Epithelial Tumors
Nonepithelial Tumors

The World Health Organization (WHO) classification of tumors of the colon and rectum include:[1]

Adenoma

• Tubular.
• Villous.
• Tubulovillous.
• Serrated.

Intraepithelial neoplasia (dysplasia) associated with chronic inflammatory diseases

• Low-grade glandular intraepithelial neoplasia.
• High-grade glandular intraepithelial neoplasia.

Carcinoma

• Adenocarcinoma.
• Mucinous adenocarcinoma.
• Signet-ring cell carcinoma.
• Small cell carcinoma.
• Adenosquamous carcinoma.
• Medullary carcinoma.
• Undifferentiated carcinoma.

Carcinoid (well-differentiated neuroendocrine neoplasm)

• Enterochromaffin (EC)-cell, serotonin-producing neoplasm.
• L-cell, glucagon-like peptide and pancreatic polypeptide/peptide YY (PYY)-producing tumor.
• Others.

Mixed carcinoma-adenocarcinoma

• Others.

• Lipoma.
• Leiomyoma.
• Gastrointestinal stromal tumor.
• Leiomyosarcoma.
• Angiosarcoma.
• Kaposi sarcoma.
• Melanoma.
• Others.

Malignant lymphomas

• Marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue type.
• Mantle cell lymphoma.
• Diffuse large B-cell lymphoma.
• Burkitt lymphoma.
• Burkitt-like/atypical Burkitt lymphoma.

Adenocarcinomas account for the vast majority of rectal cancers. Other histologic types of colorectal cancer account for an estimated 2% to 5% of colorectal tumors.[2]

References

1. Hamilton SR, Aaltonen LA: Pathology and Genetics of Tumours of the Digestive System. Lyon, France: International Agency for Research on Cancer, 2000. 
   
   
2. Kang H, O'Connell JB, Leonardi MJ, et al.: Rare tumors of the colon and rectum: a national review. Int J Colorectal Dis 22 (2): 183-9, 2007.  [PUBMED Abstract]