Application of immunohistochemistry to the diagnosis of primary and metastatic carcinoma to the lung.

Immunohistochemistry is a very valuable and often used tool in the differential diagnosis of lung carcinomas whether primary or secondary to the lung. The most useful application is in distinguishing primary lung tumors from metastatic tumors to the lung from common sites (colon, breast, prostate, pancreas, stomach, kidney, bladder, ovaries, and uterus). Immunohistochemistry also aids in the separation of small cell carcinoma from non-small cell carcinoma and carcinoids particularly in small biopsy specimens limited by artifact. Although there is no "lung-specific tumor marker," with the help of a relatively restricted marker, thyroid transcription factor 1, it is possible to separate a lung primary from a metastasis with a reasonable degree of certainty. Another lung-specific marker on the horizon is napsin A, which appears to complement thyroid transcription factor 1 in defining a lung primary. To present a practical review and to critique commonly used markers in the differential diagnosis of lung neoplasms and to list valuable immunohistochemical prognostic markers that the pathologist is called on to perform and interpret. A comprehensive PubMed data search and personal practical experience. With a panel of immunohistochemical markers, it is possible to distinguish or narrow down most lung neoplasms and separate them into meaningful therapeutic categories. In the future as more proteomic and genomic data surface, immunohistochemical markers to newly discovered antigens may become a routine part of prognostication.