Unicystic Mucoepidermoid Carcinoma: A Pitfall for Clinical and Pathologic Diagnosis

Abstract

Unicystic mucoepidermoid carcinoma (UC-MEC) is a rare MEC variant, and its diagnosis is frequently problematic. This study is aimed at summarizing its clinicopathologic characteristics, treatment, and prognosis and proposing key points to avoid missed diagnosis and misdiagnosis in clinical and pathological conditions. This retrospective study included 30 UC-MEC cases, and the clinical findings were collected from the clinical medical records. Radiographic features, histologic behaviors, MAML2 rearrangement by fluorescence in situ hybridization (FISH), and follow-up data were analyzed. Moreover, glandular odontogenic cyst (GOC) and cytadenoma (CA) were used as controls. In the UC-MEC group, 19 patients were female (63%), and 11 were male (37%). The mean patient age was 39.5 (range, 7–72 years). The affected locations included the jaw (8 maxillary, 3 mandibular) and salivary glands (7 parotid, 11 palates, and 1 floor of the mouth). The chief complaint was swelling; the lesions were all cystic, among which 66.7% were well circumscribed and 33.3% poorly defined. Microscopic examination showed two UC-MEC histologic subtypes. Type A presented as a single cyst with mural thickening (8/30, 27%) lined predominantly by epidermoid cells with interspersed intermediate and mucinous cells, and type B (22/30, 73%) showed infiltrative tumor islands in the cystic wall or the surrounding tissue. FISH analysis suggested that approximately 66.7% of UC-MEC harbored a MAML2 rearrangement. During the median follow-up period of 42 months (range, 6–120 months), all type A patients and 68% of type B patients who underwent complete surgical resection lived without relapse. Seven cases with type B cancer that underwent curettage initially had local recurrence. Clinicians and pathologists hardly recognize UC-MEC owing to its cystic architecture. Specific epidermoid, mucous, and intermediate tumor cells, and MAML2 fusion testing, are essential to avoid potential diagnostic pitfalls. Prompting and completing resection surgery with negative margins would have a favorable prognosis.

MAML2-Rearranged Primary Central Mucoepidermoid Carcinoma of the Mandible as an Incidental Finding: A Case Report and Review of the Literature of Molecularly Confirmed Cases

“…MAML2 rearrangement in MEC is not specific to salivary gland origin, as MEC of other origins, such as thymus, lung, lacrimal gland, and uterine cervix, have also demonstrated MAML2 rearrangement [25][26][27][28]. Our literature review only identified nine reports/studies documenting the presence of MAML2 rearrangement in CMEC in 30 cases (Table 1) [10][11][12][13][14][15][16][17][18]. Of these, site and demographic information are provided for 25 cases [10,11,[13][14][15][16][17][18].…”

“…Our literature review only identified nine reports/studies documenting the presence of MAML2 rearrangement in CMEC in 30 cases (Table 1) [10][11][12][13][14][15][16][17][18]. Of these, site and demographic information are provided for 25 cases [10,11,[13][14][15][16][17][18]. The mandible represents 64% of cases, with 36% occurring in the maxilla [10,11,[13][14][15][16][17][18].…”

“…Of these, site and demographic information are provided for 25 cases [10,11,[13][14][15][16][17][18]. The mandible represents 64% of cases, with 36% occurring in the maxilla [10,11,[13][14][15][16][17][18]. There is a broad age range, from 20 to 85 years, which seems to separate into two peaks; 48% of cases are in patients under 32 years of age (average of 27.5 years), and 44% of cases are in individuals over 57 years of age (average of 68.2 years) [10,11,[13][14][15][16][17][18].…”

“…The mandible represents 64% of cases, with 36% occurring in the maxilla [10,11,[13][14][15][16][17][18]. There is a broad age range, from 20 to 85 years, which seems to separate into two peaks; 48% of cases are in patients under 32 years of age (average of 27.5 years), and 44% of cases are in individuals over 57 years of age (average of 68.2 years) [10,11,[13][14][15][16][17][18]. Twenty cases report the histopathologic grade, with 55% characterized as low-grade and 45% characterized as intermediate-grade [10,11,[14][15][16][17][18].…”

“…CRTC1/3-MAML2 fusions are specific to MEC and absent in other tumors of salivary gland origin [8,9]. Few reports have documented the presence of MAML2 rearrangement in CMEC [10][11][12][13][14][15][16][17][18].…”