Dermatology MCQ - Infiltrative and Neoplastic Disorders - Trichilemmal cysts

A 45-year-old woman presents with a smooth, mobile, firm, subcutaneous nodule on her scalp that has been present for several years. It is non-tender and measures 2 cm in diameter. The overlying skin is normal without a punctum. The lesion is surgically excised, and the pathology report describes a cyst wall composed of pale-staining epithelial cells without a granular layer, showing palisading of the nuclei. The cyst contents are described as homogeneous, eosinophilic, and compact. What is the diagnosis?

A. Epidermoid cyst
B. Pilar cyst (Trichilemmal cyst)
C. Dermoid cyst
D. Steatocystoma
E. Lipoma

Correct Answer: B. Pilar cyst (Trichilemmal cyst)

Answer and Explanation

The correct answer is B. This question provides the classic clinical presentation (scalp nodule) and the pathognomonic histology of a pilar cyst, also known as a trichilemmal cyst. The key histological features are:

• Cyst wall without a granular layer: This distinguishes it from an epidermoid cyst, which has a granular layer.

• Palisading of the nuclei of the epithelial cells.

• Homogeneous, compact (trichilemmal) keratin: This is in contrast to the laminated keratin of an epidermoid cyst.

Clinically, the lack of a punctum is also a clue, as pilar cysts typically do not have a visible central punctum like epidermoid cysts often do.

Why the Other Options are Incorrect:

• A. Epidermoid cyst: This cyst is lined by stratified squamous epithelium with a prominent granular layer and is filled with laminated keratin. It commonly has a central punctum.

• C. Dermoid cyst: This is a congenital cyst that contains adnexal structures like hair follicles and sebaceous glands within its wall. It is not typically found on the scalp in isolation and has a different histological appearance.

• D. Steatocystoma: The cyst wall of a steatocystoma is characteristically thin and lined by eosinophilic, corrugated cuticle, and it often has sebaceous glands within its wall. The contents are an oily liquid, not solid keratin.

• E. Lipoma: A lipoma is a benign tumor of adipose tissue, not a cyst. Histology would show mature adipocytes, not an epithelial cyst wall.

Additional High-Yield Information for Exams:

• Histopathology: As described above, the absence of a granular layer and the presence of trichilemmal keratinization (where the outer root sheath of the hair follicle matures directly into keratin without forming a granular layer) is diagnostic.

• Clinical Features:

  • Location: Over 90% are found on the scalp.

  • Number: Can be solitary or multiple. Multiple cysts can run in families as an autosomal dominant trait.

  • Consistency: They are often firmer and more mobile than epidermoid cysts.

• Differential Diagnosis: The main clinical differential on the scalp is an epidermoid cyst. A lipoma would be softer.

• Associated Conditions & Prognosis:

  • Benign Nature: Pilar cysts are benign.

  • Proliferating Trichilemmal Cyst (Pilar Tumor): This is a benign but locally proliferative tumor that arises from a pilar cyst. It can be locally aggressive and, very rarely, may undergo malignant transformation into a malignant proliferating trichilemmal tumor.

  • Prognosis: Excellent. Simple excision is curative.

• Management & Rationale:

  • Rationale: For an asymptomatic cyst, the goal is reassurance. For a symptomatic or cosmetically concerning cyst, the goal is complete surgical removal to prevent recurrence.

  • Asymptomatic Cyst: Observation and reassurance.

  • Definitive Treatment: Surgical excision. Due to their common location on the scalp, it is important to be aware of the underlying anatomy. The cyst wall is often more fragile than that of an epidermoid cyst, making complete removal without rupture a surgical challenge. If the cyst wall is not entirely removed, recurrence is possible.