[Eruptive epidermoid cysts during topical imiquimod treatment]

Imiquimod is a local immune-response modifier that works by stimulating innate and acquired immunity. It is frequently used to treat superficial basal cell carcinoma, the most common form of skin cancer. Marked local inflammatory reaction is common during treatment. We report a case of the rare condition, multiple eruptive milia, during topical imiquimod therapy.

A 67-year-old male patient presented infiltrating basal cell carcinoma above the left eyebrow. The patient underwent surgery and skin grafting. He presented superficial relapse at the periphery of the graft and was initially treated with Aldara^®. Fifteen days after initiation, Aldara^® was withdrawn due to a critical inflammatory reaction. A few weeks after complete healing, an erythematous annular plaque of milia, excluding the graft zone, appeared. This element was confirmed by histopathology.

The most common local side effects reported with Aldara^® are erythema, irritation and crusting. Reports of eruptive milia following Aldara^® therapy are rare and they are never mentioned in the summary of product characteristics. Application of imiquimod in fact induces local inflammatory reaction due to stimulation of local cytokines, which can result in marked reaction in the infundibular epithelium of hair follicles and thus in the production of abnormal keratin that can cause pilosebaceous duct obstruction and thus the formation of epidermoid cysts. This pathological mechanism explains the absence of lesions on the skin graft of the inner arm.

The occurrence of eruptive milia during treatment with Aldara^® is rarely described. The timing of occurrence of these eruptive milia as well as the mechanism of action of the drug made such a reaction highly probable in our patient.