Epidermoid (Inclusion) Cyst – Multimodal Imaging

Keywords: Thin capsule , subcutaneous location

🔹 Clinical:

Slowly growing, subcutaneous lesion

Located close to the skin surface

Stable for more than 2 years

🔹 CT findings (non-contrast, 2 years ago):

Central cystic appearance

Surrounding dirty fat planes → chronic inflammatory reaction

Stability over time favors a benign process

🔹 MRI findings (current):

T1: Hypointense

T2: Mildly hyperintense (not as bright as pure fluid, due to keratin/debris)

Capsule: Thin, well-defined

Base: Sitting on the subcutaneous fat

Protrusion: “Beak/drop-shaped tip” extending from dermis into subcutis

🔹 Why the “beak-shaped protrusion”?

The cyst capsule attaches at the dermis–subcutis interface.

Due to the keratin/debris inside, the cyst tries to expand according to its own volume.

Because of the flexible but limited space between the skin and subcutaneous tissue, as the cyst grows, it narrows downward, forming a pointed tip—appearing “beak/drop-shaped” on MRI.

🔹 Key teaching points:

CT: Dirty fat sign = chronic, benign process

MRI: Beak-shaped protrusion + typical signal intensity

Multimodality + stability strongly support an epidermoid cyst

Differential: lipoma, necrotic lymph node, abscess

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