Primary salivary gland–type tumors of the lung are uncommon tumors that comprise no more than 1% of all lung neoplasms. Because of their rarity as primary lung neoplasms, salivary gland–type tumors may pose problems in interpretation of findings in small biopsy specimens. They often are mistakenly identified as non–small cell carcinomas; however, their clinical behavior can be quite different from that of conventional non–small cell carcinomas. Because of the similar immunohistochemical profile that these tumors may share among themselves and with other conventional non–small cell carcinomas of the lung, an awareness of their occurrence as primary lung tumors is essential for accurate diagnosis and proper clinical management. It can be difficult to make this diagnosis from small transbronchial biopsy specimens, in which a limited amount of material is available for evaluation. Accordingly, in many instances, definitive diagnosis of this tumor type is best accomplished using material obtained at complete surgical resections.
Salivary gland–type tumors in the lung do not follow the same patterns of occurrence as those in the salivary glands. For instance, mixed tumors (pleomorphic adenomas), which are common in the salivary glands, are very uncommon in the lung. The most common salivary gland–type tumors of the lung are of the malignant type, such as mucoepidermoid carcinoma or adenoid cystic carcinoma. Also, at the histopathologic level, some differences may be observed with certain types of tissues. Because salivary gland–type tumors of the lung have no parallel with any tumors of the salivary glands, the diagnosis of salivary gland–type tumors of the lung requires careful clinical, radiologic, and pathologic correlation.
MUCOEPIDERMOID CARCINOMA
Clinical Features
Mucoepidermoid carcinoma (MEC) is the most common salivary gland–type tumor of the lung. It can occur at any age; however, most affected patients are adults. Yousem and Hocholzer, in their analysis of a large series of these tumors, reported 58 patients between the ages of 9 and 78 years, with a slight female preponderance. Clinically, patients’ signs and symptoms varied in accordance with the size and location of the tumor. Larger tumors in a central location are most likely to produce clinical signs and symptoms, including those related to obstruction, pneumonia, dyspnea, chest pain, and cough. Tumors that are not associated with the airway may produce symptoms when they reach a large size.
Macroscopic Features
MECs are classically described as exophytic endobronchial tumors that can attain a size of up to 5 cm in greatest dimension. They usually are well circumscribed, with a smooth surface ( Fig. 4-1 ). On cut surface, the tumors may be solid or cystic, and in many instances they exhibit both features. No predilection for any lung or lung segment for involvement by these tumors has been documented.
Microscopic Features
Histologically, MECs can be divided into low- and high-grade tumors. The classification as low- or high-grade is based on histopathologic features, such as the presence of necrosis, hemorrhage, cellular atypia, and mitotic activity. Because both histologic variants share similar features, careful evaluation of these neoplasms is important.
In low-grade tumors, the low-power view shows a tumor composed of cystic and solid areas in close association ( Fig. 4-2 ). The cystic component may contain acellular material within the cyst-like structures, which may be mixed with calcifications ( Fig. 4-3 ). Higher magnification shows both components in detail. The solid component is composed of sheets of round to polygonal cells with distinct cell borders, light eosinophilic cytoplasm, round small nucleus, and inconspicuous nucleoli ( Fig. 4-4 ). In some areas, this solid component may show the presence of sheets of similar cells with clear cytoplasm (intermediate cells) ( Fig. 4-5 ). Mitotic activity and cellular pleomorphism are absent. In some of the cystic areas it is possible to observe the presence of “epidermoid cells” admixed with mucus-secreting cells (so-called mucocytes) ( Fig. 4-6 ). Both of these components may be embedded in, or separated by, bands of fibroconnective tissue ( Fig. 4-7 ), or embedded in an inflammatory background composed predominantly of plasma cells ( Fig. 4-8 ). In some of these tumors, both solid and cystic components may show areas in which the cells acquire a more oncocytic appearance ( Fig. 4-9 ). MECs may show extensive areas of solid component with a glandular-like appearance, in which histochemical staining for mucicarmine may be helpful to demonstrate the presence of mucus-secreting cells ( Fig. 4-10 ). Areas with an exuberant fibroblastic proliferation, so-called sclerosing mucoepidermoid carcinoma, also may be predominant in these tumors. In some MECs, the appearance on low-power magnification may mimic that of a basaloid carcinoma; however, closer examination of the islands of tumor cells will disclose the presence of epidermoid elements admixed with mucus-secreting cells ( Fig. 4-11 ). In a minority of cases, the mucus-secreting glands predominate, with little solid component ( Fig. 4-12 ). An important feature of MEC is the absence of keratinization in both solid and cystic components ( Fig. 4-13 ).
High-grade tumors will display areas of necrosis or hemorrhage at low-power magnification. In the high-power view, the tumors will display areas of cytologic atypia in terms of nuclear pleomorphism and mitotic activity ( Fig. 4-14 ). Even in the high-grade neoplasms, however, areas of low-grade differentiation may be encountered.
Immunohistochemical Features
The use of immunohistochemical studies to diagnose or to rule out MEC is limited. The tumor may display immunohistochemical features similar to those of conventional non–small cell carcinomas. Thus, the final interpretation is based largely on morphology alone.
Differential Diagnosis
The scope of the differential diagnosis will depend primarily on the material available for review. In small biopsy specimens, low-grade tumors may pose more diagnostic difficulty. If the tumor shows cystic changes with a glandular appearance, mucous gland adenoma may be the most difficult clinical entity to rule out. If the specimen shows a more solid component, squamous cell carcinoma will be the leading consideration in the differential diagnosis. In the latter case, the presence of keratinization, nuclear atypia, and mitotic activity and presence of an in situ component may lead to a more correct interpretation. The separation of low- and high-grade tumors cannot be accomplished in a small biopsy specimen; this distinction requires careful examination of material obtained at complete surgical resection. Another possibility in the differential diagnosis is adenosquamous carcinoma; for this diagnosis, however, it is imperative to observe unequivocal areas of squamous cell carcinoma admixed with unequivocal areas of adenocarcinoma.
Clinical Behavior
The behavior of MEC is closely related to the degree of differentiation. Low-grade tumors can be managed surgically, and complete surgical resection is the treatment of choice. When complete surgical resection is not possible, the possibility of recurrence is high, and additional treatment may be considered. Tumors of high-grade histology often display aggressive behavior. In such cases, surgical resection and additional medical options should be considered.
ADENOID CYSTIC CARCINOMA
Clinical Features
Adenoid cystic carcinoma appears to be the second most common tumor in this family of neoplasms. Most of the cases described are in adults. In one of the larger series of these tumors, patient ages ranged from 29 to 79 years (mean, 54 years), with a slight predilection for men in a ratio of 2:1. As with other types of salivary gland–type tumors of the lung, when the tumors are centrally located, the patient may present with pulmonary obstructive symptoms, pneumonia, dyspnea, cough, wheezing, or hemoptysis. If the tumor occurs in the periphery of the lung, however, the patient may be asymptomatic.
Macroscopic Features
Most adenoid cystic carcinomas will manifest in a central endobronchial location. The tumors may range from 1 to 4 cm in greatest diameter ( Fig. 4-15 ). They usually are well circumscribed and of soft consistency.
Microscopic Features
Three distinct growth patterns for adenoid cystic carcinoma have been described: cylindromatous, tubular, and solid. The most common growth pattern is the cylindromatous pattern, characterized by islands of tumor cells arranged in a jigsaw pattern ( Fig. 4-16 ). Thin bands of fibroconnective tissue separate each one of these islands. At higher magnification, the cystic areas are seen to be lined by two rows of cells, which are composed of somewhat smaller cells with scant cytoplasm and round to angulated nuclei ( Fig. 4-17 ). In the lumen of these cystic areas, it frequently is possible to identify acellular material or the presence of extensive areas of hyalinization ( Fig. 4-18 ). Mitotic figures, cellular pleomorphism, necrosis, and hemorrhage usually are absent.
