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Trichilemmal Cyst


KEY FACTS

Terminology

  • Trichilemmal cyst (TC) is preferred term

    • Sebaceous cyst commonly used but incorrect

    • Variant = proliferating trichilemmal tumor

  • Keratin-containing cyst

    • Lined by stratified squamous epithelium

    • Pathology looks like root sheath of hair follicle

Imaging

  • General features

    • Most within dermis or subcutaneous tissue

    • Size varies (few millimeters to several centimeters)

    • Can be single or multiple

  • CT

    • Round/ovoid, well-delineated scalp mass

    • Multifocal punctate/curvilinear/coarse Ca++

  • MR

    • Isointense with brain, muscle on T1WI

    • Inhomogeneously hypointense on T2WI

    • Does not suppress on FLAIR

    • Multifocal “blooming” foci common on T2*

    • Simple sebaceous cysts usually do not enhance

Top Differential Diagnoses

  • Basal cell, squamous cell carcinoma

  • Dermoid cyst

  • Epidermoid cyst

  • Metastasis

  • Cephalocele

Clinical Issues

  • Classic presentation

    • Subepidermal scalp mass in women > 60 years of age

    • Hairless, rubbery, nontender, mobile subcutaneous scalp mass(es)

    • Usually nonpainful

Axial NECT in a 63-year-old man was obtained for trauma evaluation. A superficial scalp mass
that contained several hyperintense foci suggesting calcifications
was incidentally noted.

Axial bone CT in the same patient shows that the scalp mass
is sharply delineated. Multiple punctate and curvilinear calcifications
within the mass can be seen. Note the lack of bone erosion or invasion. The remainder of the scalp appears normal. This was presumed to be a sebaceous cyst.

Sagittal T1WI MR in a 68-year-old woman with headaches shows 2 large but very well-circumscribed scalp masses
. The masses are incompletely surrounded by fat and isointense with brain and muscle.

Axial T2WI MR in the same patient shows the mass
to be inhomogeneously hypointense. Both lesions demonstrated some internal “blooming” on T2* that suggested calcifications. These lesions had been slowly enlarging over many years and are benign proliferating trichilemmal cysts.

TERMINOLOGY

Abbreviations

  • Trichilemmal cyst (TC) is preferred terminology

    • “Sebaceous cyst” (SC) commonly used but incorrect

  • Variant = proliferating trichilemmal tumor (PTT)

Synonyms

  • PTT also called “pilar” or “turban” tumor

Definitions

  • Keratin-containing cyst

    • Lined by stratified squamous epithelium

IMAGING

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