Neuroendocrine Tumor Histopathological Assessment

All specimens were pathologically reviewed and assessed retrospectively by two investigators (K.I and M.K). Maximum tumor size was determined based on H.E. slides and neuroendocrine differentiation was assessed by positive immunohistochemical staining for chromogranin A (diluted 1:200, clone DAK-A3, Dako, Glostrup, Denmark), synaptophysin (ready to use, DAK-A3, Dako, Glostrup, Denmark), and CD56 in all cases (diluted 1:50, clone 123C3, Dako, Glostrup, Denmark). Tumor specimens were considered positive for neuroendocrine markers if more than 5% of tumor cells were stained. All tumors are positive for at least two of three neuroendocrine markers. Elastica and D2–40 staining was used in all cases to assess lymph-vascular invasion (diluted 1:200, clone D2–40, Acris, Herford, Germany). The Ki-67 and mitotic index was evaluated, and grading was performed according to the WHO classification 2010¹⁵ . Lymphatic invasion and venous invasion were assessed as reported previously^{27 (link)}. Lymph node metastasis was confirmed histologically using surgical specimens, and distant metastases were evaluated either radiologically or histologically.