An adenoma is a type of polyp, a growth that can form on the inner lining of the colon or rectum. While most polyps are benign, adenomas are distinct because they originate from glandular tissue and have the potential to develop into cancer over time. This potential for malignant transformation is why adenomas are considered precancerous and are typically removed when detected. Other types of polyps, such as hyperplastic polyps, are generally not considered to have this same risk.
What is an Adenoma?
Adenomas are abnormal growths that form on the inner lining of the colon or rectum. They are quite common, with roughly one-third to one-half of all individuals developing one or more adenomatous polyps in their lifetime. Most adenomas do not cause symptoms and are discovered during routine screening procedures. Their removal is important as it prevents them from potentially progressing to colorectal cancer.
Defining “Advanced” Characteristics
An adenoma is classified as “advanced” when it exhibits specific features indicating a higher likelihood of progressing to colorectal cancer. An adenoma needs only one of these characteristics to be considered advanced. These characteristics include size, a particular cellular growth pattern, and the degree of cellular abnormality.
One characteristic is size, with adenomas typically considered advanced if they are 1 centimeter (0.4 inches) or larger. Larger polyps have more cells and a greater opportunity for genetic mutations to accumulate, increasing their potential for cancerous transformation. For instance, the incidence of cancer in polyps over 25 mm can be as high as 40%.
Another characteristic is villous or tubulovillous histology, which describes the specific growth pattern of the cells within the adenoma. Villous features refer to finger-like or leaf-like projections of the epithelial cells. Tubulovillous adenomas contain both tubular (tube-shaped) and villous components, with villous architecture making up between 25% and 75% of the adenoma. These growth patterns are associated with a higher risk compared to purely tubular adenomas.
The third characteristic is high-grade dysplasia (HGD), which refers to significant abnormality in the cells’ growth and appearance. Dysplasia describes how abnormal the cells look under a microscope, with high-grade indicating that the cells are very abnormal and closer to becoming cancerous. High-grade dysplasia involves features like enlarged, pleomorphic, and hyperchromatic nuclei, loss of cell polarity, and increased mitotic activity.
Significance for Colorectal Cancer Risk
Advanced adenomas are medically significant because they represent a more substantial step along the pathway toward colorectal cancer development. Most colorectal cancers are understood to develop from pre-existing adenomas over many years, a process known as the adenoma-carcinoma sequence. This sequence involves a stepwise accumulation of genetic mutations in the cells, transforming normal colon lining into adenomas and then, over time, into cancerous tissue.
While all adenomas have the potential to become cancerous, advanced adenomas have a higher likelihood of progression. They serve as an important warning sign, indicating an individual is at increased risk for developing colorectal cancer. Advanced adenomas are not yet cancer, but their presence underscores the importance of early detection and removal to prevent full-blown cancer from developing. Removing these lesions is an effective strategy for preventing colorectal cancer, with studies suggesting a significant reduction in incidence.
Detection and Follow-Up
Advanced adenomas are primarily detected during a colonoscopy, which is a comprehensive screening procedure for the colon and rectum. During this procedure, any polyps found, including advanced adenomas, are typically removed through a technique called polypectomy. This removal is crucial, as it prevents these precancerous growths from potentially developing into colorectal cancer.
After an advanced adenoma has been identified and removed, regular surveillance colonoscopies are recommended. Individuals who have had an advanced adenoma are at an increased risk for developing new adenomas or even colorectal cancer in the future. The frequency of these follow-up colonoscopies is determined by medical guidelines, taking into account factors such as the number, size, and specific type of adenomas initially found. This personalized surveillance schedule helps manage the risk and detect any new growths early.