Mucoepidermoid carcinoma

• Histologic grade (low vs high) determines imaging appearance and treatment—low-grade tumors are well-circumscribed with cystic components, while high-grade tumors show poorly defined margins and solid appearance
• Perineural spread along cranial nerves, particularly the facial nerve (CN VII) and branches—requires dedicated fat-saturated post-contrast T1 imaging to the skull base, cavernous sinus, and inner ear
• Location within salivary glands—most common in parotid (~40%) and hard palate, but can occur anywhere salivary tissue exists including minor glands, bronchi, and lacrimal glands

• Low-grade mucoepidermoid carcinoma appears as a well-circumscribed mass with prominent cystic components, similar to pleomorphic adenoma on ultrasound (hypoechoic lesion with partial or completely cystic appearance, standing out against hyperechoic normal parotid gland).
• Low-grade tumors on CT show well-circumscribed margins with cystic components, enhancement of solid elements, and occasional calcifications; histologically characterized by well-differentiated cells, high proportion of mucous cells, and prominent cyst formation.
• Low-grade tumors on MRI demonstrate T1 signal that is low to intermediate with low signal in cystic spaces, T2 signal that is intermediate to high with high signal in cystic areas, and heterogeneous enhancement of solid components on post-contrast T1.
• High-grade mucoepidermoid carcinoma presents as a poorly circumscribed, infiltrative solid mass with infrequent or absent cystic components; histologically shows poorly differentiated cells with cellular pleomorphism, high proportion of squamous cells, and solid architecture.
• High-grade tumors on MRI show T1 signal of low to intermediate intensity, T2 signal of intermediate to low intensity, and poorly defined margins that infiltrate locally, in contrast to the well-demarcated low-grade lesions.
• Perineural spread is a hallmark feature requiring careful assessment with fat-saturated post-contrast T1 sequences from the primary tumor through the skull base, including the cavernous sinus and inner ear, as this tumor has a known predilection for perineural invasion.
• The tumor is composed histologically of a mixture of mucus-secreting cells (muco-) and squamous cells (-epidermoid), with lymphoid infiltrate often present; clear mucin-containing cells stain reddish pink with mucicarmine stain.
• MAML2 gene rearrangement/fusion is present in approximately 75-80% of mucoepidermoid carcinomas and appears to be specific for this entity, distinguishing it genetically from other salivary gland malignancies.

• Pleomorphic adenoma: can be indistinguishable from low-grade mucoepidermoid carcinoma on imaging; key distinguishing feature is that pleomorphic adenoma is benign and has no perineural spread, low recurrence, and excellent prognosis, whereas low-grade mucoepidermoid requires wide local excision with facial nerve preservation.
• Warthin tumor: benign, typically shows cystic appearance and is more common in smokers; lacks the cellular atypia and aggressive potential of mucoepidermoid carcinoma.
• Adenoid cystic carcinoma: malignant tumor that can present with either well-circumscribed or infiltrative appearance; distinguished by characteristic cribriform histology and strong predilection for perineural spread, but typically shows lower cystic component and different cytologic features.
• Lymphoma (NHL): presents as infiltrative lesion in high-grade mimics; distinguished by involvement of multiple lymph nodes, lack of salivary gland origin, and different imaging features (homogeneous appearance without cystic components typical of mucoepidermoid).
• Metastatic squamous cell carcinoma to intraparotid nodes: presents as infiltrative lesion; distinguished by clinical history of primary malignancy and lymph node location rather than primary gland origin.
• Salivary duct carcinoma: high-grade malignancy; distinguished by solid appearance without significant cystic components and more aggressive clinical behavior than even high-grade mucoepidermoid carcinoma.

Report the histologic grade based on imaging features (well-circumscribed with prominent cystic components suggests low-grade; poorly defined margins with solid appearance suggests high-grade), explicitly assess for perineural spread along the facial nerve and its branches by describing any abnormal enhancement or signal intensity along the nerve pathway on fat-saturated post-contrast sequences, specify the exact location within the salivary gland system, and note whether margins are well-defined or infiltrative, as these factors directly determine surgical approach and prognosis.

• Confusing low-grade mucoepidermoid carcinoma with benign pleomorphic adenoma based on similar imaging appearance; the key distinction is that mucoepidermoid carcinoma requires wide local excision and careful follow-up for recurrence and perineural spread, whereas pleomorphic adenoma has excellent prognosis—clinical history and pathology are essential for definitive diagnosis.
• Failing to assess for perineural spread by not imaging to the skull base and cavernous sinus; perineural invasion is a hallmark of this tumor and can influence surgical planning, staging, and prognosis, requiring dedicated fat-saturated post-contrast T1 sequences.
• Over- or under-grading the tumor on imaging by focusing on size rather than margin definition and cystic component prevalence; a large tumor can still be low-grade if well-circumscribed with prominent cysts, whereas a smaller tumor can be high-grade if poorly defined and solid.
• Missing mucoepidermoid carcinoma in unusual locations such as minor salivary glands of the palate, retromolar region, floor of mouth, buccal mucosa, lacrimal gland, or bronchi; the tumor's predilection for parotid and palate should not obscure awareness of its potential to arise in any salivary tissue.