Neuroendocrine lesions share morphologic and biochemical features with cells of the dispersed neuroendocrine cell system ( Chapter 24 ).[128] The normal lung contains neuroendocrine
cells within the epithelium as single cells or as clusters, the neuroepithelial bodies. While virtually all pulmonary neuroendocrine cell hyperplasias are secondary to airway fibrosis and/or
inflammation, a rare disorder called diffuse idiopathic pulmonary neuroendocrine cell hyperplasia appears to be a precursor to the development of multiple tumorlets and typical or atypical
carcinoids.
Figure 15-46 A, Bronchial carcinoid growing as a spherical, pale mass (arrow) protruding into the lumen of the bronchus. B, Histologic appearance of bronchial carcinoid, demonstrating
small, rounded, uniform cells.
TABLE 15-13-- Mediastinal Tumors and Other Masses
Superior Mediastinum
Lymphoma
Thymoma
Thyroid lesions
Metastatic carcinoma
Parathyroid tumors
Anterior Mediastinum
Thymoma
Teratoma
Lymphoma
Thyroid lesions
Parathyroid tumors
Posterior Mediastinum
Neurogenic tumors (schwannoma, neurofibroma)
Lymphoma
Gastroenteric hernia
Middle Mediastinum
Bronchogenic cyst
Pericardial cyst
Lymphoma
occurs most often with esophageal carcinomas and mediastinal lymphomas.
Morphology.
The pattern of metastatic growth within the lungs is quite variable. In the usual case, multiple discrete nodules (cannonball lesions) are scattered throughout all lobes ( Fig. 15-47 ). These
discrete lesions tend to occur in the periphery of the lung rather than in the central locations of the primary lung carcinoma. Other patterns include solitary nodule, endobronchial, pleural,
pneumonic consolidation, and mixtures of the above. Foci of lepidic growth similar to bronchioloalveolar carcinoma are seen occasionally with metastatic carcinomas and may be
associated with any of the patterns listed above.
Metastatic growth may be confined to peribronchiolar and perivascular tissue spaces, presumably when the tumor has extended to the lung through the lymphatics. In these cases, the lung
septa and connective tissue are diffusely infiltrated with the gray-white tumor. The subpleural lymphatics may be outlined by the contained tumor, producing a gross appearance referred to
as lymphangitis carcinomatosa. Least commonly, the metastatic tumor is not apparent on gross examination and becomes evident only on histologic section as a diffuse intralymphatic
dissemination dispersed throughout the peribronchial and perivascular channels. In certain instances, microscopic tumor emboli fill the small pulmonary vessels and may result in lifethreatening
pulmonary hypertension or hemorrhage and hemoptysis.
Pleura
Pathologic involvement of the pleura is, with rare exceptions, a secondary complication of some underlying disease. Secondary infections and pleural adhesions are particularly common
findings at autopsy. Occasionally, the secondary pleural disease assumes a dominant role in the clinical problem, as occurs in bacterial pneumonia with the development of empyema.
Important primary disorders include (1) primary intrapleural bacterial infections that imply seeding of this space as an isolated focus in the course of a transient bacteremia and (2) a
primary neoplasm of the pleura: mesothelioma (discussed later).
PLEURAL EFFUSION
Pleural effusion is a common manifestation of both primary and secondary pleural diseases. Normally, no more