Clear Cell Odontogenic Carcinoma of the Mandible Harboring EWSR1 Rearrangement: Report of a Massive Jaw Tumor and Review of Diagnostic Considerations

Clear cell odontogenic carcinoma (CCOC) is a low-grade malignant neoplasm that affects the jaws. We report an 18 cm massive case of mandibular CCOC in a 43-year-old female. The tumor was composed of nests and cords of round to polygonal monomorphic clear cells separated by prominent stromal hyalinization. Immunohistochemically, the tumor cells showed focal cytokeratin 5/6 positivity and intracytoplasmic PAS-positive granules and were negative for S100 and after diastase treatment (PAS-D). Molecularly, this case was positive for EWSR1 rearrangement by FISH. The following should be included in the histopathological differential diagnosis: hyalinizing clear cell carcinoma of the salivary gland, clear cell variant of central mucoepidermoid carcinoma, clear cell variant of calcifying epithelial odontogenic tumor, and metastatic renal cell carcinoma. CCOC is a rare entity, with only 79 cases reported in the mandible. This case highlights the propensity for CCOC to exhibit invasiveness, destructive nature, and facial disfigurement if left untreated.


Introduction
Clear cell odontogenic carcinoma (CCOC) is a rare malignant odontogenic tumor with less 120 cases reported in the jaws since it was first described by Hansen et al. in 1985 [1]. CCOC was previously known as clear cell odontogenic tumor or clear cell ameloblastoma, when it was classified as a benign neoplasm [1,2]. However, due to its aggressive clinical characteristics such as invasive growth, regional lymph node involvement, local recurrence, and distant metastasis, the WHO reclassified it as a malignant tumor of odontogenic origin in 2005. CCOC has a preference for females in the 5th decade of life and for the mandible, with 73.8% of reported cases occurring in the mandible [3].
Morphologically, CCOC presents as an infiltrative tumor composed of nests, sheets, or cords of monomorphic round to polygonal clear epithelial cells separated by variable hyalinized stroma [4]. Immunophenotypically, they are positive for cytokeratins, p63, and PAS, are diastase-sensitive (demonstrating intracytoplasmic glycogen), and are negative for mucicarmine and myoepithelial markers such as S100, SMA, and calponin [4,5]. Molecularly, CCOC has been shown to demonstrate Ewing sarcoma region 1 (EWSR1) rearrangement [5]. In this article, we report a massive case of CCOC of the mandible in a Nigerian patient, performing limited immunohistochemical stains (CK5/6 and S100), special stains (PAS and, PAS-D), and FISH molecular analysis for EWSR1 rearrangement.

Case Report
A 43-year-old female patient presented in September 2019 with a 4-year history of a mandibular mass of 18 cm in diameter associated with buccolingual expansion of the mandible (Figure 1(a)) and regional lymph node involvement. Incisional biopsy had been performed 2 years prior, and an initial diagnosis of ameloblastoma was rendered. The mass was reported to have progressively increased in size for the past 2 years prior to presentation. Imaging revealed an osteolytic  The tumor was positive for PAS but negative for PAS-D (demonstrating the presence of intracytoplasmic glycogen) (Figures 2(c) and 2(d)), showed focal expression for CK5/6, and was negative for S100 protein. FISH for EWSR1 rearrangement was positive with 65% of translocated cells in 60 cells examined (Figure 2(e)). The cut-off for a positive EWSR1 rearrangement is 12.5% of cells translocated. (e) Split of the green signal from the red signal by break-apart FISH demonstrates that the tumor harbors EWSR1 rearrangement. In contrast, the uninvolved EWSR1 allele shows a yellow signal (resulting from the fused red and green probes).

Case Reports in Dentistry
Two months after presentation, a subtotal mandibulectomy with neck dissection was performed for the resection of the CCOC (Figures 3(a)-3(c)). The patient is alive without disease and no recurrence at the last follow-up 15 months posttherapy.

Discussion
Clear cell odontogenic carcinoma (CCOC) is a rare malignant odontogenic neoplasm characterized by EWSR1 gene rearrangement. Less than 120 cases have been reported in the literature. A recent review of the literature of 107 patients diagnosed with CCOC showed a predilection for females, the 5th decade of life, and the mandible [3]. In the same review, 13.6% (12/88) of CCOC patients manifested regional nodal involvement, and 11.2% (12/107) of CCOC patients died of disease [3]. In this report, we present a case of CCOC in a 43-year-old female with progressive growing mass, 18 cm in size in its largest diameter, located in the mandible associated with regional lymph node involvement and alive without disease and no recurrence 15 months posttherapy.
EWSR1 rearrangement in CCOC was first described by Bilodeau et al.; the fusion partner identified was the activating transcription factor 1 gene (ATF1) on chromosome 12q13 [5]. An alternative fusion partner, CREB1, on chromosome 2q34 has been described [6]. ATF1 and CREB1 both belong to the cAMP response element-binding protein (CREB) family of transcription factors. ATF1 remains the most common fusion partner to EWSR1 identified in CCOC to date. Most cases of CCOC reported in the literature were described before EWSR1 rearrangement was identified in the tumor. EWSR1 gene rearrangement has been reported in numerous tumors, such as Ewing sarcoma, extraskeletal myxoid chondrosarcoma, clear cell sarcoma, desmoplastic small round cell tumor, myxoid liposarcoma, angiomatoid fibrous histiocytoma, primary pulmonary myxoid sarcoma, myoepithelial tumors of soft tissue, skin, and salivary glands, and hyalinizing clear cell carcinoma. EWSR1-ATF1 translocation has been reported in other tumors, such as  Case Reports in Dentistry angiomatoid fibrous histiocytoma, hyalinizing clear cell carcinoma, and clear cell sarcoma [7][8][9]. Additionally, EWSR1-CREB1 translocation has been reported in myxoid mesenchymal tumors, angiomatoid fibrous histiocytoma, primary pulmonary myxoid sarcoma, and gastrointestinal-clear cell sarcoma [10][11][12][13].
The differential diagnosis of CCOC includes hyalinizing clear cell carcinoma (HCCC) of the salivary gland, clear cell variant of central mucoepidermoid carcinoma (MEC), clear cell variant of calcifying epithelial odontogenic tumor (CEOT), and metastatic renal cell carcinoma. HCCC of the salivary gland is a rare low-grade malignant neoplasm that typically affects the minor salivary glands [14]. It has been argued that CCOC and HCCC are essentially the same analogous tumors with different locations [4,5]. CCOC occurs centrally in the bone while HCCC occurs peripherally in the submucosa. CCOC and HCCC share similar morphologic, immunophenotypic, and molecular features [5].
MEC is the most common malignant salivary gland tumor that needs to be differentiated from CCOC, despite the presence of intercellular bridging of true epidermoid areas. MEC is composed of epidermoid, mucinous, and intermediate cells in variable proportion depending on its grade, forming solid and cystic patterns. MEC has been described in the jaw bone [15]. Clear cells have been described in MEC, and when predominant, they are called the clear cell variant of MEC. Immunohistochemical stains are not helpful in differentiating MEC from CCOC. However, PAS-D shows positive intracytoplasmic globules in MEC, which are negative in CCOC [4,16]. MEC is negative for EWSR1 rearrangement but may be positive for CRTC1/CRTC3-MAML2 fusion [17].
CEOT is a rare invasive epithelial odontogenic tumor also known as Pindborg tumor. CEOT is composed of sheets, nests, and/or cords of polygonal eosinophilic epithelial cells with a prominent stroma [18]. These epithelial cells may present with clear cytoplasm, making it challenging to differentiate it from CCOC. However, cellular pleomorphism and the presence of amyloid and Liesegang calcified rings in the stroma that define CEOT are absent in CCOC. The eosinophilic amyloid material can be highlighted with Congo red stain, showing the classic apple-green birefringence with polarized light.
Metastasis of renal cell carcinoma to jaw should also be considered in the differential diagnosis of CCOC. These patients may have a history of a renal mass, or the metastasis to the oral cavity may be the first indication of a malignancy [19]. The metastatic renal cell carcinoma will be positive for PAX8, CD10, and CAIX in contrast to CCOC and negative for EWSR1 rearrangement.
The case described here originated from the jaw, negative for PAS-D, and positive for PAS and EWSR1 rearrangement, ruling out the aforementioned differential diagnoses. In conclusion, we report a rare form of jaw neoplasm "clear cell odontogenic carcinoma" with a propensity for invasiveness, destructive nature, and facial disfigurement if left untreated in a Nigerian patient harboring EWSR1 rearrangement.

Data Availability
No data were used to support this study.

Disclosure
The funding source has no role in conceiving and performing the study.