Numerous large nodules on scalp

Diagnosis: Pilar cysts

Pilar cysts (PC), also known as trichilemma cysts, wen, or isthmus-catagen cysts, are benign cysts that manifest as smooth, firm, well-­circumscribed, pink nodules. PCs originate from the follicular isthmus of the hair’s external root sheath¹ and are found in 5% to 10% of the US population.² Possible sites of appearance include the face, neck, trunk, and extremities, although 90% of PCs develop on the scalp.¹ They tend to have an autosomal dominant pattern of inheritance with linkages to the short arm of chromosome 3.³ PCs can occasionally become inflamed following infection or trauma.

Characteristic histology of PCs demonstrates semisolid, keratin-filled, subepidermal cysts lined by stratified epithelium without a granular layer (trichilemmal keratinization). Lesions excised from this patient’s scalp showed 2 subtypes of PCs: nonproliferating (FIGURE 2A) and proliferating (FIGURE 2B). Subtypes appear similar on exam but can be differentiated on histology.

With gradual growth, proliferating PCs can reach up to 25 cm in diameter.¹ Rapid growth, size > 5 cm, infiltration, or a non-scalp location may indicate malignancy.⁴

Differential diagnosis includes lipomas

The differential diagnosis for a lesion such as this includes epidermal inclusion cysts, dermoid cysts, and lipomas. Epidermal inclusion cysts have a punctum, whereas PCs do not. Dermoid cysts are single congenital lesions that manifest much earlier than PCs. Lipomas are easily movable rubbery bulges that appear more frequently in lipid-dense areas of the body.

For this patient, the striking turban ­tumor–like presentation, with numerous large cysts on the scalp, initially inspired a differential diagnosis including several genetic tumor syndromes. However, unlike the association between Gardner syndrome and numerous epidermoid cysts or Brooke-Spiegler syndrome and spiradenomas, no syndromes have been linked to numerous trichilemmal cysts.

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