The use of immunohistochemistry in the diagnosis of composite and collision tumors: exemplified by pleural mesothelioma and carcinoid tumor of the lung.

A case of a collision lymph node metastasis of a mesothelioma and a carcinoid tumor in a 73-year-old man with a history of asbestos exposure is reported. An interesting finding in this case was that both the mesothelioma and its lymph node metastases exhibited a wide variety of histologic patterns, including one characterized by a solid growth of large cells with abundant, clear, and foamy cytoplasm and another exhibiting deciduoid features. Pathologists should be aware that mesotheliomas can present very unusual morphologic features, such as those seen in the present case, and therefore, should be included in the differential diagnosis of those tumors that can display similar morphology and can metastasize to the serosal membranes. Reexamination of the pneumonectomy specimen in the current case identified a primary peripheral carcinoid tumor. The recognition of a nonasbestos-related tumor in a patient with mesothelioma is important since its presence may have an impact on the patient's life expectancy and, therefore, may affect any compensation settlement.