Histopathological and immunohistochemical study of neuroendocrine tumors of the rectum

A clinicopathological and immunohistochemical study was carried out on 32 cases of neuroendocrine tumors of the rectum. Typical carcinoids consisted of 27 cases, histologically showing uniform round to columnar cells forming solid alveolar nests and ribbon-like or trabecular arrangement. Neuroendocrine carcinomas consisted of 5 cases in which tumor cells with prominent nuclear atypism were arranged in a ribbon-like or trabecular fashion and formed gland-like structures. There were also small round tumor cells resembling lymphocytes. The prognosis of neuroendocrine carcinomas is very poor with marked tumor invasion of lymphatics and veins resulting in liver metastases and death within one year after operation. Thirty cases out of the 32 showed a positive argyrophil reaction, while immunohistochemistry of 29 cases revealed more than one peptide hormone in 23 cases. The most common hormone was somatostatin being present in 18 of the 23 tumors and glucagon in 16 of the 23 tumors. Gastrin/CCK and calcitonin were proven in 6 of the 23 tumors and in 4 of the 23 tumors, respectively. On the other hand, more than two hormones was present in 15 of the 23 tumors examined. Histologically, neuroendocrine tumors have a very wide spectrum. Histogenetically, typical carcinoids and neuroendocrine carcinomas are considered to be of the same origin with the former showing morphological and functional differentiation to endocrine cells and the latter being more undifferentiated.