Epidemiology and risk factors
Colorectal cancer predominantly affects older age groups: nearly 75% of cases are diagnosed in patients aged 65 years and older. A positive family history is present in approximately 20% of cases.
Major risk factors include obesity, alcohol abuse, smoking, and chronic inflammatory bowel disease (IBD). At the same time, there are protective factors that reduce the likelihood of developing the disease: aspirin use and a diet low in red meat but high in fiber (fibre).
The rectum alone accounts for about one-third of all cases. Key prognostic factors include the local tumor stage, presence of vascular or lymphatic invasion, histological grade (grade), and preoperative elevation of carcinoembryonic antigen (CEA).
TNM staging and prognosis
The 8th edition of the TNM classification is used to assess disease extent, which correlates with the historical Dukes system (Dukes).
- Stage I (Dukes A): Tumor is confined to the submucosa (T1) or muscularis propria (T2). Lymph nodes are not involved. Survival is 85%–95%.
- Stage II (Dukes B): Tumor invades through the muscularis propria (T3), reaches the visceral peritoneum (T4a), or invades other organs (T4b). No distant metastases. Survival is 60%–80%.
- Stage III (Dukes C): Characterized by regional lymph node involvement (N1 for 1–3 nodes, N2 for >3 nodes). Survival drops to 30%–60%.
- Stage IV (Dukes D): Presence of distant metastases (M1a/M1b) or peritoneal carcinomatosis. Survival in this group does not exceed 10%.
Diagnostic imaging methods
A wide range of imaging modalities is used for the detection and staging of CRC:
- CT colonography (CTC): The sensitivity of the method is comparable to traditional colonoscopy. It clearly visualizes annular lesions ("shouldered ends" sign shouldered ends). Flat cancer manifests as local wall thickening. Important: classic barium enema misses twice as many tumors and should be abandoned.
- Conventional CT: Effective for detecting extracolonic wall invasion (T3–T4) and signs of intraperitoneal dissemination (ascites, omental nodules). However, CT has low efficacy in assessing N-stage, as more than 50% of metastatic nodes are smaller than 1 cm.
- MRI: The gold standard for local staging of rectal cancer. High-resolution T2-weighted sequences are used strictly perpendicular to the lesion.
- PET-CT: Tumors show high 18F-FDG uptake. The method outperforms MRI in diagnosing recurrences. An exception is mucinous tumors, which may show low radiopharmaceutical uptake (however, they are well visualized on MRI and tend to calcify on CT).
Features of rectal cancer and surgical tactics
The mainstay of CRC treatment is surgical resection. For rectal cancer, the standard is total mesorectal excision (TME) — en bloc removal of the tumor along with the rectum and mesorectum.
A critically important parameter assessed by preoperative MRI is the circumferential resection margin (CRM), which runs along the mesorectal fascia. If the tumor is within 1 mm of the fascia, the margin is considered involved (high-risk group). Such patients require preoperative chemoradiotherapy to achieve downstaging (downstaging) and prevent local recurrence.
Another important prognostic factor is extramural venous invasion (EMVI), defined as direct tumor invasion into a blood vessel. On MRI, it is recognized as nodular abnormal dilation of perirectal veins and indicates a high risk of hematogenous metastasis.
Anal canal cancer, whose epicenter is located caudal to 2 cm above the dentate line, is considered separately. It is a rare squamous cell carcinoma, often associated with HPV, for which MRI is also the preferred imaging modality for staging.
Briefly: answers to topical questions
What are the histological types of colon cancer?
Histological types of malignant neoplasms of the colon include adenocarcinomas and lymphomas.
- Adenocarcinoma (adenocarcinoma) — may have a homogeneous or heterogeneous structure (in large tumors).
- Mucinous tumor (mucinous tumor) — a variant of adenocarcinoma with a prominent tendency toward calcification; residual mucin can cause pseudomyxoma peritonei.
- Lymphoma (lymphoma) — a rare type (less than 0.5%), typically represented by Burkitt lymphoma or MALT lymphoma.
What does bowel preparation for CT colonography entail?
Bowel preparation for CT colonography consists of optimal cleansing and oral contrast administration.
- Fecal tagging (faecal tagging) — the use of oral contrast to tag residual stool.
This is necessary to increase the sensitivity and specificity of the method. Tagged feces acquire high X-ray attenuation, allowing them to be reliably differentiated from true soft-tissue polyps.
What are the indications for preoperative chemoradiotherapy in rectal cancer?
Indications for preoperative chemoradiotherapy in rectal cancer include advanced disease stages and aggressive local growth.
- Advanced stages — therapy has been proven to reduce local recurrence rates.
- Mesorectal fascia involvement (mesorectal fascia involvement) — infiltration of the fascia makes it impossible to obtain clear resection margins during total mesorectal excision. In this case, neoadjuvant therapy is used for tumor downstaging (downstaging).
Class and exam questions
Why is conventional CT poorly suited for assessing lymph node involvement (N-stage)?
CT criteria for lymph node size lack sufficient specificity and sensitivity: according to pathomorphological data, more than 50% of metastatic nodes are smaller than 1 cm.
How do annular malignant lesions appear on CT colonography?
They manifest as luminal narrowing and the characteristic "shouldered ends" sign.
What is the circumferential resection margin (CRM) and why is it so important?
CRM is the plane of surgical dissection along the mesorectal fascia. If MRI shows the tumor is within 1 mm of the fascia, the margin is considered involved, requiring preoperative chemoradiotherapy to prevent recurrence.
Is colorectal cancer always well visualized on PET-CT?
As a rule, tumors show high 18F-FDG uptake. An exception is mucinous tumors, which may show low uptake or no uptake at all.
What else to study on this topic
- Criteria for extramural venous invasion (EMVI) on MRI
- Watchful waiting strategy in case of complete clinical response to radiation therapy
- Diagnostic imaging of anal canal cancer
- Application of faecal tagging in CT colonography
- Diffusion-weighted MRI (DWI) in lymph node staging
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