The process of diagnosing colorectal cancer (CRC) involves a combination of histopathologic analysis, endoscopic evaluation, radiologic imaging, and selective laboratory testing. Accurate and timely identification is essential for optimal treatment and improved patient outcomes. This article provides a detailed overview of the initial investigations used to diagnose CRC, incorporating all key diagnostic components.
Histological Diagnosis: The Foundation
The confirmation of CRC is based on microscopic evaluation of cancerous tissue, typically obtained through a biopsy performed during lower gastrointestinal endoscopy or from a surgical specimen. The vast majority of CRCs are adenocarcinomas, originating from the glandular epithelium of the colon or rectum.
Colonoscopy: Primary Diagnostic Modality
Diagnostic Accuracy and Capabilities
Colonoscopy is the most reliable and comprehensive tool for diagnosing CRC. It allows for:
• Direct visualization of the entire colon
• Biopsy of suspicious lesions
• Detection of synchronous tumors (defined as two or more primary tumors found at the same time or within six months, located in separate parts of the colon, and not due to local spread or metastasis)
• Polyp removal, aiding in prevention and diagnosis
Synchronous CRCs occur in 3% to 8% of patients. When individuals with Lynch syndrome are excluded, the incidence decreases to about 2.5%. The discovery of multiple simultaneous tumors should raise clinical suspicion for hereditary cancer syndromes like Lynch syndrome or MUTYH-associated polyposis.
Appearance of CRC Lesions
Most colorectal tumors appear as endoluminal masses protruding from the mucosal surface into the bowel lumen. These may take on exophytic or polypoid forms. Tumors often appear friable, ulcerated, or necrotic, and may be associated with visible bleeding.
Nonpolypoid Lesions
Some colorectal neoplasms, especially in asymptomatic individuals, present as flat or depressed (nonpolypoid) lesions. These are more difficult to identify during colonoscopy but may have a stronger association with invasive cancer compared to polypoid lesions. Despite their subtle presentation, colonoscopy remains more sensitive than CT colonography for detecting these types of lesions.
Tissue Sampling and Tattooing
Visible lesions are sampled via biopsy or removed entirely through polypectomy, endoscopic mucosal resection (EMR), or endoscopic submucosal dissection (ESD). When a lesion is completely excised and shows invasive features on pathology, tattooing is performed to mark the area for potential future surgery. Tattoos are usually placed justdistal to the lesion, and their location is documented in the colonoscopy report.
Miss Rate and Right-Sided Lesions
Despite its accuracy, colonoscopy may miss between 2% and 6% of CRCs, even in experienced hands. Missed tumors are more frequently found in the right side of the colon.
Incomplete Colonoscopy
In cases where complete colonoscopy is not feasible, particularly due to a malignant obstruction, the rest of the colon should be examined after surgical resection. This can be accomplished through a postoperative colonoscopy or, alternatively, an intraoperative colonoscopy performed immediately following tumor removal.
Flexible Sigmoidoscopy: Limited Use in Diagnosis
Flexible sigmoidoscopy is not sufficient as a standalone diagnostic test in individuals suspected of having CRC. The limited range of this procedure may miss proximal lesions, including those in the cecum. Although left-sided tumors are more common in young-onset CRC, a full colonoscopy is still essential to evaluate for synchronous tumors and polyps throughout the entire colon.
CT Colonography (Virtual Colonoscopy)
Technique and Application
Computed tomographic (CT) colonography, also known as virtual colonoscopy, generates a computer-based endoluminal view of the colon using helical CT imaging and post-processing software. The technique requires full bowel preparation, similar to conventional colonoscopy, to differentiate between true polyps and retained stool.
Use in Incomplete Colonoscopy
CT colonography is particularly useful when conventional colonoscopy is incomplete, which occurs in approximately 10% or fewer of symptomatic patients in clinical trials. Completion may be hindered by technical difficulties, such as:
• Tumor obstruction
• Colon tortuosity
• Poor bowel prep
• Patient intolerance
In such cases, CT colonography can provide valuable radiographic insights. However, it is limited by:
• Inability to perform biopsies or therapeutic interventions
• Potential for false positives, especially in poorly distended or inadequately prepared colons.
This modality should only be used in patients who can tolerate oral bowel prep and pass intestinal gas. For patients with bowel obstruction, a CT scan with a gastrointestinal protocol is more appropriate.
Colon Capsule Endoscopy (PillCam 2)
The colon capsule endoscopy, marketed as PillCam 2, has been approved by both the European Medicines Agency and the U.S. Food and Drug Administration for use in cases of incomplete colonoscopy.
Current Role
While its utility in CRC screening remains under investigation, it may be considered for:
• Patients who had an incomplete colonoscopy
• Patients without bowel obstruction
Despite being noninvasive, capsule endoscopy lacks the ability to obtain tissue samples, limiting its diagnostic capability compared to traditional methods.
Laboratory Investigations
Iron Deficiency Anemia
Iron deficiency anemia is frequently associated with CRC, often reflecting chronic blood loss. However, the absence of anemia does not exclude the presence of cancer.
Liver Function Tests
Routine liver function tests (LFTs) are not reliable for detecting hepatic metastases, given their low sensitivity. They should not be used as primary diagnostic tools for CRC.
Tumor Markers: CEA and CA 19-9
Carcinoembryonic Antigen (CEA)
CEA is a serum marker often associated with CRC, but it is not recommended for use in screening or diagnosis due to:
• Low sensitivity: Estimated at 46% (95% CI: 0.45–0.47)
• Limited specificity: Estimated at 89% (95% CI: 0.88–0.92)
CEA levels can be falsely elevated due to non-malignant conditions including:
• Peptic ulcer disease
• Diverticulitis
• Gastritis
• Liver disease
• COPD
• Diabetes
• Inflammatory conditions
Smoking is also associated with elevated CEA levels, further reducing its diagnostic specificity.
Carbohydrate Antigen 19-9 (CA 19-9)
Like CEA, CA 19-9 has low sensitivity (approximately 30%) for diagnosing CRC and does not outperform CEA in detection rates.
Conclusion
The evaluation of suspected colorectal cancer requires a comprehensive diagnostic strategy. Colonoscopy remains the primary investigation, offering direct visualization, biopsy, and therapeutic intervention. When colonoscopy is incomplete or contraindicated, CT colonography or capsule endoscopy may be considered in select patients. However, their limitations must be acknowledged. Flexible sigmoidoscopy, although useful in some contexts, is not adequate for a complete diagnostic workup. Laboratory tests and tumor markers may support the clinical picture, but are insufficient for establishing the diagnosis. A meticulous, evidence-based approach is crucial for accurate and timely detection of CRC.
