| [ Summary ] |
Rectal carcinoid tumors are the most frequent carcinoid tumor of the colorectal region, accounting for more than 90 % of such tumors in Japan. Macroscopically, these tumors exhibit a submucosal tumor-like appearance and occasionally reveal tumor progression on the mucosal surface. Histologically, these tumors show trabecular, ribbon-like, tubular, small alveolar, or solid architecture. Tumor cells contain uniform round or oval nuclei with coarse chromatin and slightly eosinophilic fine-granular cytoplasm. Immunohistochemically, synaptophysin and CD56 are usually positive, but chromogranin A shows negative or focally positive results. In the practical histopathological diagnosis, WHO grades should be described as well as conventional findings such as tumor depth, lymphovascular invasion, and resection margin. In the mucosal biopsy diagnosis, it is occasionally difficult to obtain a sufficient amount of tumor cells for diagnosis because of the submucosal tumor-like shape, therefore larger sample or multiple samples are recommended. There is a difference in diagnostic criteria for neuroendocrine tumor between Japan and WHO, that is, the former in based on the histologic findings and the latter is based on the proliferative index. |