Adenoma refers broadly to any benign tumor of glandular tissue. This article will address specifically adenomas of the colon, occurring within polyps. Colon polyps are a common entity, increasing in prevalence with age. Adenomatous polyps are the most common type of polyp in the colon, accounting for about 60% to 70% of all colonic polyps. Conventional type adenomatous polyps can be classified as tubular, villous, or tubulovillous. Villous adenomas are characterized by more than 75% villous features, where villous refers to finger-like or leaf-like epithelial projections. Tubulovillous adenomas have between 25% and 75% villous features. Less than 25% villous features indicate a tubular adenoma. Adenomas are usually asymptomatic and found on routine colorectal cancer screening. Adenomas with villous features may be associated with slight increase in development of more advanced neoplasia or dysplasia compared to other types of adenomas.
The risk of developing colorectal polyps is related to both environmental and genetic factors. The majority of polyps in the colon arise sporadically, even though genetics play the most important role in influencing the individual risk of developing colon polyps and subsequent colorectal cancer. Personal history of colon adenomas places a patient at increased risk for future development of colon cancer.
Colorectal cancer has been shown to arise mainly from pre-existing adenomatous colon polyps. Adenomatous polyps form as mentioned above primarily in a sporadic fashion. The sporadic nature of these polyps is a result of either a mutation in the adenomatous coli pathway or DNA mismatch repair.
Familial adenomatous polyposis (FAP) is a genetic disorder which, in particular, predisposes affected patients to the development of multiple adenomatous polyps. FAP is an autosomal dominant disorder caused by a mutation in the tumor suppressor APC gene, located on chromosome 5q. This mutation can either be inherited or in as many as one out of three patients, can present as a new germline mutation. The frequency of FAP is from 1 in 8,000 to 1 in 14,000, with large numbers of adenomas occurring beginning in late childhood. Classically, patients with FAP have over 100 colorectal adenomatous polyps, but many patients have several hundred, or even thousands, of polyps. These patients usually develop adenocarcinoma by the age of 30 to 40. Even given the likely development of colon cancer in these patients, FAP only accounts for around 1% of colon cancers.
Adenomas are defined as possessing at least the characteristics of low-grade dysplasia. Some adenomas may progress over an extended period from low-grade dysplasia to high-grade dysplasia, carcinoma in situ, or invasive adenocarcinoma. Although physicians debate this, and the evidence does not completely confirm, thus far, if villous features indicate an increased risk of developing colorectal cancer, multiple studies demonstrate that histologic villous features of adenomas may be associated with developing advanced forms of neoplasia. Size at baseline of adenomas has been shown in some studies to influence future advanced disease; lesions greater than 1 cm increase a patient's risk. One source states that villous lesions have an increased risk of containing adenocarcinoma correlated to size specifically, with a 10$ to 20% risk in adenomas larger than 2 cm, and a 5% risk in adenomas from 1 cm to 2 cm in size. However, these findings are inconsistent when considered across multiple studies regarding the subject. A recent study proposes using the overall sum of all adenoma diameters (called the adenoma bulk) at initial surveillance colonoscopy to stratify patients as either high or low-risk for predicting the development of advanced neoplasia. This study states that the adenoma bulk is comparatively predictive as the conventional current model using histology features when using a risk stratification cutoff of 10mm for the adenoma bulk, with high-risk adenoma bulk being greater than or equal to 10 mm.
The incidence of colon polyps increases in direct correlation with age. By age 50, colorectal cancer screening studies have demonstrated a prevalence of 25% to 30%, increasing to 50% by age 70 in high-risk Western countries such as the United States. Colon polyps are rare in the younger population, present in only one to four percent of 20 to 30-year-olds. Villous adenomas account for 5% to 15% of all adenomas.
Before discussing what constitutes villous features of adenomas, it is important to differentiate degree of dysplasia present in adenomas. The degree of dysplasia present in an adenoma is determined by both cytological and architectural features. Adenomas are tumors of dysplastic epithelium which can be characterized as having a low grade or high-grade dysplasia, which indicates the level of maturation of the epithelium. By definition, adenomas have at least low-grade dysplasia.
The cytological features of low-grade dysplasia include crowded, pseudo-stratification to early stratification of spindled or elongated nuclei which occupy the basal half of the cytoplasm. Pleomorphism and atypical mitoses should be absent or minimally present. Mitotic activity and minimal loss of cell polarity are allowed. Architecturally, the crypts should maintain a resemblance to normal colon, without significant crowding, cribriform, or complex forms.
High-grade dysplasia cytologically boasts increased nucleus to cytoplasm ratio, more significant loss of polarity, more "open" appearing nuclei with increasingly prominent nucleoli. Other features which distinguish high grade from low-grade dysplasia include significant pleomorphism, rounded nuclei, atypical mitoses, and significant loss of polarity. Mitotic figures may be observed as in low-grade dysplasia. Cribriform and crowding, back to back glands indicate high-grade dysplasia and can be important architectural features to aid in differentiating some cases of low vs. high-grade dysplasia which may have less distinct cytological features.
The villous component of adenomas refers to epithelial finger-like projections away from the muscularis mucosae. There should be deep crypts, and the projections should contain fibrovascular cores lined by dysplastic epithelium. As stated above, the amount of villous differentiation distinguishes villous adenomas (over 75% villous features) versus tubulovillous adenomas (mixed tubular and villous features with 25% to 75% villous features).
The majority of patients will not present symptomatically and subsequently, have a colonoscopy which identifies colorectal polyps. Rather, polyps are found on screening or surveillance colonoscopy. If patients do present with symptoms, commonly they will endorse a history of blood per rectum.
Current US Preventive Services Task Force recommendation for average-risk adults is that beginning at age 50 patients should initiate regular colon cancer screening. Methods include a fecal occult blood test, fecal immunochemical test, and direct visualization methods. Direct visualization through colonoscopy, biopsy, and histologic examination of colon lesions is necessary to examine and diagnose colorectal polyps and cancers.
Surgeons should excise adenomatous polyps of any type completely, and confirm clear margins. Based on the morphology and number of lesions removed, surveillance guidelines according to the American Cancer Society are as follows: