What are the types of colon polyps?
Not all colon polyps are the same. There are different histologic types, that is, the cells that make up the polyp have different characteristics when viewed under the microscope. They also vary in size, number, and location. Most importantly, they vary in their tendency to become cancerous (malignant).
The most common type of polyp is the adenoma or adenomatous polyp. It is an important type of polyp not only because it is the most common, but because it is the most common cause of colon cancer. The likelihood that an adenoma will develop into (or has already developed into) cancer is partially dependent on its size; the larger the polyp, the more likely it is that the polyp is or will become malignant (concern about the malignant potential increases with a polyp greater than one centimeter in size). It also matters if there is a single polyp or multiple polyps. Patients with multiple polyps -- even if they are not malignant when examined under the microscope -- are more likely to develop additional polyps in the future that may become malignant. Concern about this increasing malignant potential begins when there are three or more polyps. Finally, the malignant potential of an adenomatous polyp is related to the manner in which the cells of the polyp organize themselves as seen under the microscope. Cells that organize themselves into tubular structures (tubular adenomas) are less likely to become cancerous than cells that organize themselves into finger-like structures (villous adenomas).
Most adenomatous polyps are considered sporadic, that is, they do not stem from a recognized genetic mutation that is present at birth (are not familial). Nevertheless, the risk of having colon polyps greater than one centimeter in size or developing colon cancer is two-fold greater if a first degree relative has colon polyps greater than one centimeter in size. Therefore, there is likely to be a genetic factor working even in sporadic adenomatous polyps.
Genetic adenomatous polyp syndromes
There are several familial, genetic conditions in which the mutations or the development of mutations are programmed into an individual's genes from before birth, passed down from parent to child. In the most common of these conditions, hundreds to thousands of adenomatous polyps form (familial adenomatous polyposis or FAP) as a result of a mutation in the APC gene. It is important to recognize these polyposis syndromes and the exact genetic abnormality that causes them, if possible since the malignant potential of these polyps is much greater than that of individuals without the genetic abnormality. (Eighty percent or more of these patients develop colon cancer.) Even though these syndromes are responsible for only a few percent of all colon cancers, recognition of a polyposis syndrome identifies patients in whom screening for additional polyps needs to be done more frequently so that new polyps and cancers can be discovered and treated early. It may even be recommended that the entire colon be removed to prevent cancer. In addition, genetic testing can be done for relatives of the patient to determine whether or not the relative has the same mutation as the patient and, therefore, is very likely to develop polyps and cancer. Relatives with the same mutation then can be screened for the presence of polyps and cancer, preferably starting the colorectal cancer screening at an earlier age than would normally be done because cancers in these syndromes develop at an earlier age than cancers not associated with a syndrome. Because of the autosomal dominant mode of transmission of the gene and its effects, only one parent needs to have the FAP gene to pass on to his or her children, and therefore, there is a 50/50 chance that each of his or her children will have FAP.
There is an uncommon form of FAP in which the number of polyps is less than classic FAP -- less than 100 -- called attenuated FAP. The mutation in the APC gene in attenuated FAP is different than the mutation in classic FAP. Patients with many polyps but not the numbers seen in FAP should be identified and tested for the mutation. Unlike FAP, which is an autosomal dominant syndrome, attenuated FAP is a recessive mutation so that an individual needs to inherit one mutated gene from each parent to develop polyps and colon cancer, and because of the rarity of the mutation, this occurs rarely.
Another syndrome of polyps and colon cancer is the MYH polyposis syndrome. Individuals with MYH polyposis develop less than 100 polyps at a young age and are at high risk for developing colon cancer. It is caused by mutations in a different gene than FAP, the MYH gene; however, the mutation occurs sporadically due to spontaneous mutations and, therefore, a hereditary pattern is not apparent in parents, although it may be seen in siblings. Because it is an autosomal recessive gene that requires a mutated gene from each parent, the MYH polyposis syndrome is rare.
The second most common type of colon polyp is the hyperplastic polyp. It is important to recognize these polyps and to differentiate them from adenomatous polyps since they have little or no potential to become cancerous unless they are located in the proximal (ascending colon), or show a particular histologic pattern under the microscope (a serrated appearance). Nevertheless, there are uncommon genetic syndromes in which patients form many hyperplastic polyps. These patients may be at a similar risk for developing colon cancer as patients with multiple adenomatous polyps, particularly if the polyps are large, serrated, located in the ascending colon, and there is a family history of colon cancer. Hyperplastic polyps may coexist with adenomatous polyps.
Other types of colon polyps
Much less common types of colon polyps exist, and their potential for becoming cancerous varies greatly, for example, hamartomatous, juvenile, and inflammatory polyps.