Unicystic Ameloblastoma Revisited

Ameloblastomas with a prominent cystic component or a unilocular radiographic appearance are often labeled as “unicystic ameloblastoma¹ without meeting the original description of this unique entity reported by Robinson and Martinez². We grouped ameloblastomas evaluated and treated at Massachusetts General Hospital (MGH) as unicystic according to the original description and hypothesized that the recurrence rates would be lower than that for ameloblastomas in general and that the variability in reported recurrence rates may be due to misclassification. The specific aims were to compare clinical presentation and outcomes of unicystic ameloblastoma (UA) treated by our group with the outcomes reported by Robinson and Martinez².

This was a retrospective cohort study of MGH patients who had diagnosis of unicystic ameloblastoma between January 2000 and January 2015. A specimen list was created by searching the electronic database of the Department of Pathology. Subjects were included if they had a confirmed histologic diagnosis of unicystic ameloblastoma on biopsy or final specimen, complete records, and follow-up > 1year. Demographic, clinical, radiographic and histological variables were collected and compared to the original description of UA. The predictor variables included age, gender, radiographic appearance (uni- vs. multilocular), type of operation and pattern of ameloblastic epithelium. The outcome variable was recurrence rate.

Twenty-one subjects (age 31.6 ± 19.1 years) were included in the MGH group compared to 20 subjects in Robinson (2) group (age 27.7 ± 21.1 years). Tumors were predominantly in the mandible in both studies: MGH, 20/21 lesions in the mandible (6 anterior, 14 posterior; 1 in the posterior maxilla); Robinson² 20/20 (1 anterior, 19 posterior). In current study, 12 tumors were associated with an impacted tooth and 14 were unilocular compared to 14 and 19 respectively in the Robinson² group. Tumors with unilocular radiographic appearances were younger in both groups (MGH age 27.1± 19.1years, Robinson² age 26.1± 21.1 years) compared to the multilocular ones (MGH age 40.6 ± 16.8 years, Robinson (2) age 47 ± 0 years); were more frequent in the mandible (13 in MGH group and 18 in Robinson² group) and were two times more frequently associated with an impacted tooth.

MGH subjects were treated by resection (n= 13, 62%, 1 marginal resection, 12 segmental resection), enucleation alone (n=5), enucleation with peripheral ostectomy (n=2), and enucleation with cryotherapy (n=1). None of the subjects undergoing resection (n=13) exhibited recurrence after follow-up of 4.1 ± 4.5 years. The recurrence rate after enucleation was 62.5% (n=5/8) after follow-up of 3.7 ± 3.0 years. All tumors that recurred in the enucleation group demonstrated mural invasion of ameloblastic epithelium. In Robinson² group, all subjects were treated with enucleation and the recurrence rate was 15% (n=3) after follow-up of 6.6 ± 4.3 years. Histologic location of ameloblastic epithelium was not documented in the Robinson² paper.

Although demographic, clinical and radiographic data for subjects with UA at MGH were similar to those in the Robinson² paper, the recurrence rate after enucleation, was substantially higher. The finding most associated with recurrence was mural invasion of ameloblastic epithelium, which was not documented by Robinson². The results of this study are consistent with Robinson² concept of UA as a distinct entity. However, treatment by enucleation alone of a unicystic ameloblastoma should be reserved for unilocular lesions with lack of mural invasion of ameloblastic epithelium.

References:

1. Gardner DG: Some current concepts on the pathology of ameloblastomas. Oral Surg Oral Pathol Oral Radiol Endod 82:660-669, 1996

2. Robinson L, Martinez MG: Unicystic ameloblastoma: A prognostically distinct entity. Cance 40:2278-2285, 1977