A Proposed Classification for Follicular Involvement by Melanoma

Background Folliculotropism in melanoma is poorly characterized and standard categorization for follicular involvement by melanoma is unavailable. We propose a logical categorization system. Methods We conducted a search of our archives over a 24-year period for cases mentioning the terms follicle, follicular, folliculotropic, folliculocentric and melanoma. Results We identified 90 cases of melanoma with involvement of the hair follicle. Distinct patterns were identified. The invasive patterns were primary follicular, folliculotropic and invasive arising from melanoma in situ (MIS) with extensive follicular involvement. Follicular involvement by MIS was either lentiginous, nested or a combination of both. A total of 29 invasive melanomas were identified. Of these 12 had invasive melanoma around the hair follicle, 2 were primary follicular melanomas, 7 showed folliculotropism and 3 were invasive melanomas arising from MIS around the follicle. Seventeen invasive melanomas had follicles only involved by MIS (9 nested, 6 nested and lentiginous and 2 lentiginous). A total of 61 cases of MIS with follicular involvement were identified; of these 33 were lentiginous, 10 nested and 18 both lentiginous and nested. Conclusion We propose that the three distinct patterns of follicular involvement by invasive melanoma and the three distinct patterns of MIS will be valuable for logically categorizing involvement of the hair follicle by melanoma.