Bronchopulmonary Carcinoid Tumors

Pulmonary carcinoid tumors account for 1 to 2 percent of all lung malignancies in adults and approximately 25 to 30 percent of all carcinoid tumors. In a recent review of data from the SEER registry, Modlin et al demonstrated annual incidence rates of bronchial carcinoids at 0.52 and 0.89 per 100,000 population in white males and females, respectively (the corresponding values for black males and females were somewhat higher, at 0.39 and 0.57, respectively). Pulmonary carcinoid tumors are thought to arise from Kulchitsky cells disseminated throughout the bronchopulmonary mucosa. Though these tumors share certain morphologic and biochemical characteristics, confusion has plagued reasonable attempts to devise a classification system for these tumors. In 2004, the WHO devised a new classification for bronchopulmonary carcinoids based upon a spectrum of clinicopathologic entities ranging from hyperplastic neuroendocrine cell lesions (carcinoid tumorlets, diffuse idiopathic pulmonary neuroendocrine cell hyperplasia [DIPNECH]) to high-grade neuroendocrine tumors (small cell carcinoma [SCLC] and large cell neuroendocrine carcinoma).

Pulmonary neuroendocrine cell hyperplasia and tumorlets (typical carcinoid tumors that measure ≤ 5mm in greatest dimension and extend beyond the basement membrane) represent perhaps the earliest manifestations of neuroendocrine disease in the bronchopulmonary system. The 2004 WHO classification of lung tumors applies the term DIPNECH to a proliferation of scattered single cells, small nodules (neuroendocrine bodies), or linear proliferations of pulmonary neuroendocrine cells confined to the bronchial and bronchiolar epithelium. The clinical relevance of these lesions is controversial. Initially identified in the clinical context of chronic pulmonary diseases like obliterative bronchiolitis, bronchiectasis, and idiopathic pulmonary fibrosis, carcinoid tumorlets were thought to represent benign hyperplastic lesions of little clinical relevance. However, recent data suggest the potential malignant nature of these lesions, as carcinoid tumorlets have been demonstrated in association with typical carcinoid tumors of the bronchopulmonary system and with lymph node metastases.

Representing nearly two-thirds of pulmonary carcinoids, well-differentiated neuroendocrine tumors of the lungs and bronchi (typical carcinoids) are composed of cytologically bland cells that exhibit minor cellular atypia and rare mitoses. Based on the 2004 WHO classification, these tumors must demonstrate <2 mitoses per 10 high power fields (10HPH), lack necrosis, and are larger than 5mm. They usually present as perhilar masses in the fifth decade of life and demonstrate a relatively indolent disease course; however, some patients may present with recurrent pneumonia, cough, hemoptysis, and chest pain. Ectopic secretion of biologically active hormones is not uncommon, as these tumors may secrete corticotrophin and growth hormone with relative frequency; however, carcinoid syndrome occurs in fewer than 5% of patients with these tumors due to the relative paucity 5-HT secretion. Metastases from typical pulmonary carcinoids have been reported in approximately 12% of cases and overall survival is thought to exceed 90%.

Approximately one third of bronchopulmonary carcinoids are atypical tumors, intermediate grade neuroendocrine tumors whose pathologic criteria depends on the presence of carcinoid morphology and at least one of the following: (1) necrosis or (2) 2 to 10 mitoses per 10HPH. Unlike typical pulmonary carcinoids, atypical carcinoids usually occur in the periphery of the lungs of older patients. Their clinical course is aggressive, with a relatively high incidence of mediastinal lymph node metastases (exceeding 50%) and an abbreviated 5-year survival rate of only 40% to 75%.

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