Colorectal cancer - Screening and Diagnosis

Colorectal Cancer Screening and Diagnosis Introduction Colorectal cancer screening aims to detect early disease when treatment is most effective. This section covers the key screening modalities, diagnostic approaches, and staging systems that are essential for understanding colorectal cancer management. The quality of screening and accurate staging directly impact patient outcomes and treatment planning. Screening and Early Detection Colorectal cancer screening uses several complementary approaches to identify disease in its earliest, most treatable stages. The choice of screening method depends on sensitivity, specificity, patient preference, and availability. Stool-Based Tests Fecal Occult Blood Test (FOBT) detects hidden (occult) blood in the stool that is not visible to the naked eye. This can indicate bleeding from colorectal lesions, including polyps and early cancers. Fecal Immunochemical Test (FIT) is more modern and more sensitive than FOBT. It specifically detects hemoglobin in stool using antibodies. The key advantage is that FIT is more specific to colonic bleeding and less likely to react to bleeding from the upper gastrointestinal tract, reducing false positives. An important limitation of stool-based tests is false-negative results with right-sided lesions. Lesions on the right (cecum and ascending colon) may bleed intermittently rather than continuously. If a patient happens to be screened during a non-bleeding period, the lesion will be missed. This is why stool tests must be repeated regularly. Endoscopic Methods Flexible Sigmoidoscopy examines the distal (lower) portion of the colon—the sigmoid colon and rectum. While it can only visualize about one-third of the colon, regular sigmoidoscopy has been shown to reduce colorectal cancer mortality. The advantage is that it's shorter, less uncomfortable, and doesn't require full bowel preparation. Colonoscopy provides complete visualization of the entire colon, from the rectum to the cecum. Beyond visualization, colonoscopy allows the endoscopist to remove polyps during the same procedure and obtain biopsies. Because it can detect and remove precancerous polyps before they become cancer, colonoscopy provides the greatest reduction in colorectal cancer incidence of any screening method. A practical limitation of colonoscopy is that the procedure has a miss rate for small polyps, particularly those less than 5 mm. This miss rate depends heavily on the quality of bowel preparation (cleansing) and the experience of the endoscopist. Inadequate preparation or rushes during the procedure can cause lesions to be overlooked. Computed Tomography Colonography CT colonography (also called virtual colonoscopy) creates a three-dimensional reconstruction of the colon using CT imaging without requiring insertion of a scope into the colon. This is a non-invasive alternative that may be useful for patients who cannot tolerate or have contraindications to colonoscopy. Screening Recommendations Average-risk adults should begin colorectal cancer screening at age 45 years and continue through age 75. For individuals with strong family history or known genetic predispositions (such as hereditary nonpolyposis colorectal cancer), screening may begin at age 40 or even earlier. Screening intervals vary by modality: Colonoscopy: every 10 years FIT: every 1-2 years Flexible sigmoidoscopy: every 10 years These intervals are based on the time it typically takes for a polyp to progress to cancer and the performance characteristics of each test. Benefits of Screening Population-based colorectal cancer screening programs have demonstrated substantial improvements in public health: Decreased colorectal cancer incidence (by preventing cancers from developing) Decreased colorectal cancer mortality (by detecting existing cancers earlier) Early detection typically means earlier stage disease, which is associated with better prognosis and less intensive treatment requirements. Diagnosis and Staging Once screening detects an abnormality, diagnosis and staging determine the extent of disease and guide treatment planning. Diagnostic Endoscopy When colonoscopy identifies a suspicious lesion, the endoscopist performs a biopsy—removing a small tissue sample for histopathologic examination. Biopsy allows definitive diagnosis of cancer and provides information about tumor grade and type. For early cancers confined to the mucosa (the innermost layer of the colon), specialized endoscopic techniques can sometimes remove the entire lesion: Endoscopic mucosal resection (EMR) removes mucosa-confined lesions Endoscopic submucosal dissection (ESD) is a more advanced technique allowing deeper and more controlled resection These approaches can be curative for very early disease while avoiding surgery. Histopathologic Evaluation The pathologist examines the tissue under a microscope and provides crucial information: Adenocarcinoma is the predominant histologic type, accounting for over 95% of colorectal cancers. Other types (mucinous, signet ring) are less common. Tumor grade describes how abnormal the cancer cells appear and how rapidly they likely grow (well-differentiated, moderately differentiated, or poorly differentiated). Lymphovascular invasion (LVI) and perineural invasion (PNI) are markers of more aggressive biology. Their presence indicates higher risk of recurrence and typically influence treatment decisions, particularly whether chemotherapy should be added to surgery. TNM Staging System Colorectal cancer is staged using the TNM classification: T = Tumor depth: how deeply the cancer has invaded through the colon wall N = Nodes: whether lymph nodes are involved and how many M = Metastasis: whether cancer has spread to distant organs The TNM classification provides a standardized language for describing cancer extent and predicting prognosis. The current staging criteria follow the American Joint Committee on Cancer (AJCC) 8th edition (published in 2018). Tumor depth (T stage) is assessed at multiple margins: Proximal surgical margin (toward the beginning of the colon) Distal surgical margin (toward the end of the colon) Radial (circumferential) margin (the outermost surface) Adequate margins ensure that the cancer has been completely removed. Molecular Testing Modern colorectal cancer diagnosis includes molecular testing that guides treatment: Microsatellite Instability (MSI) testing identifies tumors with defects in DNA mismatch repair. MSI-high tumors behave differently and may benefit from immunotherapy. KRAS mutation testing is critical because cancers with KRAS mutations do not respond to anti-EGFR (epidermal growth factor receptor) targeted therapies. Knowing KRAS status prevents ineffective treatment. NRAS and BRAF mutations are additional mutations that affect treatment options. BRAF V600E mutation is associated with worse prognosis. These molecular features, combined with stage, guide chemotherapy selection and targeted therapy choices. Imaging for Staging After diagnosis, imaging determines the extent of disease: Computed tomography (CT) of the chest, abdomen, and pelvis detects whether cancer has spread (metastasized) to the liver, lungs, or other organs. This is essential information for treatment planning. Magnetic resonance imaging (MRI) is particularly valuable for rectal cancer because it provides excellent detail of: How deeply the tumor invades the rectal wall Whether it extends through the mesorectal fascia (a membrane surrounding the rectum) This local staging information is critical for deciding whether neoadjuvant (pre-operative) radiation or chemotherapy should be given before surgery. Biomarkers for Prognosis Carcinoembryonic antigen (CEA) is a blood marker that can be measured before and after treatment. Elevated CEA before surgery predicts higher risk of recurrence. After curative surgery, a rising CEA may indicate recurrent disease and prompt imaging or more intensive follow-up. Summary Effective colorectal cancer management begins with screening using stool tests, endoscopy, or imaging. When cancer is diagnosed, comprehensive staging using TNM classification, pathologic features, and molecular testing provides the information needed for treatment planning. The combination of accurate staging and molecular characterization allows physicians to tailor therapy to the individual tumor's biology and extent.