Introduction: Hybrid odontogenic tumours, where distinct regions of more than one odontogenic tumour tissue type can be identified, are rare. Their biological behaviour usually reflects that of the predominant tumour type and malignant transformation is extremely rare.
Case report: We report a case of 30‐year‐old male presenting with an expansile lesion of the mandible. A biopsy at the time was reported as adenomatoid odontogenic tumour (AOT). The lesion was subsequently removed with clear margins and the histology confirmed AOT. The patient was aware of a bony swelling 5 years later, which continued to expand, until he eventually sought advice. Radiographs showed an extensive multilocular radiolucency with buccal and lingual expansion. Incisional biopsy showed an odontogenic tumour with features of AOT, calcifying epithelial odontogenic tumour (CEOT) and aneurysmal bone cyst. The subsequent surgical specimen contained zones of AOT and CEOT, but the bulk of the specimen showed a mesenchymal neoplasm with marked cellular atypia and an infiltrative growth pattern. Within the mesenchymal region there was calcification, some of which resembled dentinoid, but most was osteoid‐like and was surrounded by pleomorphic epithelioid cells, including multinucleate forms. These cells were cytokeratin negative and vimentin positive. The working diagnosis is osteosarcoma.
Conclusion: The potential pathogenesis of this unusual situation will be discussed.