Around 98% of all cancers in the colon and rectum are adenocarsinomas. The others are spinocellular carsinomas, carsinoids, lymphomas and sarcomas.
The intestinal wall includess three layers:
- Mucosa (three layers)
- Submucosa (may be divided into three levels of depth)
- Muscularis propria (the muscle layer)
Cancer in the colon and rectum generally originate from epithelial cells in the mucosa and starts as a polyp (adenoma) with irregular cell. When the cell division is no longer under control and the cells obtain properties that enable them to infiltrate the surrounding tissues a cancer has developed. A diagnosis of cancer is possible when atypical cells have infiltrated the submucosa.
Precursor lesions are benign intramucosal neoplasias often appearing as polyps or adenomas. Adenomas are divided into tubular villous and tubulo-villous adenomas according to the presence of glands (tubules), villous structures or a combination of both. In addition the adenomas are classified according to the degree of cellular atypia and changes in morphology.
- Low grade intraepithelial neoplasia – previously called slight or moderate dysplasia
- High grade intraepithelial neoplasia – previously called grov dysplasia
- Intramucosal neoplasia – prevoiously called intramucosal carcinoma: the changes are found in lamina propria, but not through the muscularis mucosa and is not manifest cancer
Serrated adenoma is a special variant of hyperplastic polyps and can develop into cancer.
Adenocarcinomas produce abnormal glands that infiltrate the submucosa and further through the muscle layer and into the surrounding tissues (extramurally). In the submucosa, the tumor can infiltrate lymphatic vessels and thus spread to lymph nodes or infiltrate veins and develop metastasis in the liver or other organs.
|Photomicrograph with adenocarcinoma infiltrating the submucosa. Click to enlarge.
||Photomicrograph with adenocarcinoma infiltrating the muscularis mucosa. Click to enlarge.
||Photomicrograph with adenocarcinoma infiltrating perirectal tissue. Click to enlarge.
Adenocarcinomas are graded according to deviation from normal glandular tissue (differential grade). About 20% of the tumors are poorly differentiated and have a worse prognosis than the well to moderately differentiated which compose the other 80%.
Mucinous adenocarcinomas are the second most common type of cancer (about 5%) and are diagnosed when mucus occurs in > 50 % of the tumor tissue. These have a worse prognosis than non-mucinous carsinomas.
|Photomicrograph of a primary mucinous adenocarcinoma. Click to enlarge.
Other cancer types
The following variants are infrequent: Signet ring-cell carcinoma, Small cell carcinoma, Squamous cell carcinoma, Adenosquamous carcinoma, Medullary carcinoma, Undifferentiated carcinoma.
Other types of cancer can also appear in colon and rectum, although they are more common in other parts of the gastrointestinal tract: Lymphoma, Endocrine tumors and Sarcomas (Gastrointestinal stromal tumor -GIST).