Epidermal Cyst: Ultrasonography, Color Doppler Ultrasonography, and Strain Elastography Features. A Pictorial Essay

Purpose or Learning Objective: To describe ultrasonography (US), color Doppler, and strain elastography (SE) features of epidermal cysts (ECs) and the differential diagnosis between ECs and malignant soft tissue tumors based on patterns reported in a literature review and from our experience.

Methods or Background: Epidermal inclusion cysts, also called epidermoid cysts, infundibular cysts, or sebaceous cysts, are benign cutaneous or subcutaneous cysts lined with stratified squamous epithelium and filled with keratin debris. ECs often arise in areas where hair grows and are distinguished as congenital or acquired. Congenital cysts are formed by entrapment of embryonic epithelial remnants, often developing at the head and neck. In contrast, acquired ones may result either from obstruction of the pilosebaceous unit or by implantation of epidermal fragments in the dermis after traumatic injury. They usually have slow growth, are asymptomatic, and rarely transform into malignant tumors. A literature review revealed few descriptions about the role of sonoelastography in the evaluation of an EC.

Results or Findings: In B-mode, ECs can take several forms but often have an oval/spherical shape, with well-defined margins surrounded by a hypoechoic halo. ECs appear as hypo-anechoic formations with inhomogeneous echo texture due to the presence of keratin debris, manifested with linear low-echogenic areas and scattered areas of bright linear echoes and anechoic clefts.

Clinically, it is possible to observe a punctum on the surface of the lesion that corresponds, on US, to a specific sign called the “submarine sign,” described by a focal projection of the hypoechoic portion, regardless of the width, depth, and morphology, toward the epidermis.

ECs do not have a vascular signal on color Doppler as opposed to malignant tumors that are often characterized by the presence of internal or peripheral vascular signals. Sometimes a twinkle artifact can be observed and correlates to the presence of calcification or keratin debris.

SE identifies four color patterns, from red (softer, pattern 1) to blue (harder, pattern 4), based on the stiffness of the lesion and of the pericystic tissue. ECs are equally distributed among these patterns.

Malignant tumors showed higher SE scores (3–4, hard nature) than ECs.

Conclusion: According to our data and references in the literature, we summarized the main features of ECs in US B-mode, color Doppler, SE, and their respective contribution to the differential diagnosis with malignant tumors.

Publication History

Article published online:
26 May 2023