Background. Just for over 100 years, adamantinoma has been recognized as a primary enigmatic bone tumour with epithelial characteristics and predominantly involving the tibia. Several similarities between osteofibrous dysplasia has been recognised, differentiated adamantinoma and classic adamantinoma. The purpose of this study was to compare these lesions and to define their histogenesis and mutual relations. Methods and Results. Comparison of the clinical, radiologic, histological, immunohistochemical and electron microscopic findings was carried out on 6 cases of classic adamantinoma, 2 cases of differentiated adamantinoma and 2 cases of osteofibrous dysplasia. This study confirmed the epithelial nature of long bone classical and differentiated adamantinomas irrespective of their wide-ranging morphologic pattern that can mimic tumours of various origins. Both types of adamantinoma were positive for cytokeratins in coexpression with vimentin. The epithelial component of the differentiated adamantinoma was much smaller than in classic adamantinoma and was present in scattered islands or single cells distributed within fibrovascular stoma. In one case the scattered epithelial cells had abundant eosinophilic cytoplasm and they resembled rhabdoid elements.On the basis of distinct histological pattern, a new variant of differentiated adamantinoma was described – a rhabdoid variant. In our cases of osteofibrous dysplasia occurring in two children with deformity of the tibia no epithelial cells were identified by the imunohistochemical methods. The lesions were composed of variably shaped spicules of woven and lamellar bone separated by a fibrovascular stroma. The woven bone spicules were surrounded by a uniform rim of plump osteoblasts. The lesion exhibited a zonal phenomenon with maturation of woven bone to bone with a lamellar configuration at the periphery of the lesion. The similar zonal phenomenon was also observed in the cases of differentiated adamantinoma. Conclusions. On the basis of the clinical, histological, immunohistochemical and electron-microscopical studies, a common histogenesis for classic adamantinoma, differentiated adamantinoma and osteofibrous dysplasia seems likely. The epithelial cell components of these lesions exhibit similar cytokeratin immunoprofiles and ultrastructural features. However, the prognosis for classic adamantinoma is much less favorable than that for cases of osteofibrous dysplasia and differentiated adamantinoma.

        Key words: classic adamantinoma, differentiated adamantinoma, osteofibrous dysplasia, histopathology, immunohistochemistry, electron microscopy, histogenesis.