Adenoid Cystic Carcinoma

Adenoid cystic carcinoma is the most frequent large airway bronchial gland tumor, occurring in the trachea and bronchi. It accounts for approximately one-third of tracheal carcinomas. They present in adults with symptoms of bronchial obstruction such as cough, wheezing, dyspnea, and/or hemoptysis. Although they may be difficult to detect by radiograph due to their central location, they are easily biopsied with the flexible fiberoptic bronchoscope. They affect adults with slight male predominance.

Gross Appearance

Grossly, adenoid cystic carcinomas present as exophytic or annular lesions of the bronchial tree. Resected lesions range in size from 1 to 5 cm. The tumor is gray-white with ill defined borders and infiltration into the adjacent tissues. Bronchial mucosa usually covers the lesion, but it may be ulcerated or replaced with metaplastic squamous epithelium.

Microscopic Features

Histologically, the tumors are composed of regular mucus-containing glands lined by epithelial and myoepithelial cells.

There is also an abundance of hyaline basement membrane material.

The tumors frequently exhibit a cribriform architecture with infiltration into adjacent tissues, although they may produce solid or tubular patterns. Individual tumor cells are hyperchromatic with increased nuclear cytoplasmic ratios. The tumor may exhibit occasional mitoses and focal areas of necrosis.

Lymphatic and perineural invasion are frequent. This microscopic appearance is identical to adenoid cystic carcinomas of the salivary gland and breast, and primary lesions in the lung cannot be distinguished from more common metastatic lesions on the basis of histopathologic features.

The use of special stains such as Alcian blue, mucicarmine, and period acid-Schiff (PAS) can be helpful in demonstrating the mucinous material within the glands.

Positive immunohistochemical staining of the tumor cells with S-100, low molecular weight cytokeratins, vimentin, and actin can prove the presence of myoepithelial cells. Glial fibrillary acidic protein staining is consistently negative and can be used to differentiate this tumor from the pleomorphic adenoma, which also has a myoepithelial component.

Other tumors that must be differentiated from adenoid cystic carcinomas include adenocarcinomas and small cell carcinomas. Occasionally adenocarcinomas may exhibit focal areas of differentiation with a cribriform pattern like adenoid cystic carcinomas. However, the diagnosis of adenoid cystic carcinoma should be limited to tumors with only the predominance of the cytological and architectural features of adenoid cystic carcinoma (described earlier). Immunohistochemical stains can also make the distinction between adenoid cystic carcinoma and adenocarcinoma by identifying the myoepithelial cells present only in adenoid cystic carcinomas.

The small hyperchromatic cells of adenoid cystic carcinoma may resemble those of small cell carcinoma, especially in specimens from small biopsies, which don't allow evaluation of the overall tumor architecture.

However, the nuclei of adenoid cystic carcinoma lack features of neuroendocrine differentiation, such as homogeneous chromatin and nuclear molding.

Adenoid cystic carcinomas have a relatively good prognosis with 5- and 20-year survival rates of 85% and 20%.

They are generally treated with a lobectomy.

These resections can be complicated by the tumor's growth along the bronchi and infiltration, which may make it difficult to obtain clear resection margins.

Despite their generally low grade appearance, behavior cannot be accurately predicted by the histology. Therefore, staging appears to be the most important factor in predicting the clinical outcome. The role of adjuvant chemotherapy and radiation is not well defined.