#1: Primary site
Source 1: ICD-O-3
Primary site--Colon: What is the correct topography code for appendiceal orifice? See Discussion.
From a number of definitions reviewed, it seems unclear if it's part of the appendix or the cecum of the colon. For example: The cecum is usually located in the right iliac fossa. In the pole of the cecum, there is often the appearance of fusion of the three teniae coli around the appendix, giving rise to the tri-radiate fold (Mercedes Benz sign), but the anatomy can be variable. The most reliable landmarks of the cecum are the appendiceal orifice and ileocecal valve. The appendiceal orifice is usually an unimpressive slit, often crescentic in shape. The ileocecal valve is made up of the superior and inferior lips (usually not seen en face) and is the gateway leading into the terminal ileum. It is located on the prominent ileocecal fold encircling the cecum, between 3 and 5 cm distal to the cecal pole. (https://www.sciencedirect.com/science/article/pii/S2212097113701730)
Assign C180, Cecum, when the neoplasm originates in the appendiceal orifice. The appendiceal orifice is a landmark in the cecum. During colonoscopy, visualization of the appendiceal orifice indicates that the entire colon was examined, from the anus to the cecum.