Colon cancer begins when cells in the large intestine grow uncontrollably. While often discussed as a single disease, it encompasses several distinct types and classifications. Understanding these differences is important because they influence how the cancer behaves and how it is treated.
Understanding the Predominant Type
The most common type of colon cancer is adenocarcinoma, accounting for approximately 95% of all colorectal cancer diagnoses. This cancer originates in the glandular cells that line the inside of the colon, which are responsible for producing mucus to help waste move through the large intestine. Adenocarcinoma typically develops from growths on the inner lining of the colon called polyps. Certain types of polyps, known as adenomas, have the potential to become cancerous over time.
Pathological diagnosis of adenocarcinoma involves examining tumor tissue to determine its characteristics, such as how deeply it has grown into the colon wall and how abnormal the cancer cells appear. The tumor’s grade, indicating how closely cancer cells resemble normal cells, also helps describe its potential behavior.
Exploring Less Common Types
While adenocarcinoma is prevalent, several other, less common types of colon cancer exist. Carcinoid tumors, also known as neuroendocrine tumors, originate from neuroendocrine cells found throughout the gastrointestinal tract. These tumors are generally slow-growing and represent less than 1% of all colorectal cancers. Gastrointestinal stromal tumors (GISTs) are another rare type, developing from specialized cells called interstitial cells of Cajal, which regulate muscle contractions in the digestive tract. GISTs are uncommon in the colon, making up about 5% of all GIST cases.
Lymphomas, cancers of the immune system, can sometimes originate in the colon, though this is rare, accounting for less than 1% of all colon cancers. Squamous cell carcinomas are exceptionally rare in the colon, forming from squamous cells that line certain areas of the digestive tract. This type is considered aggressive and usually presents similarly to adenocarcinoma.
Molecular Markers and Their Significance
Beyond their histological type, colon cancers are increasingly classified by their molecular characteristics, which are genetic changes within the tumor cells. These molecular markers provide insights into how a cancer might respond to specific therapies.
One such marker is Microsatellite Instability (MSI), which indicates defects in the DNA mismatch repair (MMR) system. Approximately 15% of colorectal cancers exhibit high microsatellite instability (MSI-H), meaning their DNA repair mechanisms are faulty, leading to many genetic mutations. MSI-H tumors often respond well to immunotherapy.
Mutations in the RAS gene family, specifically KRAS and NRAS, are common molecular markers. KRAS mutations, found in about 40-45% of colorectal cancer patients, can cause uncontrolled cell growth and may indicate that certain targeted therapies, like EGFR inhibitors, may not be effective. The BRAF gene mutation, present in approximately 10-15% of colorectal cancers, is another important marker. The most frequent BRAF mutation, V600E, is associated with more aggressive tumor growth and can influence treatment choices.
Impact on Diagnosis and Treatment Approach
Diagnostic methods, such as pathology reports, differentiate between histological types like adenocarcinoma versus rarer forms. These distinctions influence initial staging and prognostic outlook for patients. For example, MSI-H status can be associated with a better prognosis in early-stage disease.
The presence of specific molecular markers directly guides treatment strategies. For instance, testing for RAS and BRAF mutations is recommended for patients with metastatic colon cancer to determine sensitivity to targeted therapies. Cancers with RAS mutations may not respond to EGFR inhibitors, while BRAF mutations can predict a less favorable response to certain treatments. Conversely, MSI-H status can predict a positive response to immunotherapies.