**3. Discussion**

OGCL are considered challenging to diagnose, as COC, DGCT, and GCOC have similar histological features [11]. Diagnosis of DGCT in this rhinoceros was largely made based on the histological and immunohistochemical features from human oncology and pathology as compiled in Table 1. From the differential diagnoses, PIOSCC, SCC and ameloblastoma were ruled out, as these tumors do not feature ghost cell lesions [12]. GCOC was ruled out mainly by the fact that histopathological examination showed low mitotic activity, suggestive of a benign cellular status. Furthermore, it did not invade adjacent tissues, lacked necrosis, had pleomorphic neoplastic cells, and the immunohistochemistry for p53 was negative. Although about 30% of GCOC may show negativity for p53, it has been reported that the diagnosis of GCOC versus DGCT should be largely based on p53 positivity [3,13]. Formation of dentinoid material in craniopharyngioma is extremely rare. If present, these dentinoid materials are described as not obvious [14], so craniopharyngioma was ruled out. From this case and bibliographical review, it was stated that differentiating DGCT, GCOC, and other differential diagnoses based on the epithelial histological and immunochemical features can be difficult. The reason for this is that they may show similar epithelial features ranging from palisading columnar (resembling ameloblastoma) to basaloid (resembling squamous epithelium) formations. The presence of foreign body giant cells has been reported in both DGCT and GCOC [15,16]. Because of the rarity of OGCL and the many synonyms for each OGCL neoplasm, available data pertaining to their immunohistochemical characteristics may be difficult to access.


**Table 1.** Summary of histological and immunohistochemical features of COC, DGCT, and GCOC in humans.

> In general, DGCT more commonly occurs in the posterior maxilla and mandible. A slight predilection for the mandible has been reported, where 53% of DGCT occurs in the mandible [3]. Two variants of DGCT, namely, central and peripheral, have been described [26]. Central DGCT, the more common of the two, is a locally invasive intraosseous tumor, whereas peripheral DGCT is a non-invasive extraosseous tumor [27]. In most cases of central DGCT, the radiographic features are unilocular with a mixture of radiolucent and radiopaque or only radiolucent lesions [3]. It is unfortunate that no sample from the maxillary cys<sup>t</sup> was collected and examined in this case. The radiographic observation of mandibular cys<sup>t</sup> in this rhinoceros suggested that this case involved a central DGCT.

> Cases of other OGCL in animals have been previously reported, such as epithelial ghost cells and dentinoid material in rats with odontogenic tumors [28]. Another report involved a Bengal tiger (*Panthera tigris tigris*), wherein only a few ghost cells and some keratin were observed in a mandibular mass. However, no formation of dentinoid material was observed. That case was diagnosed as calcifying epithelial odontogenic tumor [29]. It is possible that the lack of reports of GCOC is due to the rarity of the tumor or the general lack of classification of odontogenic tumors in veterinary medicine [30]. Histological similarities were observed with the previously reported odontogenic tumors in rats, wherein no ameloblastoma-like tumor cells were seen and ovoid neoplastic epithelial cells predominated [28]. But this was very different from DGCT in humans, wherein ameloblastomatous proliferation is typically obvious [3,17].

> In megavertebrates, oral and facial proliferative lesions have been previously reported. This includes cases of gingivitis, tooth root abscessation, and SCC [31–33]. It is important

to conduct routine clinical examinations and detailed histopathological examinations to properly diagnose these lesions. Despite its rarity, OGCL should be considered in cases of oral and facial proliferative lesions in megavertebrates and animals in general.
