Well-differentiated Neuroendocrine Tumor : Gross
The most common locations for well-differentiated neuroendocrine tumors (WDNETs; Carcinoids) in the large intestine are rectum and cecum. Most tumors are solitary; however, a small number of patients may harbor multiple carcinoids, usually in the rectum. WDNETs arising in the setting of inflammatory bowel disease may be multicentric. The size ranges from 0.2 cm to more than 5 cm; about 80% of rectal NETs are smaller than 1 cm. Larger tumors are usually found in the proximal colon. The risk of metastasis is exceedingly small for tumors less than 1 cm in size but much greater for tumors > 2 cm. They appear as a submucosal nodule/polyp or as flat and slightly depressed plaque. The overlying mucosa may be intact (smaller tumors) or ulcerated (larger tumor; high-grade neuroendocrine carcinomas). The cut surface is homogenous and yellow-tan or white (as seen in this appendiceal carcinoid). The presence of hemorrhage or necrosis usually indicates high-grade tumor.