Malignant polyps of the large intestine can be divided into polypoid cancers, which are largely cancerous lesions with a polypoid shape, and polyps that are mostly benign but have a focus of cancer. Polypoid cancers are generally unsuited to endoscopic treatment and are resected along with the segment of bowel in which they lie. Malignant polyps that are mostly benign but have a focus of cancer. They can sometimes be managed endoscopically, but in some cases, formal bowel resection is required. The way to make this often-difficult decision is discussed in this chapter.
Malignant neoplasms of the colon and rectum arise from a pre-existing benign polyp by the “adenoma-carcinoma” or “serrated polyp-carcinoma” sequence. These stepwise progressions of normal epithelium to dysplastic adenoma or of a normal to serrated polyp to carcinoma occur as a result of an accumulation of multiple genetic and epigenetic abnormalities; thus all colorectal polyps are “genetic.” The process of carcinogenesis is often indolent, taking many years, during which time a benign polyp gradually acquires the characteristics of malignancy. Family history, gender, smoking, inflammatory bowel disease, inherited syndromes, and obesity are all risk factors for colorectal carcinogenesis, but age and family history are the basis of current screening guidelines. Effective screening programs have contributed to the declining incidence of colon cancer by identifying and removing precancerous lesions. However, there comes a time in the history of a neoplasm when it transforms from being benign to malignant, defined as the passage of neoplastic cells through the muscularis mucosae into the submucosa. Here the cells gain access to blood vessels and lymphatics, acquiring the ability to spread outside the organ of their origin. The submucosa responds to the presence of severely dysplastic cells by generating a desmoplastic reaction, which is a clue to the presence of invasive neoplasia.
Assessment of Polyps
When a polyp is seen during a colonoscopy, an assessment is made of the likelihood that malignancy is present. The risk of malignancy varies with polyp size, shape, mobility, and appearance. The larger the polyp, the higher the risk. Up to 46% of polyps greater than 2 cm in diameter may contain microscopic cancer, but other characteristics can refine the risk. Asymmetrical polyps, fragile polyps, hard polyps, and polyps that are not mobile on the submucosa are very likely to contain a cancer and thus should either be resected formally or at least marked with an endoscopic tattoo. The shape of the polyp dictates treatment because the stalk of pedunculated polyps offers the opportunity of snare excision in one piece with a good margin. Large sessile polyps may require piecemeal excision or treatment with an advanced technique such as endoscopic submucosal resection. Endoscopic polypectomy should be complete and ideally performed in a fashion that allows complete histologic evaluation. If a large polyp cannot be removed endoscopically, a segmental colectomy is necessary because the presence of a malignancy cannot be assessed without removal of the entire polyp. A biopsy alone is not sufficient.
Endoscopic polypectomy techniques include snare polypectomy, endoscopic mucosal resection, endoscopic submucosal dissection, and laparoscopic-assisted endoscopic resection. An appropriate polypectomy should include some submucosa to allow optimal histologic evaluation of the deep margin. Piecemeal removal of a polyp complicates the histologic interpretation of the margin and should be avoided if possible. If a piecemeal technique is used, all the polyp tissue should be recovered for pathologic examination. Other techniques such as coagulation of the polyp bed with argon plasma coagulation/electrocautery, although possibly effective in achieving clear margins, does not provide any direct evidence that clear margins have been achieved. Therefore, recent guidelines recommend early follow-up colonoscopy in 1 to 3 months.
When a polypectomy is performed for a suspicious polyp, the site of the polypectomy should be tattooed on three sides of the colon wall to identify the site for close follow-up and for surgical resection if necessary.
Adenomatous polyps are noninvasive glandular neoplasms with dysplastic epithelial cells that have variable malignant potential. They can be viewed as a continuum of carcinogenesis, and those with villous histologic features or high-grade dysplasia are closer to cancer than are those without these characteristics. High-grade dysplasia (carcinoma in situ) is defined as severely dysplastic cells in the epithelium, bounded by the basement membrane. When these cells traverse the basement membrane and enter the lamina propria, the definition is intramucosal carcinoma, which is an unfortunate term, because it implies that cancer is present when in fact the chance of these cells metastasizing is extremely low. Such polyps should not be treated radically, but such reports should be checked by a review of slides. Malignant polyps are defined by the presence of severely dysplastic cells invading through the muscularis mucosa into the submucosa. They sometimes appear benign endoscopically, and if invasive malignant cells are recognized only after pathologic evaluation, management may be difficult and controversial. If the chances of residual or metastatic cancer are small, then polypectomy alone is adequate treatment. If the chances are high, then formal resection of the bowel is necessary. The decision represents a balance of the chance of residual cancer versus the complications of treatment.
The chance of residual cancer focuses on two issues: whether the neoplasm has been completely removed via a polypectomy, and whether lymph node metastases are present.