Application of Immunohistochemistry to the Diagnosis of Primary and Metastatic Carcinoma to the Lung

Archives of Pathology & Laboratory Medicine,  Mar 2008  by Jagirdar, Jaishree

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Context.-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.

Objective.-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.

Data Sources.-A comprehensive PubMed data search and personal practical experience.

Conclusions.-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.

(Arch Pathol Lab Med. 2008;132:384-396)

LUNG-SPECIFIC MARKERS

The quest has been to find lung-specific tumor markers, but we have not yet identified any marker that has high fidelity for the lung such as prostate-specific antigen for the prostate. Perhaps one does not exist, but we will not know that unless an exhaustive search is made. Recently, a unique lung carcinoma microRNA molecular profile has been identified showing promise in both diagnosis and prognosis.1 A novel antibody, ES1, is claimed to stain lung carcinomas that are not picked up by thyroid transcription factor 1 (TTF -1) such as poorly differentiated lung carcinomas including large cell carcinoma, whereas nonlung carcinomas are negative or show weak staining. It is a single domain antibody raised against lung carcinoma cell line A549. ES1 antibody recognizes a variant form of carcinoembryonic antigen-related adhesion molecule 6. However, ES1 is not ready for prime-time diagnostic use.

Surfactant is exclusively produced by the lung and appears, intuitively, to be a good lung-specific marker. However, this has not been the case. Although displaying some specificity for lung and lung tumors, antibodies to surfactants used for immunohistochemistry lack sensitivity and specificity.2-4 In fact according to some authors, only 63% of primary lung carcinomas stain with surfactant protein A and B. Conversely, 46% of metastatic carcinomas, including primary breast cancers, stain with surfactant A and B antibodies.5 Enteric type of lung adenocarcinomas pose yet other problems because they tend to aberrantly express "colonic-type" markers. In the enteric type of pulmonary adenocarcinomas, lung-specific markers such as surfactant proteins, TTF -1, and napsin A (see later) tend to be lost, whereas the enteric-type (colonic) markers, CDX-2, cytokeratin (CK) 20, and MUC3, are aberrantly expressed. Cytokeratin 7 is retained in these primary enteric type lung adenocarcinomas and in this context it appears to be most valuable in distinguishing lung cancer from colon cancer, which is CK7 negative.6

Another lung-specific marker worth considering is napsin A. Napsin A is a functional aspartic proteinase that is expressed in the normal lung parenchyma in type II pneumocytes and the proximal and convoluted tubules of the kidney.7 It is present in the lysosomes of type II pneumocytes and alveolar macrophages (probably secondary to phagocytosis) and to a lesser degree in pancreatic acini and ducts.8 Napsin A is first detected at the outset of embryogenesis of type II pneumocytes and continues to be expressed on mature type II cells.8 Napsin may be involved in maturation of the biologically active SP-B peptide. 9 In the lung, napsin A is identical to the protein spots TAO1 and TAO2 detected by 2-dimensional gel electrophoresis of lung adenocarcinoma.10 It is expressed in the cytoplasm and is strongly positive in up to 80% of primary lung adenocarcinomas by immunohistochemistry. The poorly differentiated cancers do not stain as well as the well-differentiated ones. Squamous cell carcinomas and small cell carcinomas of the lung have been negative for napsin A (personal observation, September 2007). In our laboratory, a preliminary evaluation of more than 1000 cases suggests that napsin A is superior to TTF-1 in that its expression is stronger, more diffuse, and more sensitive (Figure 1, A through D). The strongest staining is seen in lung carcinoma and appears to be specific for a lung primary. However, ~10% of renal cell carcinomas and thyroid carcinomas are also positive. Unlike lung adenocarcinomas, many renal and thyroid cancers end up being false positives most likely because of the presence of intrinsic biotin, which can be readily detected on negative controls (Figure 1, E and F). Less than 5% of assorted adenocarcinomas including those from the breast, pancreas, biliary tract, and colon stain with napsin A. Expression, when present, in breast and colonic adenocarcinomas, appears to be granular unlike that in the lung. In a handful of studies conducted by Japanese and Swedish workers, metastatic carcinomas except for renal cell carcinoma have been negative for napsin A.11-13 In our study, napsin A stained 11% more lung adenocarcinomas than did TTF-1.