The pathological evaluation was performed at the National Center of Pathology, Vilnius, Lithuania. Final tumor histology was provided ypTNM and staged according to the American Joint Committee on Cancer Staging, 8th edition. The histological type of tumors was classified according to the WHO Classification of Tumors of the Digestive Tract (2010) and Lauren classification of gastric carcinoma. Regional lymph nodes were macroscopically identified in surgical specimens. All lymph nodes were longitudinally sectioned through the hilus and embedded into paraffin blocks. All slides were stained with hematoxylin-eosin, additional immunostaining was performed if necessary. For the study, all slides were recalled from the institutional archive. They were reviewed by the senior pathologist trainee and experienced gastrointestinal pathologists to evaluate histologic regression grade after preoperative chemotherapy in the primary tumor and metastatic lymph nodes. Regression in the tumor was graded as described by Becker et al. 8. For nodal regression, we adapted the same grading system. Histological signs of regression in the primary tumor and metastatic lymph nodes were similar and included: areas of fibrosis, necrosis, calcifications, acellular mucin pools, cholesterol deposits, and histiocytic reaction with hemosiderin-laden and foamy macrophages (Figure (Figure1).1). Regression was graded: Grade 1, complete (0% residual tumor; Grade 1a) or subtotal tumor regression (<10% residual tumor per tumor bed; Grade 1b); Grade 2, partial tumor regression (10-50% residual tumor per tumor bed), and Grade 3, minimal or no tumor regression (>50% residual tumor per tumor bed). Lymph nodes without metastasis or signs of nodal regression were classified as negative nodes.

Representative pictures of lymph nodes presenting signs of histological regression (Haematoxylin-eosin staining; original magnification 20x). A - Lymph node with residual carcinoma (▲), foci of fibrosis (f) and calcifications (↑); B - Lymph node with few residual carcinoma aggregates (▲), fibrosis (f), and acellular mucin pools (↑); C - Lymph node with residual carcinoma (▲), foamy macrophages (mc), and areas of necrosis (n).

For the purpose of the study, patients were grouped according to the regression scores recorded in the lymph nodes. Patients who had all negative nodes were allocated to the node-negative (lnNEG) group. Patients with a regression score of 1a-2 detected in at least some of the retrieved metastatic nodes were categorized as nodal responders (lnR). Non-responders (lnNR) had a score of 3 in all metastatic LN.

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