## Histological Types of Colorectal Carcinoma **Key Point:** Adenocarcinoma accounts for approximately 95% of all colorectal cancers, making it by far the most common histological type. ### Breakdown of Colorectal Cancer Histology | Histological Type | Frequency | Key Features | |---|---|---| | Adenocarcinoma | 95% | Arises from glandular epithelium; includes tubular, mucinous, and signet-ring variants | | Squamous cell carcinoma | 2–3% | Rare; usually occurs in lower rectum; worse prognosis than adenocarcinoma | | Neuroendocrine (carcinoid) | 1–2% | Arises from enteroendocrine cells; often detected late | | Mucinous adenocarcinoma | 10–15% of adenocarcinomas | Subtype of adenocarcinoma; produces excess mucin; intermediate prognosis | | Signet-ring cell carcinoma | 1–3% of adenocarcinomas | Subtype of adenocarcinoma; poor prognosis; often presents at advanced stage | **High-Yield:** Adenocarcinoma is subdivided into tubular (most common subtype), mucinous, and signet-ring variants based on differentiation and mucin production. All three are adenocarcinomas and fall within the 95% figure. **Clinical Pearl:** Mucinous and signet-ring variants, though adenocarcinomas, carry a worse prognosis than well-differentiated tubular adenocarcinoma and are associated with earlier lymph node involvement. ### Why Other Types Are Rare - **Squamous cell carcinoma:** Arises from squamous metaplasia of the rectal mucosa; associated with chronic irritation, HPV infection, or prior radiation; prognosis worse than adenocarcinoma. - **Neuroendocrine carcinoma:** Arises from Kulchitsky cells; often presents at advanced stage with carcinoid syndrome; poor prognosis. - **Other rare types:** Lymphoma, sarcoma, and melanoma can occur in colon but are not carcinomas of epithelial origin. [cite:Robbins 10e Ch 17]
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