Ⅰ. INTRODUCTION

Dentigerous cyst constitutes approximately 15%-30% of the developmental odontogenic cysts, and represents the second most common cystic lesion^1-3). It is formed by fluid accumulation between the enamel epithelium and the crown of an unerupted tooth^4-6). Radiographically, a well-defined and unilocular radiolucency surrounding the crown of an impacted tooth is found^5-8). An epithelial lining of flattened non-keratinizing cells is microscopically observed around a dentigerous cyst^6-7). Occasionally, small islands of odontogenic epithelial rests are present in the fibrous wall of dentigerous cysts^5,9). Enucleation and extraction of the unerupted tooth are usually performed for dentigerous cysts, and recurrence is uncommon^6,8). However, a dentigerous cyst has been reported to have neoplastic potential to convert to other odontogenic tumors (i.e., ameloblastoma and adenomatoid odontogenic tumor) or malignant tumors (i.e., squamous cell carcinoma and mucoepidermoid carcinoma) ^4,8-11). Particularly, this case report presents an uncommon case characterized by histopathological conversion from a dentigerous cyst to an ameloblastoma and further investigates the factors contributing to its conversion. Additionally, the differential diagnosis and postoperative follow-ups are emphasized for dentigerous cysts despite their low recurrence rates.

Ⅱ. CASE REPORT

A 16-year-old female patient presented to the Kyungpook National University Dental Hospital in Korea with chief complaints of consistent swelling and pain in her right cheek that persisted for a month. Radiographic examination revealed a large radiolucent lesion with a well corticated margin extending from the right mandibular body to the entire ramus area (Fig. 1A). The right mandibular third molar was unerupted and displaced by the cystic lesion. Furthermore, bone and dental root resorption were observed. Cone-beam computed tomography (CBCT) revealed buccolingual expansion and thinning of the cortical bone of the mandibular body (Fig. 1B, C). Under general anesthesia, the bony enlarged lesion was first marsupialized to reduce its size, and incisional biopsy was concurrently performed. To prevent mandibular fracture, arch bar fixation was performed before marsupialization. A slit was made on the cystic wall, and a penrose drain was inserted into the slit to keep it open and to ensure free drainage. The cystic lesion was diagnosed as a dentigerous cyst. Histopathological examination revealed thin, non-keratinized epithelial linings (Fig. 2A, B). Some part of the linings was thicker with hyperplastic rete ridges due to inflammation.

Four months later, radiographic examination was repeated. The cyst size had slightly reduced, but no dramatic change was observed (Fig. 1D). Buccal expansion of the cortical bone of the right mandibular body had reduced compared with that at her first visit (Fig. 1E, F). However, marsupialization had a limitation in decreasing the cyst size. Thus, enucleation and extraction of the right mandibular third molar were followed by marsupialization. The final biopsy led to the diagnosis of an ameloblastoma. Microscopically, a combination of characteristics of dentigerous cysts and ameloblastomas was observed. The flattened, non-keratinized epithelial lining and several small islands of inactive-appearing odontogenic epithelial rests in the fibrous wall, which represent dentigerous cysts, persisted. Conversely, the histopathological features of ameloblastomas were more evidently noted. The thicker, loosely cohesive overlying epithelial cells were visible with a hyperchromatic and polarized basal layer. Islands of ameloblastic follicles in the fibrous connective tissue wall were another noticable characteristic of ameloblastomas. Although inflammation led to palisades and polarity in weaker basal layers, obvious stellate reticulum clearly indicated the lesion to be an ameloblastoma (Fig. 2C, D).

Wide resection is an optimal treatment for ameloblastoma. Nonetheless, additional operation was not followed considering the patient’s young age. Instead, regular follow-up care was planned. At seven months after enucleation, bone healing in the mandible was favorable (Fig. 1G). Furthermore, thinning and expansion of mandibular cortical bone gradually resolved. To date, no dental symptoms and recurrence occurred. The close follow-up observation was continued to monitor its recurrence.

Ⅲ. DISCUSSION

Dentigerous cyst is considered one of the developmental odontogenic cysts enclosing the crown of an unerupted tooth^4,6). It is the most common cause of pericoronal radiolucency, which is formed by fluid accumulation between the crown of an impacted tooth and the follicular epithelium^4-6). The unilocular radiolucency with cortical expansion of buccolingual bone and thinning of the outer radiopaque border is one of the main radiographic features of dentigerous cysts^6,8,12). Additionally, the resorption of bone and adjacent teeth and the displacement of teeth often occur simultaneously ^6,12). It is commonly observed in individuals in the twenties and thirties and most commonly occurs in the impacted mandibular third molars⁹⁾.

Ameloblastomas are benign, slow-growing epithelial neoplasms that are locally aggressive and infiltrative^7,9,12). They are the most common odontogenic tumors and represent approximately 10% of all such tumors^6,9). They generally occur in the third to fifth decades of life⁶⁾. The radiographic characteristics of ameloblastoma range from unilocular radiolucency to multilocular radiolucency with internal septa and a honeycomb or soap bubble appearance^6,8). Particularly, ameloblastomas with unilocular and well-corticated radiolucent lesions enclosing the crown of unerupted teeth are radiographically indistinguishable from dentigerous cysts. Additionally, the epithelial linings of untreated dentigerous cysts are often reported to be converted into ameloblastomas ⁹⁾. Thus, a differential diagnosis between dentigerous cysts and ameloblastomas greatly relies on histopathological evaluation.

In this case, the cystic lesion represented indistinct radiographic aspects between dentigerous cysts and ameloblastomas. Hence, histopathological examination was required for differential diagnosis. At the first microscopic examination of marsupialization, most of the epithelial cell linings were thin and non-keratinized, which defined the cystic lesion as a dentigerous cyst^4,6,7). The rete ridge proliferation due to inflammation might lead to its misinterpretation as other similar odontogenic cysts or tumors; however, the absence of polarity of nuclei and stellate reticulum- like basal layers indicated it to be a dentigerous cyst^6,7). However, during the four months of cyst size reduction by marsupialization, the walls of the dentigerous cyst altered to ameloblastic linings. The flattened, non-keratinized linings and islands of inactive epithelial rests partially persisted, but the majority had developed hyperchromatic, reversely polarized basal layers with loosely arranged epithelial cells^4,7). Although the peripheral palisading of columnar cells was weakly expressed owing to inflammation, some tumoral islands and stellate from epithelial cells clearly demonstrated ameloblastic characteristics. Consequently, the transformation from a dentigerous cyst to an ameloblastoma was noted based on histopathological examinations.

Approximately 15%-20% of ameloblastomas develop from the walls of dentigerous cysts^7,9,12). Kahn et al. proposed the etiopathogenesis of ameloblastomas arising from dentigerous cysts¹⁰⁾. Furthermore, numerous factors, including non-specific irritating factors (extraction, caries, trauma, infection, inflammation, and unerupted tooth), nutritional deficit disorders, and viral infection have been suggested^4,8-10). In our case, unerupted mandibular third molar and inflammation were the major doubtable etiologic factors. Microscopic images showed inflammatory infiltrates with the development of rete ridges and thicker epithelial linings¹³⁾. Additionally, weak palisading basal layers and polarity of nuclei caused by inflammation were verified. Overall, this transition of lesion was assumed to be owing to irritating factors.

Because ameloblastoma has an aggressive potential for local invasion and a high recurrence rate, the treatment method of ameloblastoma has been controversial¹³⁾. According to several literatures, conservative treatments such as enucleation, marsupialization, and curettage resulted in a high recurrence rate of 55%-90%, whereas radical treatments resulted in a recurrence rate of less than 13%¹³⁾. Thus, radical treatment such as wide resection was recommended for ameloblastomas^11,13). Nevertheless, mandibular resection was not performed in this case to conserve the facial appearance, to allow the mandibular growth, and to reduce the functional sequelae, considering the patient’s young age. Accordingly, regular follow-up was ensured for seven months, and no recurrence has been noted to date.

Ⅳ. CONCLUSION

This case report represented the conversion from a dentigerous cyst to an ameloblastoma to emphasize the neo plastic potential of dentigerous cysts and the importance of histopathological evaluation for the differential diagnosis of cystic lesions. Although dentigerous cysts typically have a low recurrence rate, regular postoperative follow-ups are necessary because dentigerous cysts, particularly in the presence of irritating factors, nutritional deficiency, and viral infections, may undergo neoplastic changes to develop into odontogenic tumors or malignant lesions.