Melanotrichoblastoma Arising on Nevus Sebaceous: A Rare Occurence

Nevus sebaceous (NS) is a congenital hamartoma of the pilosebaceous follicular unit typically occurring in the head and neck region. It is an organoid nevus over which numerous neoplasms of adnexal origin can arise. Syringocystadenomapapilliferum and trichoblastoma (TB) are the most common neoplasms.[1] We report an unusual variant of trichoblastoma, arising from NS. A 33-year-old woman gave a history of a long-standing hairless plaque since childhood with a slow-growing mass since 3 years. On examination a skin-colored, well-circumscribed, pedunculated irregularly pigmented tumour of size 4 cm on the left side of the scalp with cerebriform surface overlying a velvety plaque [Figure 1]. The lesion was excised under local anaesthesia with the clinical diagnoses of trichoblastoma and basal cell carcinoma. Histopathological examination on haematoxylin and eosin-stained sections revealed thinned out epidermis with dermis showing multiple well-circumscribed nodules of basaloid cells with peripheral palisading separated by eosinophilic hyalinised stroma. A large amount of intracellular and extracellular melanin pigment was seen. Also seen were a few undifferentiated hair structures and mitotic figures [Figure 2a and b]. An incisional biopsy from the velvety plaque revealed increased number and size of mature sebaceous glands with few areas showing cystic dilatations. Immunohistochemistry for S100 and HMB45 revealed the presence of abundant dendritic melanocytes within the basaloid nests [Figure 2c and d]. Based on clinicohistological and immunohistochemical findings, it was diagnosed as melanotrichoblastoma developed on an NS.Figure 1: Melanotrichoblastoma arising from nevus sebaceousFigure 2: Nests of basaloid cells of melanotrichoblastoma (a), (b) HE ×40 with corresponding immunohistochemistry stain for HMB45 and S100 showing pigment laden melanocytes (c), (d) (x40)Trichoblastomas (TB) are benign trichogenic tumours consisting of the proliferation of epithelial and mesenchymal cells.[2] They occasionally develop in association with other adnexal hamartomas, commonly NS as in our case. It may present at any age but commonly in the fifth and seventh decades of life with equal sex distribution.[3] Several variants described are nodular, cribriform, racemiform, retiform, pigmented, giant, subcutaneous, rippled-pattern, and clear-cell type.[4] The pigmented subtype is quite rare. It was first described in 2002[3] and has been reported in less than ten cases worldwide [Table 1]. Dermoscopy shows blue ovoid nests, arborizing telangiectasias, and white structures.[5] Histologically, these tumours are well-circumscribed and unencapsulated located in the dermis with no connection to the epidermis. It is typically composed of nests of basaloid cells with cellular stromal components. The histological differential diagnosis includes pigmented basal cell carcinoma, which shows retraction artefacts, fibromucinous stroma, and a connection with the epidermis while trichoepitheliomas show features of hair follicle differentiation with small keratinous cysts and hair bulb structures while TB shows minimal changes with hair follicle origin.[3]Table 1: Cases reported on melanotrichoblastoma with nevus sebaceous in the literatureDeclaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.