Neuroendocrine tumors of the lung share the common neuroendocrine morphologic features including organoid nesting,
palisading,
rosettes,
or a trabecular growth pattern.
They can be classified radiologically and pathologically into subtypes.
Typical carcinoids are relatively well-differentiated tumors and atypical carcinoids are highly aggressive poorly differentiated tumors; both have similar gross pathologic and imaging features,
but typical carcinoids are less aggressive.
Carcinoids manifest as well defined pulmonary nodules or masses that are often closely related to central bronchi.
They may exhibit intrinsic calcification and contrast material enhancement at CT,
and patients with carcinoids may have postobstructive atelectasis and pneumonia.
Large cell neuroendocrine carcinoma and small cell lung cancer manifest with large peripheral or central pulmonary masses.
Local invasion,
intrathoracic lymphadenopathy,
and distant metastases are frequent at presentation.
At expiratory high-resolution CT,
multifocal pulmonary micronodules with or without associated mosaic attenuation or air trapping are usually seen. High- resolution CT may indicate the presence of associated constrictive bronchiolitis.
The micronodules are typically ground glass or solid in attenuation,
and a centrilobular distribution has been described.
Although the association of small nodules with CT features of constrictive bronchiolitis should suggest the diagnosis,
nodular neuroendocrine cell hyperplasia cannot be readily differentiated from carcinoid tumorlets,
small carcinoid tumors,
or even small pulmonary metastases.
Bronchiectasis,
bronchial wall thickening,
and atelectasis have also been seen.
The majority of asymptomatic patients exhibit pulmonary micronodules without significant mosaic attenuation.
Multidetector CT is useful for evaluating suspected carcinoid tumors.
CT typically demonstrates a soft-tissue nodule,
which often exhibits a relationship to an adjacent airway.
CT allows identification of associated bronchiectasis,
mucoid impaction,
atelectasis,
and air trapping.
CT visualization of the bronchus sign (bronchus leading directly to the tumor) may allow the radiologist to suggest the diagnosis prospectively.
Contrast-enhanced chest CT may demonstrate variable degrees of tumor enhancement because of the vascular nature of the lesions and may allow differentiation between the lesion and adjacent atelectasis or consolidation.
Calcification is present histologically in 30% of carcinoid tumors.