Which of the following immunohistochemical marker is positive in neuroendocrine tumor?
Ref. Robbins Pathology. 9th edition. Page. 1099
Though Cytokeratin is also positive in neruroendocrine carcinoma. For neuroendocrine differentiation more specific marker is synaptophysin and chromogranin
 
WHO classification

Typical carcinoids (tcs)
Atypical carcinoids (acs)
Small cell carcinomas (sccs)
Large cell necs (LCNECs)

 
Carcinoid grade I – ruled out because it lacks comedo-like necrosis, has < 0.2 mitotic figures/HP
 
Atypical carcinoid:

Increased mitotic activity -  per 2-10/10 HPF
Pleomorphism and irregular nuclei with hyperchromatism and prominent nucleoli
Areas of increased cellularity with loss of the regular, organized architecture observed in typical carcinoid
Areas of necrosis within the tumor

 
Small cell carcinoma – has extensive necrosis (not comedotype as shown in the picture), mitotic figure usually very high (>20/10hpf) and weak staining by neuroendocrine markers
 
IHC  - Neuroendocrine specific – Synaptophysin, chromogranin, CD56
Cytokeratin
Synaptophysin
Calretinin
GEAP