Fig. 3

Microscopic findings; hematoxylin and eosin stain and immunohistochemical stain. a The transition area from the adenocarcinoma with a tubulopapillary structure (left upper part) to the neuroendocrine carcinoma (NEC) with a solid structure (right lower part). Hematoxylin and eosin stain (H&E, ×60). b The area of NEC consisting of large tumor cells with irregular-shaped hyperchromatic nuclei (H&E, ×200). c Ki-67 labeling index was over 90 % solid component (×100). d Tumor cells are positive for chromogranin-A in the solid component, thereby revealing neuroendocrine differentiation (×100)