The significance of pathohistological analysis of operative material and tumor immunophenotype in differential diagnosis of sarcomatoid carcinoma and carcinosarcoma rare malignant lung neoplasms. Introduction: Lung carcinosarcoma, biphasic tumor, composed of non-small caracinomatous and sarcomatous heterogeneous components, is a rare neoplasm and represents less than 1% of all malignant lung tumors. Case report: A 64-year old man reported to physician complainig of fever that lasted for two months. Chest CT showed signs of right lung infiltration, close to the hilus and mediastinal lymphadenopathy. After bronchoscopy and biopsy of the suspected changes pathohistological finding was: carcinoma squamosum invasivum. Lobectomy of upper right lobe and mediastinal lymphadenectomy were performed. On gross examination, distanced 5 mm from the resection margin, withish tumor (22x7x5mm) infiltrating the wall of the lobar bronch was found. Histological analysis found that tumor tissue was partly consisted of atypical epithelial cells, with larger, mitotically active nuclei with focuses of keratinization and partialy of atypical oval and spindle cells,with hyperchromatic, pleomorphic nuclei, with high mitotic index. Lymph nodes were negative for tumor tissue. Results of immunohistochemical tumor tissue analysis: Cytokeratin - positive in carcinomatous component, Vimentin - positive in sarcomatous component, Actin, Desmin and S100 - negative. According to the results of morphological and immunohistochemical analysis of the tumor tissue lung carcinomsarcoma was diagnosed. Conclusion: For establishing definitive pathohistological diagnosis in biphasic tumor cases, a pathohistological examination of operative material is necessary, and morphological assessment should be suported by immunohistochemical analysis.

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