[Differential diagnosis of primary lung tumors and pulmonary metastases]

The lungs are frequent metastatic targets for metastases of extrapulmonary cancer with or without known primary tumor. Metastases of extrapulmonary tumors are found in 20 to 50%. Metastatic carcinoma of unknown primary localizations contributes to about 4% of all diagnosed carcinomas. Within this group adenocarcinomas can be found in 60%, squamous cell carcinomas in 15%, unspecified tumors including small cell and undifferentiated tumors in 25% histogenetically. Due to overlapping histologic features a histopathological distinction of primary and secondary tumors might be difficult. Topography, size and form of pulmonary growth pattern are important aspects for the differential diagnosis. Primary lung tumors are mainly localized in upper lobes as singular nodules, metastases in lower lobes as multiple lesions. For the distinction of endobronchial primary or secondary squamous cell carcinomas findings of severe dysplasia and CIS of the bronchial epithelium are important. Stroma and vascularisation pattern of primary and secondary lung tumors differ in distribution, arrangement and extracellular matrix components. Pulmonary vascular changes in primary tumors are characterized by vascular infiltration, destruction and occlusion whereas in secondary tumors intravascular embolisation and extravasation predominate. Immunohistochemical techniques using different algorithms of antibodies (cyto-keratins, neuroendocrine markers, TTF-1, vimentin, HMB 45, LCA, oncogenes, tumor suppressor genes etc.) give vital clues as to the origin of the primary tumor. Metastases of thyroid or prostate carcinomas, melanomas, sarcomas, lymphomas etc. can be reliably differentiated. Molecular investigations are up to now only subject in research.