Osteonevus of Nanta: A Histopathological and Morphometric Case Report of a Rare, but Otherwise Benign Lesion

Background and Clinical Significance: Osteonevi, originally described by Heidingsfeld in 1908 and later by Nanta in 1911, because of whom it is known as osteonevus of Nanta, is a rare condition with not yet fully established etiopathogenesis; Case Presentation: Herein, we report a case of a 33-year-old female patient who presented to our institution with a papilliform pigmented lesion located on the projection of the left mandibular angle, measuring 2 × 1.5 cm. The lesion had been present since childhood; however, it had increased in size by approximately 5 mm over the previous month and had become painful. Surgical excision was performed, which went uncomplicated. Histology of the resected specimen showed a dermally based, symmetrical melanocytic proliferation, without signs of dysplasia, and an underlying keratocyst with rupture, accompanied by a surrounding foreign-body-type granulomatous reaction around inert keratin flakes. A third component of the lesion was also noted, represented by foci of osteoid and myeloid metaplasia underneath the melanocytic proliferation, without direct relation to the ruptured keratocyst. Based on the morphological findings, the diagnosis of osteonevus of Nanta was established; Conclusions: Oseonevus of Nanta is an extremely rare, benign morphological finding. The etiopathogenesis of these rare lesions is not yet fully established, despite several proposed mechanisms. The differential diagnosis, while typically straightforward, is broad.