MUCOEPIDERMOID CARCINOMA: CLINICAL FEATURES, HISTOPATHOLOGY & TREATMENT
Author: Sanketh DS, MDS
INTRODUCTION AND ETIOLOGY
Mucoepidermoid carcinoma (MEC), so called because it comprises of “mucus” cells and “epidermoid (squamous)” cells histologically, is the most common malignant salivary gland tumor. This lesion was previously called “mucoepidermoid tumor” since it was thought that there were two subsets of these lesions – a benign and a malignant counterpart. However, it is now recognised that low grade tumors, that were previously designated as “benign” could recur and metastasize to regional lymph nodes and the term “mucoepidermoid carcinoma” is more appropriate.
Though the etiology and pathogenesis is not clear, radiation has been recognised to be a contributing factor. Translocation of genes MECT1 and MAML2 at chromosomes 19 and 11 respectively have been frequently identified and could be an early event in the lesion’s pathogenesis. This genetic alteration has been reported in 70% of cases of MEC.
MEC is the most common malignant salivary gland tumor in adults and children accounting for 30% of all malignant salivary gland neoplasms occurring in the major and minor salivary glands. The age range is wide and MECs can occur from the 1st to 9th decade of life with the the 4th decade being most common. It has a slight male predilection although lesions in the tongue and retromolar areas are more common in women.
The lesion occurs predominantly (60%) in major salivary glands with the parotid gland being the most common major gland (45%). When MEC affects the minor glands, palate and the buccal mucosa are most frequently affected, with the lesion rarely occurring in the lower lip, tongue, retromolar area and the floor of mouth.
Low-grade and intermediate-grade tumors present as slow growing swellings, high-grade tumors are rapidly growing and 75% of the lesions are asymptomatic. Very rarely, do patients experience symptoms like pain, dysphagia and trismus. Facial paralysis may sometimes occur with high-grade parotid tumors. Fixation to the skin, ulceration and numbness of adjacent areas may also accompany high-grade tumors.
MECs of the minor salivary glands are least suspicious and may be confused for other benign neoplasms or reactive lesions. Those occurring on the palate may also mimic mucoceles with their blue and fluctuant appearance.
As the name indicates, MEC is composed of mucus cells and epidermoid/squamoid cells. In addition there is a 3rd type of cell called the intermediate cell that is thought to be a progenitor to the mucus and epidermoid cell.
1. Mucus cells are columnar, goblet shaped cells that have a foamy cytoplasm due to the presence of mucin. They are slightly basophilic and have a frosted-glass appearance. They may line cystic structures or may be scattered in islands composed of epidermoid cells.
2. Epidermoid cells are squamoid in appearance and occur in nests or solid islands. They have a stratified appearance and very rarely show keratinization or keratin pearls.
3. Intermediate cells are round to ovoid with very little eosinophilic cytoplasm and a darkly staining nucleus. It may form nests and may appear merged with other cells.
Based on their histopathological appearance, MECs are classified as a)low-grade b)intermediate grade and c)high-grade tumors.
These lesions show numerous mucin filled cysts of varying sizes in the stroma that are lined by mucous, epidermoid or intermediate cells. Cysts and mucus cells are more prevalent in low-grade than intermediate and high grade tumors. Solid nests and islands are not very prominent. The stroma is fibrous and may be mildy inflamed. Cells are bland looking and do not show pleomorphism.
These lesions show prominent solid nests and islands comprising of epidermoid and intermediate cells. Cystic spaces and mucus cells are rarely seen and the tumor may show considerable pleomorphism, mitotic figures and necrosis. Epidermoid cells predominate and the lesion may look like squamous cell carcinoma.
These tumors fall between a low-grade and a high-grade tumor. Cysts may be less conspicuous than a low-grade tumor although solid islands are more prominent. Pleomorphism to a certain degree may be observed with a few mitotic figures. Though all 3 types of cells are present intermediate cells predominate.
TREATMENT AND PROGNOSIS
Treatment of MEC depends on the grade and location of the tumor. Low and intermediate- grade tumors of the parotid is managed by a sub-total parotidectomy with preservation of the facial nerve while high grade tumors are managed by total removal of the gland. Affected sub-mandibular glands are removed entirely. A radical neck dissection is advocated when there is clinical evidence of cervical node metastasis.
Prognosis depends on the histopathological grade of the tumor and several grading systems are in use. Low and intermediate grade tumors have a good prognosis while the high-grade tumors have a bad prognosis with only 30-54% of patients surviving.
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