Mucoepidermoid Tumors

Mucoepidermoid tumors of the lung and trachea are rare salivary gland-type tumors arising from the excretory duct reserve cells along the tracheobronchial tree, and are identical to those originating from the salivary glands. Not surprisingly, there are no pathologic features by which a primary lung tumor can be differentiated from a metastatic tumor, and a careful clinical history and examination is needed to make this distinction. Mucoepidermoid carcinomas have been reported in all age groups and equally in both sexes, with less aggressive forms being present in patients younger than 30 years.

Most mucoepidermoid tumors arise from the main or segmental bronchi, producing a polypoid tumor which may produce obstructive symptoms due to its growth in the bronchial lumen. At the time of presentation chest radiograph generally find a solitary mass or postobstructive pneumonic consolidation.

Gross Appearance

They range in size from a few millimeters to over 6 cm in diameter.

Most are covered by respiratory mucosa and grossly appear well circumscribed.

On cut section they are composed of a mixture of solid and cystic mucinous areas.

Microscopic Features

The tumors are composed of solid nests and tubular structures, formed by a variable mixture of squamous, mucus-secreting and intermediate cells. Mucoepidermoid carcinomas are subclassified based upon their degree of differentiation in to low and high grades. Low grade tumors have a predominance of tall columnar mucus secreting cells, often form in glandular spaces. The focal areas of squamous differentiation are recognized by the presence of intercellular bridges. Although low grade tumors often appear well circumscribed by gross inspection, the majority exhibit invasion of the bronchial submucosa. In contrast, high grade tumors contain predominately squamous cells with increased cytologic atypia and pleomorphism, forming more solid areas with few mucus secreting cells. Higher grade tumors also show more frequent and deeper invasion of the adjacent lung parenchyma. Grade generally correlates with the behavior, with low grade lesions being less aggressive. However, there is a subset of low grade tumors which behave in a very aggressive fashion. Immunohistochemistry can demonstrate the presence of cytokeratins, and mucicarmine or periodic acid-Schiff (PAS) stains can be used to demonstrate the presence of intracytoplasmic mucin.

The diagnosis of mucoepidermoid carcinomas can be difficult because there are several lesions with similar features that must be excluded.

A low grade mucoepidermoid carcinoma appears very similar to a mucous gland adenoma. The low grade mucoepidermoid carcinoma has a very low mitotic rate and minimal or no cytologic atypia. Mucous gland adenomas may contain numerous glands, like a mucoepidermoid carcinoma, however, they lack the areas of squamous differentiation and intermediate cells possessed by the mucoepidermoid carcinoma.

The poorly differentiated squamous cell carcinoma may be confused with a high grade mucoepidermoid carcinoma unless intermediate and mucin secreting cells are appreciated.

Perhaps the most difficult distinction is the differentiation of a mucoepidermoid carcinoma from an adenosquamous carcinoma, which is a highly aggressive tumor. The following features of mucoepidermoid carcinoma have been suggested as criteria for differentiating the two:

An exophytic lesion with an endobronchial component located in the proximal bronchial tree.

An absence of carcinoma in situ changes in the bronchial epithelium overlying the tumor.

The absence of squamous pearls and individual cell keratinization.

The presence of a random mixture of squamous and adenomatous elements forming solid and glandular areas.

The presence of low grade mucoepidermoid areas if there are areas of poor differentiation.

Other features helpful in the recognition of adenosquamous carcinomas are their more peripheral location, association with scars, and gross areas of pigmentation.

Low grade mucoepidermoid carcinomas are generally treated by surgical resection without adjuvant radiation or chemotherapy. The extent of the resection is based upon the extent of tumor at the time of resection. High grade tumors are treated in a similar fashion, but these patients may receive postoperative radiation therapy.

Low grade tumors have a very good prognosis, except for the widely metastatic variant, and are treated by complete surgical resection alone. High grade tumors behave in a fashion and have survival rates equivalent to similarly staged well differentiated non-small cell carcinomas. For these tumors, the combined use of surgery and radiation therapy is suggested