Lung cancers are the most common cause of morbidity and mortality from malignant tumors in the World. The neodjuvant therapy in patients with locally advanced (IIIA-IIIB) lung cancer and affected N2 lymph nodes is one of the modes of multimodal treatment of patients with non-small cell lung cancer (NSCLC) in order to improve the outcome of their treatment. This involves converting patients from a higher to a lower stage of the disease - “downstaging”. There has been no significant connection between some forms of tumor response and types of therapy. Given the importance of complete pathological responses and tumor regression in the prediction of treatment outcomes, finding this relationship is of importance for the design of future neoadjuvant trails. In determining the histological tumor regression is very important measurement of area of residual tumor (ART). As the size of the tumor is one of the prognostic factors in patients with NSCLC who did not receive neoadjuvant therapy so the measurement of ART, as opposed to the macroscopic size of the tumor, one of the prognostic factors in patients with NSCLC, who had received neoadjuvant therapy. The ultimate goal of neoadjuvant therapy should be resectability and “downstaging” that could provide overall oncology benefit in specific clinical situations. The main objectives of this research were: to objectively estimate the size of ART in tumor tissue of lung and lymph nodes; to estimate the relation between the surface of ART with the size of the tumor on postoperative surgical material after neoadjuvant therapy; to analyze and estimate the relation between histomorphological parameters in tumor regression induced by neoadjuvant therapy and spontaneous tumor regression in tumors of the lung and lymph nodes in the postoperative surgical material and depending on the histological type of cancer; to estimate the relation between clinical response to neoadjuvant therapy according to criteria of the World Health Organization and histological parameters in lung tumors and lymph nodes in the postoperative surgical material after neoadjuvant therapy; to estimate the correlation of the pathological ypTN with clinical ycTN stage of the disease and the degree of tumor regression induced by neoadjuvant therapy and pathological ypTN and estimation of the relation between clinical and pathological involvement of N2 lymph nodes after neoadjuvant therapy. Measurement of the total size of the preserved ART is the most important objective parameter in the assessment of the grade of tumor regression. Size of residual tumor did not correlate with the size of the tumor after neoadjuvant therapy. There was a significant difference in the histological picture of tumor regression induced by neoadjuvant therapy and spontaneous tumor regression. There was no significant difference between the histologic type of tumor and histological tumor regression. There is no significant correlation between clinical response and the grade of tumor regression after neoadjuvant therapy. There is no correlation between clinical and pathological staging of the diseaSPECIAL SESSION: DEPARTMENT OF PATHOLOGY, MEDICAL FACULTY, UNIVERSITY NOVI SAD, SERBIA 34 MATERIA MEDICA • Vol. 34 • Issue 1, suplement 1 • april 2018. se after neoadjuvant therapy. There is no correlation between the grade of tumor regression induced by neoadjuvant therapy and ypTN stage of the disease. There is no correlation between the clinical and the pathological involvement of the N2 lymph nodes to neoadjuvant therapy. The grade of tumor regression and measurement ART after neoadjuvant therapy determined by histopathological analysis of the resected tumor is the most objective criterion for evaluation of chemotherapeutic response and prediction of treatment outcome in patients.

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