Sebaceous carcinoma arising in a sebaceous cyst: Impossible, because ‘sebaceous cyst’ is a histogenetic misnomer

We read with interest the letter by Kumar et al. on malignant transformation in a sebaceous cyst.¹ Though they state that such a transformation is ‘uncommon but not impossible’, we most emphatically state that it is indeed impossible. As Nigel Kirkham states, ‘It seems impossible to get across to general surgeons that ‘sebaceous cyst’ does not exist.’²

The term ‘sebaceous cyst’ is misleading and is in fact, a misnomer. Most ‘sebaceous cysts’ are either epidermal cysts with an epidermal lining or are pilar/trichilemmal cysts with keratinization.² This misnomer is frequently used in place of more appropriate terminologies such as epidermoid cyst or epidermal inclusion cyst.^2,3 Histologically, these cysts are lined by squamous epithelium which produces and forms the cyst’s lamellated keratin content. There is no component of a sebaceous gland in these cysts.

Although rare, there are several case reports of malignant transformation in an epidermoid cyst into squamous cell carcinoma.^3,4

However, these cysts cannot develop into sebaceous carcinomas because they lack sebaceous glands, as previously stated.

A truly sebaceous cyst would have a lining of sebaceous glands. One such example is the Steatocystoma simplex, which is a cyst derived from the pilosebaceous junction and contains lobules of sebaceous glands within the lining of the cyst, along with squamous epithelium.⁶

Sebaceous carcinomas, on the other hand arise from the epithelium of the sebaceous gland. Histologically, they display sebaceous differentiation coupled with basaloid cells and clear cells.⁵ Given the components of a sebaceous carcinoma, it is clear that an epidermoid cyst cannot evolve into a sebaceous carcinoma.

We hypothesise that a malignant conversion of a long-standing sebaceous adenoma or a steatocystoma complex or disordered sebaceous hyperplasia into sebaceous carcinoma could be an alternate explanation for the case reported by Kumar et al.¹

We note that there is no pathologist among the authors and that their diagnosis is not supported by any histology image. Epidermoid cyst and sebaceous carcinoma are both diagnosed on histopathology and have distinct morphological characteristics.^3–5 Had there been a pathologist as a co-author and if an image had been provided, we suggest that such an error might have been easily avoided.

We further observe that the authors have used the abbreviation ‘SC’ for sebaceous carcinoma and then proceed to use the same abbreviation for ‘common, benign, intradermal or subcutaneous dermatological lesions’, which we presume are epidermoid cysts or as they refer to it, ‘sebaceous cysts’.