Abstract
Trichogerminoma (TG) is a rare cutaneous adnexal neoplasm of hair follicular differentiation. Microscopically, it is characterized by basaloid cellular morphology along with a lobular architectural growth pattern. While it is currently classified as a spectrum of trichoblastoma in the latest WHO classification of 2019, the presence of pale cell balls on histology, along with its inherent cytokeratin staining pattern make this neoplasm unique. Herein, we discuss a case of trichogerminoma in a 42-year-old woman who presented with a slowly growing papule on her back. On physical examination, the cutaneous lesion was 3 mm in greatest dimension, flesh color papule, that was non-tender and mobile. A tangential biopsy of the target papule was offered which showed a well-circumscribed nodular lesion within papillary dermis. The lesion was separate from the overlying epidermis, surrounded by pseudo-capsular tissue composed of fibroblasts and collagen bundles. The inner aspect of the neoplasm was marked by basaloid cells organized into a lobular architecture, forming multiple interconnected budding islands. The outer rim of lobules showed a palisading pattern of basophilic cells. In the center, paler cellular clusters streamed to form round nests, known as cell balls. The tumor cells were diffusely positive for CK5/6. Based on the histomophology, the diagnosis of trichogerminoma was made. TG is an extremely rare follicular neoplasm that shows peripheral palisading of the basaloid cells. Due to its histological similarity, differentials include basal cell carcinoma, trichoepithelioma, trichoblastoma. The presence of pale staining cell balls is a unique and key histologic feature of TG.