutaneous cysts that contain keratin and are lined by stratified squamous epithelium. Epidermoid cysts are either found incidentally or present as a firm non-tender lump. US features of epidermoid cysts are well-circumscribed, oval or lobulated, heterogeneously mildly echogenic masses with posterior acoustic enhancement. [3]

Previously, the association between epidermoid cysts and the twinkling artefact was described by Clarke et al. [4] In this retrospective study, the twinkling artefact was observed in 17 histologically proven superficial epidermoid cysts. It is not fully understood what causes the twinkling artefact in epidermoid cyst, however it was suggested to be caused by the strong reflecting interfaces arising from keratin layers. [4]

In this particular patient, the twinkling artefact on ultrasound mimicked a hypervascular lesion and the lesions was subsequently suggested to be hypervascular tumour. Additional MRI images revealed the lesion to be a cyst without enhancement. Closer inspection of the ultrasound images showed that the “flow” was in fact caused by the twinkling artefact.
Radiologists should be familiar with the twinkling artefact in epidermoid cyst to avoid misinterpretation and unnecessary diagnostic follow-up.

Differential Diagnosis List

Final Diagnosis

Epidermoid cyst mimicking hypervascular lesion on ultrasound caused by twinkling artefact.

URL:	https://www.eurorad.org/case/15639
DOI:	10.1594/EURORAD/CASE.15639
ISSN:	1563-4086