Colorectal Anatomy and ICD-O-3

Priority order for coding primary site

  • ICD-O
  • SEER Program Coding Manual (Including Coding Guidelines in Appendix C)
  • Solid Tumor Rules (Colorectal)

NAACCR Item # 400: Primary Site (2580 Text-Primary Site Title) (See illustration below)

ICD-O-3 Codes ICD-O-3 Preferred Term Synonyms Location
C180 Cecum Appendiceal orifice
Ileocecal valve
Ileocecal junction		 Part of the right colon, 6 x 9 cm pouch covered with peritoneum
C181 Appendix   A vermiform (wormlike) diverticulum located in the lower cecum
C182 Ascending colon Right colon Part of the right colon, approximately 20-25 cm long, located behind the peritoneum. Supported by peritoneal folds and mesentery
C183 Hepatic flexure of colon   Part of the right colon
Lies under right lobe of liver
C184 Transverse colon   Right half of transverse colon (right colon); left half of the transverse colon (left colon).
Lies anterior in abdomen, attached to gastrocolic ligament
C185 Splenic flexure of colon   Part of the left colon. Near tail of pancreas and spleen
C186 Descending colon Left colon Part of the left colon, approximately 10-15 cm long, located behind the peritoneum. Supported by peritoneal folds and mesentery
C187 Sigmoid colon Sigmoid, NOS
Sigmoid flexure of colon
Pelvic colon		 Part of the left colon. Loop extending distally from border of left posterior major psoas muscle
C188 Overlapping lesion of colon    
C189 Colon, NOS Large intestine (excludes rectum)
Large bowel, NOS		  
C199 Rectosigmoid junction Rectosigmoid, NOS
Rectosigmoid colon
Colon and rectum
Pelvirectal junction		 Approximately 10 to 15 cm from anal verge
A tumor is classified as rectosigmoid when differentiation between rectum and sigmoid is not possible.
C209 Rectum Rectal ampulla Approximately 12 cm long; upper third covered by peritoneum; no peritoneum on lower third which is also called the rectal ampulla. About 10 cm of the rectum lies below the lower edge of the peritoneum (below the peritoneal reflection), outside the peritoneal cavity
C210* Anus, NOS (excludes skin of anus and perianal skin [C445])    
C211 Anal canal Anal sphincter Most distal approximately to 4-5 cm to anal verge, which marks the junction of the anal canal and the external hair-bearing skin.
The anal verge is a critical area during a colonoscopy, as it is where the anal canal meets the rectum.
C212 Cloacogenic zone    
C218 Overlapping lesion of rectum, anus and anal canal Anorectal junction
Anorectum		  

*The sites codes for anus and the anal canal are listed. This module does not include any other information about the anus and anal canal.

Anatomic drawings of the colon. Source: Young JL Jr, Roffers SD, Ries LAG, Fritz AG, Hurlbut AA (eds). SEER Summary Staging Manual - 2000: Codes and Coding Instructions, National Cancer Institute, NIH Pub. No. No. 01-4969, Bethesda, MD, 2001.

Updated: June 24, 2025