Introduction: Who Should Seek Diagnostic Testing

Rectosigmoid cancer typically develops in the area between 9 and 20 centimeters from the anal verge, which is the opening of the anus. This location sits at an important boundary in the digestive system, where the sigmoid colon transitions into the rectum.^[1] Because of its unique position, diagnosing rectosigmoid cancer requires careful attention to detail and specific measurements that help doctors determine the best treatment approach.

People who should consider seeking diagnostic testing include those experiencing warning signs such as blood appearing in their stool, which might make the stool look dark maroon or bright red. Changes in bowel habits that persist—such as ongoing diarrhea, constipation, or feeling like the bowel doesn’t empty completely after using the bathroom—are also important signals.^[2] Other symptoms that warrant medical attention include unexplained abdominal pain that doesn’t go away or hurts significantly, a bloated stomach lasting more than a week, stools that appear narrow or stringy like a pencil, unexplained weight loss, persistent tiredness, or discovering a lump in the rectum during examination.^[5]

It’s important to understand that adults aged 45 and older should talk to their doctor about screening even if they have no symptoms at all. Regular screening can detect problems before cancer develops.^[4] Those with higher risk factors—such as a close family member (parent, sibling, or child) who had colon or rectal cancer, a personal history of colon polyps or other cancers, inherited genetic conditions like Lynch syndrome or familial adenomatous polyposis (FAP), or long-standing inflammatory bowel diseases like Crohn’s disease or ulcerative colitis—may need to start screening earlier and more frequently.^[7]

Certain populations face higher risks. People who are Black are statistically more likely to develop rectal cancer, though the reasons aren’t fully understood yet. Males are also slightly more likely than females to develop this type of cancer.^[14] Lifestyle factors such as smoking tobacco, having obesity, eating large amounts of red and processed meats, or consuming three or more alcoholic drinks daily can increase the chance of developing colorectal cancers.^[7]

Classic Diagnostic Methods for Rectosigmoid Cancer

Diagnosing rectosigmoid cancer typically begins with a physical examination and progresses through several specialized tests. The process aims not only to confirm whether cancer is present but also to determine its exact location, size, and whether it has spread to nearby tissues or other parts of the body.

Physical Examination and Digital Rectal Exam

A digital rectal examination (DRE) is often one of the first steps. During this procedure, a healthcare provider inserts a gloved, lubricated finger into the rectum to feel for abnormalities such as lumps or unusual growths.^[14] While this simple test cannot detect all cancers, especially those higher up in the rectosigmoid area, it can provide initial clues that more comprehensive testing is needed.

Colonoscopy

Colonoscopy is considered the gold standard for diagnosing colorectal cancers, including rectosigmoid cancer. This procedure uses a long, flexible tube with a tiny camera at the end, called a colonoscope. The tube is gently inserted through the anus and advanced through the rectum and entire colon, allowing the doctor to view the inner lining of the large intestine on a video screen.^[10]

Before a colonoscopy, patients receive medications to keep them comfortable and relaxed during the procedure. The examination not only allows visualization of any suspicious areas but also enables the doctor to take tissue samples for further analysis. If polyps are found—small clumps of cells that can develop into cancer over time—they can often be removed during the same procedure, potentially preventing cancer from developing.^[6]

Biopsy

A biopsy involves removing a small sample of tissue from a suspicious area for examination under a microscope. During a colonoscopy, the doctor can pass special cutting tools through the colonoscope to collect these tissue samples from inside the rectum or sigmoid colon.^[10] The samples are then sent to a laboratory where a specialist called a pathologist examines the cells to determine if cancer is present.

Beyond simply confirming cancer, the laboratory performs additional specialized tests on the tissue to learn more about the cancer cells. These tests provide information about how the cancer might behave and which treatments might work best. This detailed analysis is crucial for developing a personalized treatment plan.

Imaging Studies

Once rectosigmoid cancer is diagnosed through biopsy, imaging tests help determine the cancer’s stage—meaning how large it is and whether it has spread. These tests create detailed pictures of the inside of the body without surgery.

Computed tomography (CT) scans use X-rays taken from multiple angles and combine them with computer processing to create cross-sectional images of the body. CT scans of the abdomen and pelvis can show the size and location of the tumor and whether it has spread to nearby lymph nodes or other organs.^[10] In rectosigmoid cancer, radiologists pay special attention to the circumferential resection margin—the distance between the tumor and the edge of surrounding tissue. When this margin is close or positive on imaging, it suggests the cancer may be more difficult to remove completely with surgery.^[1]

Magnetic resonance imaging (MRI) uses powerful magnets and radio waves instead of X-rays to create detailed images. MRI scans are particularly useful for examining soft tissues in the pelvis and can provide very clear pictures of the rectum and surrounding structures. This helps surgeons plan the best approach for removing the tumor.

Understanding the tumor’s position relative to anatomical landmarks is critical. Two important landmarks are the sacral promontory (a bony landmark in the pelvis) and the peritoneal reflection (a fold of the membrane lining the abdominal cavity). Research has shown that tumors located below the peritoneal reflection may require different treatment approaches than those above it.^[1] Imaging studies help doctors identify these relationships precisely.

Blood Tests

Several blood tests support the diagnostic process. A complete blood count (CBC) measures different types of cells in the blood. This test can reveal anemia—a low red blood cell count—which might result from bleeding caused by a tumor in the rectosigmoid area.^[10]

The carcinoembryonic antigen (CEA) blood test measures a protein that can be elevated in people with colorectal cancer. While not used as a standalone diagnostic test—since many conditions can raise CEA levels and some cancers don’t elevate it at all—this marker is valuable for monitoring treatment response and watching for cancer recurrence after treatment.^[14] If CEA is high before treatment and drops to normal afterward, it suggests the treatment worked. If it rises again later, it may signal that cancer has returned.

Measuring Tumor Location

Accurately determining where the tumor sits is particularly important for rectosigmoid cancer. Doctors measure the distance from the anal verge to the lower edge of the tumor, typically during colonoscopy or on imaging studies. Rectosigmoid cancers are generally considered to be between 9 and 20 centimeters from the anal verge.^[1]

This measurement matters because cancers in different locations may require different treatment strategies. The rectum sits in a tight space in the pelvis, surrounded closely by other organs and structures, making surgery more complex than for tumors higher up in the colon. The sigmoid colon, which connects to the rectum, has more room around it.^[2] A tumor right at the junction presents unique challenges.

Diagnostic Testing for Clinical Trial Qualification

When patients with rectosigmoid cancer consider participating in clinical trials—research studies testing new treatments—they typically undergo additional diagnostic testing. Clinical trials have specific requirements, called eligibility criteria, that determine who can participate. These criteria ensure the study can properly evaluate the treatment being tested and keep participants safe.

Staging Assessments

Clinical trials often require precise staging information before enrollment. The stage describes how advanced the cancer is, based on the size of the primary tumor (T stage), whether it has spread to nearby lymph nodes (N stage), and whether it has spread to distant organs (M stage). This is called the TNM staging system.

For rectosigmoid cancers, trials may specifically look for patients with T3-T4 tumors (meaning the cancer has grown through multiple layers of the bowel wall) and/or node-positive disease (cancer cells found in nearby lymph nodes).^[1] Determining this requires the imaging studies mentioned earlier—CT scans and MRI—along with careful review by radiologists who specialize in cancer.

Molecular and Genetic Testing

Modern clinical trials increasingly use molecular testing to match patients with treatments most likely to help them. After a biopsy, the cancer tissue can be tested for specific genetic mutations or protein markers. For example, some colorectal cancers have mutations in genes called KRAS, NRAS, or BRAF, or they may have something called microsatellite instability (MSI) or problems with mismatch repair (MMR) genes.

These molecular characteristics help predict how the cancer will behave and which treatments might work. Some clinical trials specifically seek patients whose tumors have particular molecular features, while others exclude certain features. Testing for these markers has become part of standard diagnostic workup for many patients with colorectal cancer, both for clinical trial participation and for regular treatment planning.

Performance Status Evaluation

Clinical trials assess a patient’s overall health and ability to perform daily activities, known as performance status. Doctors use standardized scales to rate how well patients can function. This helps ensure that participants are healthy enough to tolerate the experimental treatment being studied. Diagnostic procedures don’t directly measure performance status, but the overall diagnostic workup—including physical examination and blood tests—helps doctors evaluate whether a patient’s general health is suitable for trial participation.

Baseline Measurements

Before starting any clinical trial treatment, doctors establish baseline measurements. These might include the exact size of the tumor on imaging, current CEA levels if elevated, complete blood counts, tests of liver and kidney function, and sometimes special imaging like PET scans (positron emission tomography), which show metabolic activity in tissues and can help detect cancer spread.^[14]

These baseline measurements are crucial because they provide a starting point for comparison. As the trial treatment proceeds, doctors repeat these same tests to see if the tumor is shrinking, staying stable, or growing. This helps researchers determine whether the new treatment is working.

Exclusion Criteria Testing

Just as trials have specific requirements for who can participate, they also have exclusion criteria—conditions or circumstances that prevent participation. Diagnostic testing helps identify these. For instance, trials might exclude patients whose cancer has spread extensively to the liver or lungs, or those with certain other medical conditions that could complicate the study. Complete diagnostic imaging and medical history review help determine if any exclusion criteria apply.