Cytodiagnosis of simple and proliferating trichilemmal cysts

To elucidate the cytologic findings in simple trichilemmal (pilar) cysts and proliferating trichilemmal cysts (pilar tumors) and the clinical importance of these lesions.

Aspirates from 12 simple pilar cysts and three pilar tumors, all histologically confirmed, were analyzed with a view to elaborating on specific cytologic features enabling a distinction from epidermal cysts and other adnexal tumors.

Aspirates from pilar cysts showed two different pictures, depending on the age of the cyst. Young pilar cysts showed aspirates with an abundant background of blotchy keratin with or without calcification and inflammation. Older (degenerating) cysts showed oily fluid debris with cholesterol crystals and inflammatory cells. The epithelial component was sparse, and only an occasional syncitial cluster of small squamoid cells was seen. As opposed to this, epidermal cysts usually showed a cleaner background, with very cellular aspirates containing many nucleate and anucleate squames, keratin flakes, platelike crystals and no calcification. The pilar tumors showed a lesser amount of keratin as compared to pilar cysts and yielded large and small clusters of squamoid and basaloid cells, a few of which showed an abrupt association with anucleate, keratinized globules.

The cytologic diagnosis of a pilar cyst should be made especially in scalp cysts, which yield either abundant, blotchy keratin or oily, cholesterol-rich debris with a sparse epithelial component and which lack a mixture of anucleate and nucleate squames. On aspiration, pilar tumors yield comparatively more cells and less keratin and show small, basaloid or squamoid cells abruptly associated with keratin globules. The cytologic diagnosis of pilar cysts is important because these cysts recur if incompletely excised and often undergo transformation to pilar tumors. Similarly, pilar tumors often clinically mimic squamous carcinomas, and hence there is a need to outline definitive cytologic features.