Spindle cell and pleomorphic ("sarcomatoid") carcinomas of the lung: an immunohistochemical analysis of 86 cases.

Spindle cell and pleomorphic carcinomas are currently grouped among sarcomatoid carcinomas of the lung. Because of their unusual occurrence, these tumors have not been properly assessed by immunohistochemistry. We performed a comprehensive immunohistochemical analysis of 86 of these tumors. Seventy-four pleomorphic carcinomas (57 with differentiated elements) and 12 spindle cell carcinomas were subjected to immunohistochemistry with CAM5.2, cytokeratin (CK) 7, thyroid transcription factor 1, napsin A, CK5/6, p40, desmocollin 3, Sox2, calretinin, and D2-40. The percentage of positive tumor cells as well as the staining intensity were evaluated and scored. The spindle/giant elements were positive for CAM5.2 (93%), CK7 (79%), thyroid transcription factor 1 (41%), napsin A (20%), calretinin (20%), Sox2 (13%), CK5/6 (9%), p40 (8%), D2-40 (6%), and desmocollin 3 (3%). Of 29 cases in which immunohistochemistry was performed on spindle/giant cell and corresponding differentiated elements, 21 (72%) showed a consistent staining pattern in both components, whereas in 8 cases (28%), the immunophenotype in the spindle/giant cells was less lineage-specific than in the differentiated component. Therefore, we consider that 42% of neoplasms otherwise classified as sarcomatoid carcinoma can be reclassified as adenocarcinoma and 14% as squamous cell carcinoma, while the remaining 44% failed to show a more specific immunophenotype. The use of a comprehensive immunohistochemical panel allows reclassification of the majority of sarcomatoid carcinomas as poorly differentiated variants of adenocarcinoma or squamous cell carcinoma. Such reclassification will facilitate clinical management and allow molecular testing and pursuit of targeted treatment strategies. Application of immunohistochemistry should become the standard in the workup of pulmonary sarcomatoid carcinomas.