Bronchial adenoma is a descriptive but misleading term for a diverse group of respiratory tract neoplasms that arise beneath the bronchial epithelium or in bronchial glands. They are characterized by a clinical course that is usually more benign than that of bronchogenic carcinoma. Three types make up approximately 95% of bronchial adenomas. Carcinoids (2 types of bronchial neuroendocrine tumors) account for 85% of bronchial gland tumors and 1-2% of all lung malignancies. Adenoid cystic carcinoma (cylindroma), which commonly arises in a salivary gland, accounts for 10% of bronchial adenomas. Mucoepidermoid carcinomas account for 1-5% of all bronchial gland tumors (0.1-02% of all lung tumors). The common bronchial gland adenoma is a truly benign mucous gland adenoma and mesodermal and other lesions can arise in the tracheobronchial tree.
Laennec described a bronchial carcinoid in 1831. In 1907, Oberndorfer introduced the term karcinoide to indicate “resembles carcinoma.” In 1930, Kramer grouped bronchial carcinoids and cylindromas as bronchial adenomas because of their better prognosis and less aggressive behavior compared with bronchogenic carcinoma and mucoepidermoid tumors were described in 1952. In 1972, Arrigoni identified a more aggressive subset of carcinoids and classified them as atypical carcinoids, as opposed to the less aggressive typical carcinoids. Bronchial carcinoids are part of a spectrum of neuroendocrine tumors (see below), of which only the first 2 are considered bronchial adenomas.
Bronchial neuroendocrine tumors, including tumor type and level of malignancy, are as follows:
Typical carcinoid (Kulchitsky type I) - (+)
Atypical carcinoid (Kulchitsky type II) - (++)
Large cell neuroendocrine carcinoma - (+++)
Small cell carcinoma (Kulchitsky type III) - (++++)
Treatment is surgical, often using conservative techniques. In 1932, Bigger performed the first bronchoplastic procedure, a bronchotomy for removal of a left mainstem endobronchial lesion. In 1939, Eloesser performed a bronchotomy with simple excision and fulguration of an adenoma of a left lower lobe orifice. In 1947, Price-Thomas performed the first sleeve resection for an adenoma originating in the right mainstem bronchus.
Bronchial carcinoids are thought to arise from Kulchitsky cells. These neuroendocrine cells, formerly classified as amine precursor uptake and decarboxylation cells, produce and store biogenic amines and peptides. Typical carcinoids originate as clusters of monotonous polyhedral cells in a fibrovascular stroma. Ultrastructurally and immunoreactively, carcinoids share characteristics with small cell neuroendocrine carcinoma of the lung.
Adenoid cystic carcinoma originates from salivary gland tissue. Occasionally, some tumor cells in this variant are of myoepithelial origin. These tumors have several other names, including cylindromas, adenoid cystic basal cell carcinomas, adenomyoepitheliomas, and pseudoadenomatous basal cell carcinomas.
Mucoepidermoid carcinomas originate from trachea and, more commonly, the proximal bronchi. These tumors are of squamous and intermediate elements, with intercellular bridges. They have the same microscopic appearance as mucoepidermoid carcinoma of the salivary glands, arise in glandular submucosa, and manifest as submucosal lesions.
Mucous gland adenomas (ie, bronchial cysts, papillary cystadenomas) are rare submucosal tumors arising from mucous glands and truly are benign tumors.
A related Medscape Reference article is Carcinoid Lung Tumors.
Symptoms develop from the growth of the tumors within the tracheobronchial tree, with consequent obstruction leading to cough, hemoptysis, atelectasis, or pneumonia. 
Adenoid cystic carcinoma behaves very similar to major and minor salivary gland tumors. An important aspect of these tumors is that they tend to spread in a submucosal plane along the perineural lymphatics, beyond the obvious endoluminal margins of the tumor. Most do not metastasize; however, total excision by tracheal resection or tracheobronchial resection is not always possible because of extensive submucosal spread, and local recurrence remains a possibility.
See the list below:
- Lobar or segmental location - Approximately 60%
- Main bronchus - Approximately 20%
- Peripheral - Approximately 20%
- Carinal or tracheal – Infrequent
- Multiple sites – Rare
Adenoid cystic carcinoma – Predilection for trachea
Mucoepidermoid carcinoma – Usually proximal bronchi and may be more common on the left side of the tracheobronchial tree
Endocrinopathies associated with bronchial carcinoids include Cushing syndrome (with increased corticotropin levels), hyperpigmentation (excess melanocyte-stimulating hormone), syndrome of inappropriate excretion of antidiuretic hormone, and hypoglycemia. In addition, bronchial carcinoids may be associated with multiple endocrine neoplasia syndrome in up to 4% of patients, the majority of whom are female.
Carcinoid syndrome is a clinical entity that includes cardiovascular, gastrointestinal, respiratory, and cutaneous manifestations. Carcinoid syndrome occurs most commonly when gastrointestinal carcinoids metastasize to the liver and less frequently when due to bronchial carcinoids. Serotonin seems to play a major role in the manifestations of carcinoid syndrome. When released into the blood stream from gastrointestinal carcinoids, serotonin is broken down in the liver. However, in the presence of liver metastasis, serotonin has a diminished opportunity to be exposed to hepatic metabolism.
Bronchial carcinoids seem to produce diminished amounts of serotonin, and carcinoid syndrome is uncommon; however, when it does occur as a result of a bronchial carcinoid, it may be unusually severe. Carcinoid syndrome can be associated with cardiac valvular fibrotic lesions. These are usually on the right side when the syndrome is due to hepatic metastases, but they may be on the left side in the presence of a right-to-left cardiac shunt or carcinoid syndrome due to a bronchial carcinoid.
Cushing syndrome occurs is reported in as many as 6% of patients with bronchial carcinoid, and represents the second most common paraneoplastic syndrome. An occult bronchial carcinoid should be sought in a patient with Cushing syndrome and no evident adrenal or pituitary source. Cushing syndrome due to bronchial carcinoids is most often the result of peripherally located tumors, many of which may be radiographically occult. Carcinoid metastases maintain a corticotropin hypersecretory status despite resection of the primary tumor.
Bronchial adenomas represent 1-3% of pulmonary malignancies.
Carcinoids account for approximately 85% of bronchial adenomas and 1-2% of all lung malignancies. Almost all bronchopulmonary carcinoids are clearly primary tumors, rather than metastatic, accounting for 25% of all carcinoids.
Approximately 84% of bronchial carcinoids are typical and 16% are atypical.
Adenoid cystic carcinomas account for 10% of bronchial adenomas.
Mucoepidermoid carcinomas account for 1-5%.
Bronchial adenomas are uncommon in all countries, and carcinoids represent the great majority. The pattern of symptoms and treatments are similar to those seen in the United States.
The overall 5-year patient survival rate for bronchial adenomas is 96%.
Long-term follow-up reports reveal little evidence of local recurrence or distant metastases after complete resection.
Mucoepidermoid carcinoma occasionally results in intracranial metastases, even in cases with minimal bronchial wall involvement.
The racial distribution is equal, as far as can be determined for these uncommon tumors.
Men and women are equally affected.
The prevalence is highest in persons aged 30-50 years, and the mean age at presentation is 43 years. The incidence varies somewhat with the type of bronchial adenoma.
Looking specifically at bronchopulmonary carcinoids, a wide age distribution is also seen with bimodal peaks in the fourth and sixth decades of life. Individuals older than 50 years are more likely (25%) to develop atypical carcinoid than those younger than 30 years (< 10%).
Although adenoid cystic carcinoma affects persons of any age, the metastatic variety tends to occur in younger persons.
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