Introduction

Immunohistochemistry is an indispensible diagnostic tool in the study of lung tumors. It may be helpful as a supplement to morphology in classifying primary lung tumours. This subclassification is increasingly important, as emerging therapeutic options demand increasing diagnostic exactitude. The technique is also invaluable in deciding whether a tumor, particularly an adenocarcinoma, is a pulmonary primary or arises from an extra-pulmonary site. If metastatic, immunohistochemical stains can also often determine the primary site.

However, immunohistochemistry has limitations. No marker is absolutely specific or absolutely sensitive. Since the exact conditions of tissue fixation, antigen retrieval and staining vary between laboratories, diagnostic laboratories do not exactly reproduce the conditions of the published studies. Deciding whether a tumor shows positive staining also has an element of subjectivity. Cut-off levels vary between studies and often involve a combination of staining intensity and proportion of cells stained, for which there is no universally agreed scoring system. Some antibodies, when applied to some tumors, have given rise to hundreds of reported cases with consistent results. Published results with other antibodies and tumors have given varied results for reasons that may or may not be apparent. Particularly for less common pulmonary or extra-pulmonary tumors, the published data may be sparse in the extreme. The corpus of published data is immense and ever growing, particularly as tissue microarrays have the potential to examine hundreds of tumors within the scope of one study. Mastering this body of knowledge is an all but impossible task.