A rare case of a metastatic giant cell-rich osteosarcoma of the mandible: Update and differential diagnostic considerations.

A metastatic giant cell-rich osteosarcoma (GCRO) to the jaws is an exceedingly rare neoplasm. To date, fewer than 10 cases have been reported in the English language literature. In this article, we describe an additional case of a metastatic GCRO that presented the diagnostic challenge of a painless mass in the posterior mandible of a 19-year-old girl who exhibited rapid and aggressive local growth. The lesion was confirmed radiologically as an ill-defined expansive osteolytic mass showing cortical perforation. Microscopically, the presence of osteoclast-like giant cells permeated with atypical oval and rounded mesenchymal cells in a fibrovascular stroma, cellular atypia, and scarce osteoid formation were observed. Immunohistochemistry revealed the Ki-67 proliferative index in 50% of positive cells, positivity for vimentin and CD68, as well as scarce positivity for CDK4. The patient's medical history involved a GCRO in the proximal ulna. This report highlights the aggressive behavior of GCRO and its high capacity for metastasis to different parts of the body. Clinicians, pathologists, and surgeons should be aware of the giant cell-rich variant of osteosarcoma of the jaws, an imminent "wolf in a sheep's skin", because its indolent but unrelenting growth and dissemination, with radiographic and histologic characteristics that may represent a diagnostic pitfall regarding aggressive central giant cell lesions of the jaws.