Sebaceous Epithelioma in association with a Squamous Cell Tumor with “Catagen-like” Attributes:
Sebaceous epithelioma is a basal cell carcinoma in which there is histologic evidence of sebaceous differentiation. The sebaceous component may be poorly developed and easily overlooked. Epithelial tumors showing follicular differentiation can be sub-classified on the basis of a comparison of the patterns of the tumor with those of particular components of a hair follicle; in this approach, the comparisons must include anagen, catagen, and resting patterns. Patterns of the infundibular portion of a follicle are commonly manifested in common, benign follicular tumors; infundibulum-like patterns are common to both irritated seborrheic keratosis and inverted follicular keratosis (i.e., follicular infundibular patterns). As in the cyclic manifestations of hair follicles, overlaps in patterns are common in follicular tumors; many follicular tumors of the butterfly area of the face combine the patterns of both inverted follicular keratosis and tricholemmoma (i.e., compound tricholemmoma - a compromise in which patterns of outer root sheath differentiation are given precedence). Interestingly, some of the compound examples also show evidence of sebaceous differentiation. Rare sebaceous epitheliomas are characterized by patterns of an “irritated seborrheic keratosis” in continuity with a basal cell neoplasm showing evidence of sebaceous differentiation. An encounter with such a lesion might evoke either a diagnosis of sebaceous epithelioma arising in a seborrheic keratosis, or the interpretation of the combination as an example of a collision tumor. As a less likely response, an observer might offer the opinion that the tumor is organoid (compound within the confines of patterns manifested in the cyclic changes of normal follicles). Such a compound tumor then becomes a special and separate tumor requiring an expansion of diagnostic criteria and taxonomy.
In chapters 20, 21, & 22, a basaloid tumor, showing sebaceous differentiation, apparently has had its origin in a tumor with features of an “irritated seborrheic keratosis.” The patterns in the sebaceous component satisfy the criteria for the diagnosis of sebaceous epithelioma, variant compound type.