Cytokeratin 19 expression in intrathoracic neoplasms: First study utilizing cellblocks, evaluating the role of a rarely used cytokeratin for lung cancers

Background Cytokeratin 19 (CK19) is low‐molecular weight CK with widespread expression in various malignancies including those of lung, colon, and breast. Occasional studies have also indicated prognostic utility of CK19 in nonsmall cell lung carcinomas, with better prognosis among squamous cell carcinoma cases with strong CK19 immunostaining. The aim of this study was to evaluate the expression and potential roles of CK19 immunostaining in cases of carcinoma lung as well as other metastatic intrathoracic neoplasms. Methods This was a prospective single‐center study of 2 years duration, during which 233 cases of endobronchial ultrasound guided aspiration of the intrathoracic lesions were examined. After excluding the cases without cellblock, with inadequate samples, with benign diagnosis, and excluding the lymphoma cases, finally 31 neoplastic cases were evaluated for expression of cytokeratin 19. Results Diffuse cytoplasmic staining for CK19 was noted in nonsmall cell lung carcinomas (18/19 cases), while the expression in small cell carcinomas was low (2/5 cases). CK19 was also positive in mediastinal lymph node metastasis from a case each of papillary thyroid carcinoma, prostatic adenocarcinoma and infiltrating ductal carcinoma breast (1/2 cases). It showed focal faint staining in a case of mediastinal mesenchymal neoplasm and was negative in both the cases of primitive neuroectodermal tumor. Conclusions CK19 is a highly sensitive but nonspecific marker for nonsmall cell lung cancers. However, owing to diffuse staining pattern in most of these cases, role of its staining intensity as a predictor of prognosis could not be evaluated.