Introduction: When to Seek Diagnostic Testing

Stage III colon cancer means that the cancer has spread from the inner layers of the colon wall into nearby lymph nodes, which are small bean-shaped structures that help your body fight infections. At this stage, the cancer has not yet reached distant parts of the body such as the liver or lungs. Diagnosing this stage properly is essential because it determines the best treatment path and helps doctors understand how far the disease has progressed.^[1]

Anyone who experiences persistent symptoms that might indicate colon cancer should seek medical evaluation as soon as possible. These symptoms include noticeable changes in bowel habits, such as ongoing constipation or diarrhea, blood in or on the stool, unexplained belly pain, a bloated feeling that doesn’t go away, or unexplained weight loss. While these symptoms can be caused by other, less serious conditions, it is always better to get them checked out by a healthcare provider. Early detection and accurate staging can make a significant difference in treatment outcomes and long-term survival.^[3]

People with certain risk factors should be especially vigilant about seeking diagnostic tests. These include having a family history of colon or rectal cancer, a personal history of colon polyps or inflammatory bowel disease, or belonging to certain ethnic groups that have higher rates of colon cancer. Older age is also a major risk factor, although colon cancer is increasingly being diagnosed in younger adults. If you have any of these risk factors, talk to your doctor about when and how often you should be screened.^[16]

Classic Diagnostic Methods for Stage III Colon Cancer

Diagnosing colon cancer and determining its stage involves several types of tests. The process typically starts with a physical examination and a review of your medical history. Your doctor will ask about your symptoms, how long you’ve had them, and whether you have any family history of colon or other cancers. They may also perform a digital rectal exam, in which the doctor uses a gloved finger to feel inside the rectum for any unusual masses or growths.^[3]

The most important test for diagnosing colon cancer is a colonoscopy. During this procedure, a doctor uses a long, flexible tube with a tiny camera on the end to look inside your entire colon and rectum. The camera sends images to a monitor, allowing the doctor to see any polyps, growths, or areas that look abnormal. If something suspicious is found, the doctor can take a small sample of tissue, called a biopsy, right during the colonoscopy. This tissue is then sent to a lab where specialists examine it under a microscope to see if it contains cancer cells and, if so, how aggressive those cells are.^[19]

A biopsy is essential because it confirms whether cancer is present and provides information about the type and behavior of the cancer cells. Lab tests on the biopsy sample can reveal how fast the cancer is growing and whether it has certain genetic or molecular features that might influence treatment decisions. For example, some cancers have specific protein markers that can be targeted with certain drugs.^[19]

Once cancer is confirmed, imaging tests help determine the stage of the disease. Computed tomography (CT) scans are commonly used to create detailed, three-dimensional images of the abdomen and pelvis. These scans can show whether the cancer has grown through the colon wall and whether it has spread to nearby lymph nodes or other organs. CT scans use X-rays and a computer to generate cross-sectional images of the body, which help doctors see structures that would not be visible on a regular X-ray.^[3]

Another imaging technique is magnetic resonance imaging (MRI), which uses powerful magnets and radio waves instead of X-rays to create pictures of the inside of your body. MRI scans are particularly useful for looking at soft tissues and can provide detailed images of the colon, nearby lymph nodes, and surrounding organs. They are often used when doctors need more information than a CT scan can provide.^[3]

Blood tests are also part of the diagnostic workup, although they cannot diagnose colon cancer on their own. One common blood test measures the level of a protein called carcinoembryonic antigen (CEA). Some colon cancers produce this protein, and elevated CEA levels can suggest the presence of cancer. However, not all colon cancers make CEA, and some non-cancerous conditions can also raise CEA levels. For this reason, CEA tests are more often used to monitor treatment response or check for cancer recurrence after treatment rather than to make an initial diagnosis.^[19]

Other blood tests check your overall health, such as how well your liver and kidneys are working and whether you have anemia (a low red blood cell count), which can occur if a tumor is bleeding slowly over time. These tests help doctors understand your overall condition and plan the safest and most effective treatment.^[3]

Diagnostics for Clinical Trial Qualification

Clinical trials are research studies that test new treatments or new ways of using existing treatments. Participating in a clinical trial can give patients access to cutting-edge therapies that are not yet widely available. However, to join a clinical trial, patients must meet certain criteria, and this often involves undergoing additional diagnostic tests.^[13]

One of the key requirements for enrolling in a clinical trial is accurate staging of the cancer. This means doctors need to know exactly how far the cancer has spread. Standard staging tests such as colonoscopy with biopsy, CT scans, and sometimes MRI or positron emission tomography (PET) scans are used to confirm the stage. PET scans involve injecting a small amount of radioactive sugar into the bloodstream. Cancer cells, which tend to use more sugar than normal cells, absorb this substance and show up as bright spots on the scan. This can help doctors see if the cancer has spread to distant parts of the body.^[3]

In addition to staging, clinical trials may require testing the cancer tissue for specific biomarkers. Biomarkers are molecules or genetic changes in the cancer cells that can affect how the cancer grows and how it responds to treatment. For example, some trials may only accept patients whose tumors have certain mutations or protein markers. One well-known biomarker test checks for mismatch repair (MMR) deficiency, which is a genetic condition that affects how cells repair damaged DNA. Cancers with MMR deficiency may respond well to a type of treatment called immunotherapy, which helps the body’s immune system recognize and attack cancer cells.^[12]

Other biomarker tests might look for mutations in genes such as KRAS, BRAF, or MSI (microsatellite instability) status. These tests are performed on the biopsy tissue and can take several days to a few weeks to complete. The results help determine whether a patient is a good candidate for a particular clinical trial and can also guide standard treatment decisions.^[11]

Blood tests and overall health assessments are also part of the eligibility process for clinical trials. Researchers need to ensure that participants are healthy enough to tolerate the experimental treatment and that they do not have other medical conditions that could interfere with the study results. This might include tests of liver and kidney function, blood counts, and heart function tests such as an electrocardiogram (EKG).^[13]