Mucoepidermoid carcinoma

 Definition Mucoepidermoid carcinoma is a malignant glandular epithelial neoplasm characterized by mucous, intermediate and epidermoid cells, with columnar, clear cell and oncocytoid  features.

ICD code 8430/3

Synonyms Mixed epidermoid and mucus secreting carcinoma. Mucoepidermoid tumour is an inappropriate synonym since the malignant biologic behaviour of this neoplasm is well established.

 Epidemiology Mucoepidermoid carcinoma (MEC) is the most common primary salivary gland malignancy in both adults and children {1560,2681,2711}. MEC demonstrates a wide, nearly uniform age distribution, with diminution in paediatric and geriatric life {456,1850}. Mean patient age is approximately 45 years. Sixty percent of palate lesions are in patients under 40. Tongue neoplasms are reported at an older average age. There is a 3:2 female predilection, but higher female predominance for tongue and retromolar pad tumours {668}.

 Localization Approximately half of tumours (53%) occur in major glands. The parotid glands predominate, representing 45%, with 7% for submandibular glands and 1% in sublingual glands. The most frequent intra-oral sites are the palate and buccal mucosa.

Clinical features Signs and symptoms Most tumours present as firm, fixed and painless swellings. Sublingual gland lesions may demonstrate pain in spite of small size. Superficial intraoral neoplasms may exhibit a blue-red colour and mimic a mucocele or vascular lesion. The mucosa overlying palatal tumours can be papillary. Cortical bone is sometimes superficially eroded. Symptoms can include pain, otorrhoea, paraesthesia, facial nerve palsy, dysphagia, bleeding and trismus {703}.

Macroscopy Tumours are firm, smooth, often cystic, tan, white or pink with well-defined or infiltrative edges.

Tumour spread and staging Parotid gland tumours spread to adjacent pre-auricular lymph nodes, then to the submandibular region. Submandibular gland neoplasms spread to submandibular and the upper jugular lymphatic chain. Palatal lesions may extend into the upper respiratory tract and skull base. Lip lesions invade submental nodes and intraoral tumours metastasize to submandibular, post auricular and upper accessory nodes in neck level II. With advancing disease, levels III, IV and V may become involved. Distant metastases may be widespread to lung, liver, bone, and brain.

Histopathology Mucoepidermoid carcinoma is characterized by squamoid (epidermoid), mucus producing and cells of intermediate type. The proportion of different cell types and their architectural configuration (including cyst formation) varies in and between tumours.

They are usually multicystic with a solid component and sometimes the latter predominates. Some tumours have defined borders but infiltration of gland parenchyma is evident. Cystic spaces are lined by mucous cells with basaloid or cuboidal intermediate cells interspersed, and to a lesser degree, polygonal epidermoid cells, but keratinization is rare. Mucous cells are large, with pale cytoplasm and peripherally displaced nuclei. They typically constitute less than 10% of the tumour. Sialomucin content is demonstrated by mucicarmine or Alcian blue staining. Intermediate cells usually predominate. Clear, columnar and/or oncocytic cell populations may be present and occasionally are prominent {985, 1198,1996}. Clear cells demonstrate minimal sialomucin, but are diastase sensitive periodic acid-Schiff positive, indicating glycogen content {666}. Focal sclerosis and/or mucus extravasation with inflammation is common. A sclerosing variant has been described {2657}. Neural invasion, necrosis, increased mitoses or cellular anaplasia are uncommon. At the tumour edge, a lymphocytic infiltrate with possible germinal centre formation can mimic nodal invasion {83}.

Grading Several systems have been proposed to grade this neoplasm, but none has been universally accepted {86,258,695,1850, 2443}. However, one recent system using five histopathologic features has been shown to be reproducible in defining low, intermediate and high-grade tumours {86,972,1766}. In the submandibular gland low-grade tumours tend to behave more aggressively {921}.