Central mucoepidermoid carcinoma

Central mucoepidermoid carcinoma (CMEC) is a rare malignant salivary-type neoplasm arising within the jawbones, accounting for 2%–3% of all mucoepidermoid carcinomas.[1][2] The tumor is composed of variable proportions of mucous, epidermoid, and intermediate cells and is most commonly low grade.[2][3] Pathogenesis remains debated; leading hypotheses include neoplastic transformation of the epithelial lining of odontogenic cysts, entrapment of ectopic salivary gland tissue within the jaw during embryologic development, and malignant change in retromolar mucous glands.[1][4] Approximately 50% of cases are associated with an odontogenic cyst or impacted tooth, supporting the odontogenic-epithelium theory.[1][5] Radiographically, CMEC typically presents as a unilocular or multilocular radiolucency that mimics benign odontogenic lesions, creating a significant diagnostic pitfall.[1][6] Radical surgical excision is the mainstay of treatment, and the overall prognosis is favorable when managed appropriately.[1][2]

• Central mucoepidermoid carcinoma (CMEC) and intraosseous mucoepidermoid carcinoma (IMEC) are the accepted synonyms used interchangeably in the literature.[1][3]
• Primary intraosseous mucoepidermoid carcinoma is used to emphasize that the tumor originates within bone rather than extending from an adjacent soft-tissue salivary gland.[4]
• The WHO Classification of Head and Neck Tumours (5th edition) recognizes mucoepidermoid carcinoma as a distinct salivary gland malignancy; the central/intraosseous variant is classified under the same histologic type but distinguished by its intraosseous location.[7]
• Older literature occasionally refers to these lesions as "intraosseous salivary gland tumors of the jaw" or "central salivary-type carcinoma," terms that encompass other rare intraosseous salivary histotypes (e.g., adenoid cystic carcinoma) and should not be used as specific synonyms.[4]

• Frequency: Extremely rare; fewer than 200 cases had been reported in the literature as of the mid-2010s. Among all intraosseous salivary-type carcinomas of the jaws, CMEC is the most common histotype, outnumbering adenoid cystic carcinoma approximately 4:1.[1][4]
• Age: Reported across a wide range (1–79 years); the majority occur in the fourth and fifth decades, with a median age of approximately 43–49 years. One recent series reported a higher mean age of 58.6 years.[1][2][3]
• Sex: Slight to moderate female predilection; the female-to-male ratio is approximately 2:1 in most series.[1][2]
• Anatomic distribution: The mandible is affected approximately three times more often than the maxilla, with the premolar–molar–angle region being the most common site. In children, the mandible-to-maxilla ratio is closer to 1:1.[1][2]
• Association with dental pathology: Approximately 32%–50% of mandibular tumors are associated with odontogenic cysts and/or impacted teeth.[1][5]

Symptoms and presentation

• Swelling is the most common presenting complaint, often painless and slowly progressive over months to years.[5]
• Pain is present in a minority of cases and may indicate higher-grade disease or cortical perforation.[1][8]
• Occasional findings include tooth mobility, paresthesia of the inferior alveolar nerve, trismus, and nasal congestion (maxillary lesions).[1][5]
• Some tumors are discovered incidentally on routine dental imaging.[5]

Tempo

• Disease progression is typically slow, with over half of patients in one series reporting symptoms for more than six months before diagnosis. Rarely, aggressive growth with rapid expansion occurs, particularly in higher-grade tumors.[5]

Diagnostic pitfall

• Because CMEC frequently mimics odontogenic cysts both clinically and radiographically, initial management is often directed at a presumed cyst (fenestration, curettage). The correct diagnosis may be delayed until pathologic examination or lesion recurrence. Intraoperative frozen-section analysis may fail to identify the malignancy, with definitive diagnosis requiring permanent sections or molecular testing.[5]

Location and distribution

• CMEC arises centrally within the jawbones. The posterior mandible (premolar–molar–angle region) is the most common location; maxillary lesions are less frequent and may involve the premaxilla or posterior maxilla.[1][2]
• Maxillary lesions are more difficult to evaluate on panoramic radiography because of superimposition of adjacent structures; cross-sectional imaging is particularly important in these cases.[5]

Periphery and shape

• The periphery ranges from well-defined and corticated to ill-defined and irregular:
  • Approximately two thirds of cases show relatively well-defined margins.[2]
  • A well-defined sclerotic border has been described as a common feature in dedicated imaging reviews.[6]
  • Ill-defined or diffuse margins are present in roughly one third of cases and should raise suspicion for malignancy.[2][5]
• Shape is variable: unilocular or multilocular. Multilocular lesions are more common and may contain numerous small loculations.[5][6]

Internal architecture

• Radiolucent component: Most lesions (approximately 82% in one large series) present as unilocular or multilocular radiolucencies with bone destruction.[2]
• Internal septations: Bony septations are frequently observed within multilocular lesions. A characteristic feature is the presence of numerous small loculations that lack well-defined septae bordering each loculation, distinguishing CMEC from the more regular compartmentalization seen in ameloblastoma.[6]
• Internal calcification/sclerotic bone: Scattered calcification or amorphous sclerotic bone within the lesion is reported in a minority of cases (approximately 18% in one series of 39 cases) and is a relatively distinctive finding among jaw radiolucencies. Rarely, a mixed radiopaque–radiolucent pattern may simulate a fibro-osseous lesion.[2][6][9]
• Soft-tissue density: Internal soft-tissue shadows may be identified on cross-sectional imaging, representing solid tumor components within a predominantly cystic lesion.[5]

Effects on teeth and adjacent structures

• Tooth displacement occurs occasionally.[5][6]
• Root resorption has been reported but is not a consistent finding.[5][6]
• Association with impacted teeth: The lesion may surround the crown of an impacted tooth (typically a third molar), closely mimicking a dentigerous cyst.[1][5]

Cortical involvement and extent

• Cortical expansion is common and may be the only sign of aggressive behavior in early-stage lesions.[1]
• Cortical perforation with extension into surrounding soft tissue is observed in a substantial proportion of cases and is a hallmark of more advanced disease. In one series, cortical bone destruction was identified in five of eight cases evaluated with cross-sectional imaging.[5][6]
• A three-tier staging system based on cortical status has been described: stage I (intact cortex, no expansion), stage II (intact cortex with expansion), and stage III (cortical perforation, periosteal breakdown, or nodal spread).[1]
• Average lesion diameter is approximately 3.5 cm (range 2.0–5.3 cm).[5]

Growth pattern

• Growth is typically expansile, but an infiltrative pattern with irregular margins and cortical destruction may be seen, particularly in higher-grade tumors.[2][5]

Recognized variants

• Cyst-like (cystic) variant: A subset of CMEC presents as a well-defined, predominantly cystic radiolucency indistinguishable from an odontogenic cyst on conventional imaging. These lesions may be unilocular or multilocular with clear boundaries and minimal cortical disruption, making preoperative radiographic diagnosis of malignancy extremely difficult.[5]

CT

• Contrast-enhanced CT delineates the extent of bone destruction, cyst-wall irregularity, internal septations, and soft-tissue components more accurately than panoramic radiography, particularly for maxillary lesions. Three-dimensional CT reconstructions provide spatial visualization of cortical defects and the tumor's relationship to adjacent vital structures.[5]
• The cyst wall of CMEC is often irregular with variable thickness, and enhancing soft-tissue masses may be identified within or adjacent to the cystic component—features that help distinguish CMEC from simple odontogenic cysts, which typically show smooth, thin walls.[5]

MRI

• MRI is superior for evaluating bone marrow invasion, perineural tumor spread, and soft-tissue extent. The solid portions of CMEC demonstrate intermediate to slightly high signal intensity on T2-weighted sequences, whereas simple cyst fluid shows uniformly high T2 signal—a useful discriminator.[5][10][11]
• MRI is recommended when perineural spread is suspected clinically (e.g., paresthesia) or when dental amalgam artifacts degrade CT image quality.[10]

Central mucoepidermoid carcinoma is frequently misdiagnosed as a benign odontogenic lesion because of its cyst-like radiographic appearance and slow clinical course. The key diagnostic challenge is recognizing features that suggest malignancy—cortical perforation, internal soft-tissue density, amorphous sclerotic bone, and irregular septations—within an otherwise benign-appearing jaw radiolucency.[2][6]

The highest-risk diagnostic pitfall is misidentifying CMEC as a benign odontogenic cyst, leading to conservative treatment (curettage or fenestration) and delayed definitive surgery. When histologic examination of a jaw cyst reveals mucous cells, duct-like structures, or epithelial atypia, MAML2 fluorescence in situ hybridization should be performed to exclude CMEC.[3][4][5]

Microscopic features

• CMEC is composed of an admixture of mucous cells, epidermoid (squamoid) cells, and intermediate cells arranged in cystic, glandular, or solid patterns—identical to the cell types seen in conventional salivary gland mucoepidermoid carcinoma.[2][3]
• The majority of cases are low grade, characterized by predominantly cystic architecture with well-differentiated mucous and epidermoid cells and few mitoses. In the largest single-institution series (39 cases), 26 of 39 (67%) were low grade.[2][3]
• High-grade tumors demonstrate anaplasia, perineural invasion, necrosis, elevated mitotic activity, and a predominantly solid growth pattern.[2][3]
• Intracellular mucin, demonstrable with mucicarmine or PAS staining, is a diagnostic requirement.[1]

Relationship to odontogenic cysts and glandular odontogenic cyst

• A significant proportion of CMEC cases show areas histologically resembling glandular odontogenic cyst (GOC), including cuboidal lining epithelium with duct-like structures and mucous cells. In one series, half of the tumors contained GOC-like areas.[3][4]
• The cyst-like variant of CMEC may show flat or cuboidal lining epithelium with only focal areas of neoplastic proliferation, making diagnosis on curettage or fenestration specimens particularly challenging.[5]

Grading

• Histologic grading follows the same systems used for salivary gland MEC.

Molecular findings

• The t(11;19)(q21;p13) translocation producing the CRTC1-MAML2 (or CRTC3-MAML2) fusion gene is detected in 50%–100% of CMEC cases across different series.[1][5]
• MAML2 rearrangement is the most reliable molecular tool for distinguishing CMEC from GOC, as GOC is consistently MAML2-negative.[1][4][16]
• MAML2 fusion-positive tumors are generally associated with favorable prognosis in salivary MEC; the prognostic significance in the central variant requires further study given the small case numbers.[1][17]

Immunohistochemistry

• Cytokeratin profiling can aid differential diagnosis: CMEC expresses CK18 more consistently than GOC, while GOC shows stronger CK19 expression. Higher maspin expression has also been reported in CMEC compared with GOC.[4][18]

Workup and referral

• Any intraosseous radiolucency with atypical features (cortical perforation, internal calcification, soft-tissue component, irregular septations) warrants prompt incisional biopsy rather than empiric cyst management.[1][2]
• Cross-sectional imaging (CT and/or MRI) is indicated to assess the extent of bone destruction, soft-tissue involvement, and relationship to vital structures before definitive treatment.[5][10]
• Neck imaging should be obtained to evaluate for regional lymph node metastasis, reported in approximately 9%–10% of cases.[1][2]

Definitive treatment

• Radical surgical excision (segmental or marginal resection with adequate margins) is the treatment of choice.[1][2][19]
  • Conservative treatment (enucleation, curettage) carries a recurrence rate of approximately 40%, compared with approximately 4% after radical resection.[1]
• Adjuvant radiotherapy is considered for high-grade tumors, positive or close margins, perineural invasion, and nodal metastasis, although its role remains debated given the rarity of the tumor.[1][2]
• Neck dissection is indicated when clinical or imaging evidence of nodal disease is present.[1]

Recurrence and metastasis

• Local recurrence occurred in 8 of 35 patients (23%) in the largest clinicopathologic series; notably, 75% of recurrences were in low-grade tumors, underscoring the importance of adequate initial surgical margins regardless of grade.[2]
• Regional lymph node metastasis has been reported in approximately 10% of cases, and distant metastasis (lungs, brain) is rare.[1][2]
• Late recurrence (years to decades after initial treatment) is well documented, necessitating prolonged surveillance.[8][19]

Prognosis

• Overall prognosis is favorable when managed with radical surgery; the 5-year survival rate is excellent in most appropriately treated cases.[1][19]
• Adverse prognostic factors include maxillary location (risk of extension into vital structures), high histologic grade, cortical perforation with soft-tissue extension, and positive surgical margins.[1][2]

Surveillance

• Semiannual clinical and radiographic examinations (panoramic radiography and/or CT) are recommended for at least 5 years, followed by annual follow-up indefinitely given the risk of late recurrence.[1][8]