Ameloblastoma is a benign tumor of odontogenic origin, accounting for approximately 1% of all cysts and tumors occurring in the jaw [1]. Despite being classified as benign, it is known to exhibit locally aggressive characteristics. According to the 2005 World Health Organization classification, ameloblastomas can be divided into four types: solid/multicystic, extraosseous/peripheral, desmoplastic, and unicystic. Unicystic ameloblastoma shares the histological characteristics of ameloblastoma while appearing clinically and radiographically as a cystic lesion. It may show luminal or mural tumor growth, or both, within the cyst cavity epithelium [2]. Unicystic ameloblastoma is generally less aggressive and has a lower recurrence rate than solid or multicystic forms, making it more amenable to conservative treatments such as enucleation or curettage. However, in the case of large cysts, accurate diagnosis can be challenging depending on the location where the biopsy is performed. If unicystic ameloblastoma is misdiagnosed as other odontogenic cysts or inflammatory cysts, the risk of postoperative recurrence increases. In this paper, we report a case of early recurrence following decompression and enucleation of a large maxillary unicystic ameloblastoma.

This study was approved by the Institutional Review Board of Chosun University Dental Hospital (IRB no. CUDHIRB 2409 003). Written informed consent for publication was obtained from all participants included in the study.

In December 2019, a 54-year-old male patient was referred to the Department of Oral and Maxillofacial Surgery at Chosun University Dental Hospital with a suspected odontogenic cyst in the posterior maxilla and maxillary sinus, as noted by a local otolaryngology clinic. The main complaints were frequent epistaxis and nasal congestion on the right side. The patient had no underlying diseases or hematological issues. Panoramic radiographs and cone-beam computed tomography revealed a radiolucent lesion at the apices of teeth #17 and #16, occupying most of the right maxillary sinus, with significant buccal, nasal, and posterior bone expansion and thinning. With root resorption, mobility of teeth #17 and #16 was also observed (Fig. 1). Clinically and radiographically, a periapical cyst, postoperative maxillary cyst, or odontogenic keratocyst was suspected. Tooth #17 was extracted, and a biopsy was performed on the same day, with a Foley catheter inserted for decompression. The initial biopsy result indicated a radicular cyst. The Foley catheter was replaced with a resin-fixed device, and decompression was continued for approximately one year and ten months, significantly reducing the lesion size. Subsequently, curettage was performed under local anesthesia (Fig. 2). During surgery, the superior wall of the cyst was partially perforated, connecting to the maxillary sinus. The final biopsy revealed a unicystic ameloblastoma, mural type with marked wall extension (Fig. 3). Three weeks post-surgery, a strawberry-like neoplasm was observed in the gingiva, and an incisional biopsy was performed. This time, it was diagnosed as ameloblastoma, follicular type, indicating peripheral recurrence of the previously unicystic type ameloblastoma (Fig. 4). Consequently, tooth #16 was removed, which had not been extracted previously, and a wide ranging flap was opened for extensive and thorough curettage. Suspected bone areas were ground using a denture bur. Multiple biopsies were taken during the reoperation due to the risk of recurrence, revealing ameloblastic epithelial islands in the palatal and buccal areas of the maxillary sinus, in addition to the main mass. The oral defect was packed with Vaseline gauze to promote secondary healing, resulting in satisfactory soft tissue and bone healing. The surgical site has remained stable without signs of recurrence during the 5-year follow-up period (Fig. 5).

Fig. 1. Initial panoramic radiographs and cone-beam computed tomography (CBCT) showing a radiolucent lesion occupying most of the right maxillary sinus with buccal, nasal, and posterior bone expansion and thinning. (A) Panorama. (B) Axial view (CBCT). (C) Coronal view (CBCT).

Fig. 2. Panoramic radiographs and cone-beam computed tomography (CBCT) after 1 year and 10 months of decompression, showing significant reduction in lesion size and regeneration of the nasal lateral wall. (A) Panorama. (B) Axial view (CBCT). (C) Coronal view (CBCT).

Fig. 3. Histological images of the entire lesion after curettage, confirming unicystic ameloblastoma (A), mural type with invasion into the connective tissue (B). H&E stain, ×100.

Fig. 4. Clinical (A) and histological (B) images of the early peripheral recurrence 3 weeks post-surgery, showing solid epithelial islands of conventional ameloblastoma. H&E stain, ×200.

Fig. 5. Panoramic radiograph after 5 years of follow-up, showing well-healed surgical site without recurrence.

Unicystic ameloblastoma primarily occurs in the posterior mandible and ascending ramus, with a relatively rare occurrence in the maxilla at a ratio of 1:13 compared to the mandible [3]. It typically presents as an asymptomatic intraosseous lesion, making early detection challenging and often leading to delays in treatment. The maxilla lesion can cause nasal obstruction, facial swelling, and gingival or palatal swelling, sometimes accompanied by pain, although it is typically absent unless infection is involved. These characteristics could delay diagnosis, and radiographical cystic appearance of unicystic ameloblastoma often misdiagnosed as a periapical cyst in the posterior maxilla or as a postoperative maxillary cyst or odontogenic keratocyst when observed within the maxillary sinus. The peak age of occurrence is reported to be in the third decade when associated with an impacted tooth, and in the fourth decade when not associated with an impacted tooth [4]. There is a clinical tendency for the root resorptions of the affected teeth, and significant bone expansion or thinning is often observed in unicystic ameloblastoma. However, when the lesion is large and associated with multiple teeth, it becomes challenging to differentiate it from other odontogenic cysts or inflammatory cysts.

In the presented case, the lesion developed in the posterior maxilla and maxillary sinus, likely growing slowly over a long period without significant symptoms. The patient was in his sixth decade, outside the typical age range for unicystic ameloblastoma. The initial biopsy, which only sampled part of the lesion, inaccurately diagnosed it as a periapical cyst. It was only after the entire lesion was excised that a definitive diagnosis of unicystic ameloblastoma was made. Small unicystic ameloblastomas are often clinically mistaken for odontogenic cysts and are only diagnosed upon examination of the entire specimen. Large unicystic ameloblastomas can be diagnosed through incisional biopsy, but the exact subtype can only be determined by examining the entire lesion [5]. The recurrence rate of unicystic ameloblastoma necessitates careful consideration of surgical scope and options, especially if the diagnosis is not suspected preoperatively. The histological subtype, which significantly impacts recurrence rates, is often only determined postoperatively, posing a treatment dilemma.

According to Ackermann et al.’s 1988 classification [6], unicystic ameloblastoma is divided into three subgroups:

I: luminal unicystic ameloblastoma (tumor cells are confined to the luminal epithelial layer of the cyst).

II: intraluminal/plexiform unicystic ameloblastoma (tumor cells proliferate into the lumen without invading the connective tissue wall).

III: mural unicystic ameloblastoma (invasive tumor cell islands infiltrate the connective tissue wall).

The recurrence rate of unicystic ameloblastoma is associated with histologic subtype, with the mural type having a recurrence rate of 35.7%, compared to only 6.7% for other types [7].

Luminal type of unicystic ameloblastomas could be treated conservatively with curettage, expecting a relatively low recurrence rate. In contrast, the mural type has a higher recurrence rate due to its potential for medullary marrow extension, and some authors recommend radical resection for this subtype [3]. Unicystic ameloblastomas could be effectively treated with conservative curettage or enucleation, with reported recurrence rates ranging from 18% to 25% [1,4,8].

In this case, the initial biopsy during decompression reported a periapical cyst, while the postoperative biopsy confirmed a mural type unicystic ameloblastoma. Three weeks later, a re-biopsy revealed a follicular type conventional ameloblastoma, presenting clinically as a strawberry-like peripheral lesion. Unicystic ameloblastomas are known to exhibit multiple histologic types within a single tumor, such as mural, luminal, plexiform, and follicular [8]. However, the recurrence of unicystic ameloblastoma in the form of a solid tumor merely three weeks after curettage, where the grossly visible lesion had been largely excised, raises some questions. In this case, the short interval of only 3 weeks after surgery before the lesion reappeared raises the possibility that it could be a residual lesion. However, the lesion identified in this case differed from the initial lesion in histopathologic findings, and it was found peripherally in the buccal gingiva, a location where no lesion had previously existed. Based on these observations, this study defined the lesion as a recurrent lesion.

There are three main hypotheses regarding the pathogenic mechanisms of unicystic ameloblastoma. The first hypothesis suggests that ameloblastic transformation occurs in the reduced enamel epithelium of developing teeth. The second hypothesis proposes that a neoplastic ameloblastic lining arises from other odontogenic cysts, such as dentigerous cysts [8]. The third hypothesis posits that a solid tumor undergoes cystic degeneration, and multiple cysts fuse to form a unicystic ameloblastoma. In this case, the rapid recurrence as a solid tumor suggests the third hypothesis.

The treatment of ameloblastoma remains controversial. The rationale for advocating conservative treatment is that a significant amount of bone tissue regenerates during healing, allowing for easier reoperation if a small lesion recurs once some bone tissue has recovered [9]. However, the maxilla has a thinner cortical bone than the mandible, making it challenging to limit tumor expansion. Additionally, the maxilla has a more prosperous blood supply, allowing the tumor to grow more invasively and potentially spread to the orbit, infratemporal fossa, and skull base. The posterior maxilla is particularly challenging for surgical access, and if a recurrent lesion is difficult to reach, the prognosis could be very poor. Therefore, even for unicystic ameloblastoma, a radical marginal resection or partial maxillectomy is recommended occurred in maxilla [10]. In this case, the initial incisional biopsy misdiagnosed the lesion as a periapical cyst, leading to a two-stage conservative treatment approach with decompression followed by enucleation. Fortunately, the lesion responded well to decompression, significantly reducing size and allowing for the regeneration of the destroyed nasal wall. Like this case, when the lesion is extensive, conservative management can be pursued by initially reducing the size through marsupialization, followed by cyst enucleation. However, in patients with cortical bone perforation, treatment combined with decompression could lead to recurrence, and if decompression is ineffective, the risk of recurrence increases by 5.459 times. Therefore, it is essential to carefully monitor the response of unicystic ameloblastoma during decompression [11].

Fortunately, despite the conservative surgery, there was no recurrence during the 5-year follow-up period in this patient. However, ameloblastomas grow slowly and could recur even after 10 or 20 years, necessitating long-term follow-up. This case highlights the diagnostic challenges and rapid recurrence encountered after conservative surgery for a rare maxillary unicystic ameloblastoma compared to the mandible.

This study was supported by research fund from Chosun University Dental Hospital, 2022.

Ji-Su Oh and Seong-Yong Moon serve on the editorial board of the Oral Biology Research. But they have no role in the decision to publish this article. Except for that, no potential conflict of interest relevant to this article was reported.