PRIMORDIAL ODONTOGENIC TUMOR-IMMUNOHISTOCHEMICAL STUDIES OF A RARE ENTITY

Abstract

Primordial odontogenic tumor (POT) is a rare, newly described entity that recapitulates early odontogenesis. POT is a neoplasm exhibiting differentiation towards dental ectomesenchyme of a developing tooth germ. It consists of a proliferation of dental papilla-like mesenchymal tissue in a vaguely lobular pattern that is covered by a layer of odontogenic epithelium demonstrating ameloblastic differentiation. Thus far, all reported cases have presented in the posterior mandible as a well-defined radiolucency associated with an unerupted tooth in the first two decades of life. Here we report a case of POT arising in a 15-year-old female. The lesion presented as a well-demarcated radiolucency associated with developing tooth #32, measuring 3.5 × 2.0 cm. Microscopically, the bulk of the tumor consisted of a cellular mass of plump spindle- and stellate-shaped cells in delicately collagenous and myxoid stroma reminiscent of dental papilla. At the periphery, columnar cells with reverse nuclear polarization resembling inner enamel epithelium formed papillary structures and invaginations. Structures that resemble developing teeth were present. Immunohistochemical studies showed strong cytoplasmic positivity for K14 and K19 in the epithelial cells, while BRAF and calretinin were negative. Overall, Ki67 showed < 1% nuclear positivity within the mesenchymal cells. Interestingly, at the areas of mesenchymal condensation, positivity for Ki67 increased to 20%–30%, and odontogenic transcription factors, MSX1, PAX9 and LEF1 were activated in those areas, indicating the initial induction of odontogenic signaling.