Helwig's inverted follicular keratosis is a benign epithelial tumor appearing most commonly as a single lesion on the face of patients in the older age group. It is characterized by distinct histologic features. After careful study of 40 cases, I am inclined to agree with Duperrat and Mascaro that these tumors are closely related to the infundibulum of the hair follicle and are an analogue of eccrine poroma which is generally accepted as a tumor of the intraepidermal sweat duct unit. Histologic points supporting this idea are presented, and a comparison with eccrine poroma is made.
MEHREGAN AH. Inverted Follicular Keratosis. Arch Dermatol. 1964;89(2):229–235. doi:10.1001/archderm.1964.01590260067012