Most Common Cancer of the Large Intestines
Adenocarcinoma is the most common cancer of the large intestines, accounting for 95-98% of all colorectal malignancies. 1
Epidemiologic Context
Colorectal cancer (CRC) represents the third most common cancer worldwide, accounting for approximately 10% of all tumor types globally, with over 1.1 million new cases annually 1
Among colorectal cancers, colon cancer is more frequent than rectal cancer, with a ratio of 2:1 in high-risk populations 1
CRC is the second leading cause of cancer death worldwide, with approximately 600,000-930,000 deaths estimated each year 1, 2
Histologic Classification
Adenocarcinomas comprise 95-98% of all rectal and colonic malignancies, typically arising from adenomatous polyps through the adenoma-carcinoma sequence 1
The remaining 2-5% include rare histologic subtypes such as neuroendocrine tumors, lymphomas, sarcomas, and squamous cell carcinomas (though these are not specifically detailed in the guidelines)
Molecular Pathways
The majority of colorectal adenocarcinomas are characterized by chromosomal instability (CIN), which represents the common pathway observed in approximately 85% of cases 1, 3
Microsatellite instability (MSI) is rare in rectal cancer (only a few percent) but accounts for approximately 15% of all colorectal cancers, predominantly affecting the right colon 1, 3
Key molecular alterations include mutations in the WNT signaling pathway, with APC gene mutations being the most frequent, along with inactivation of tumor suppressor genes (P53, TGFβ pathways) and activation of oncogenes (KRAS, PI3KCA) 1, 4
Clinical Significance
Approximately 15-30% of patients present with metastatic disease at diagnosis, and an additional 20-50% of patients with initially localized disease will develop metastases during their disease course 1
Colorectal cancer presents as an emergency in approximately 30% of cases, with large bowel obstruction representing 80% of these emergencies (15-30% of all CRC) and perforation accounting for the remaining 20% (1-10% of all CRC) 1, 5
The sigmoid colon is the most common site for obstructing colorectal cancer, with approximately 75% of obstructing tumors located distal to the splenic flexure 1, 5