Rectal cancer

Assessment of local spread and resection margins

The basis of successful surgical treatment is the precise determination of the T stage, which directly depends on the size of the neoplasm and the degree of its penetration into surrounding tissues. On T2-weighted images (T2WI), the healthy muscular layer (muscularis propria) is visualized as a structure with a consistently low MR signal. Upon reaching the T3 stage, the neoplastic tissue, which produces an intermediate-intensity signal, breaches this dark contour and extends beyond the muscular layer.

A critically important prognostic parameter is the circumferential resection margin (CRM). If the malignant masses are located at a safe distance from the CRM, the patient's prognosis remains favorable. However, in situations where the cancer tissue abuts this boundary (forming a positive margin), the risk of local recurrence in the postoperative period increases manifold.

In the most aggressive forms of the disease, the tumor is capable of infiltrating the mesorectal fascia and extending into the lateral space. Under such conditions, achieving clean surgical margins during total mesorectal excision is technically impossible. To reduce the tumor volume and downstage the disease (downstaging), such patients are invariably prescribed neoadjuvant therapy.

Sphincter involvement and lymph drainage

Neoplasms localized in the lower ampullary segment of the rectum are characterized by extension to the level of the anorectal junction. From there, the process can penetrate deeply into the intersphincteric plane (intersphincteric plane). Even with minimal dimensions of the primary lesion, involvement of the sphincter complex dictates the necessity of chemoradiotherapy. If tumor regression is not achieved at this stage, radical treatment will require an levator-extirpation surgical approach or an disabling abdominoperineal excision (abdominoperineal excision).

The pathway of lymphatic metastasis (N stage) strictly depends on the location of the primary focus relative to the dentate line (dentate line):

• Lesions growing below the dentate line drain lymph predominantly to the inguinal nodes (inguinal nodes).
• Tumors located above the dentate line drain into perirectal (perirectal) collectors and nodes along the internal iliac vessels (internal iliac).

Lymph node staging remains an extremely challenging task for diagnostic radiology. The golden rule: one cannot rely solely on node size. Morphological characteristics play a leading role. For example, in mucinous forms of cancer, both the primary lesion itself and the metastatic lymph nodes will demonstrate a characteristic high signal on T2WI.

Treatment response assessment (MR-TRG) and vascular invasion

Following the completion of neoadjuvant treatment, MRI is used to search for residual tumor masses. Treatment efficacy is classified according to a special radiological regression grade scale (MR-TRG):

1. TRG 1 (Complete response): Complete absence of signs of tumor signal, with only dense fibrosis being determined.
2. TRG 2 (Good response): A dense fibrous scar is formed, within which only minimal islands of viable tumor remain.
3. TRG 3 (Moderate response): A mixed picture where fibrous tissue or mucinous degeneration predominates over the intermediate signal of residual cancer.
4. TRG 4 (Poor response): Partial regression is noted, areas of fibrosis are present, but residual tumor tissue clearly predominates.
5. TRG 5 (No response): Absence of visible reaction to treatment; the radiological picture is completely identical to the baseline (baseline).

This classification helps clinicians select candidates for conservative management or salvage surgery (salvage surgery).

Another crucial marker of aggressiveness is extramural venous invasion. On T2WI, it appears as marked dilatation and pathological tortuosity of the draining vein (e.g., the middle rectal vein), the lumen of which is filled with soft-tissue masses.

Differential diagnosis and associated pathology

During a comprehensive bowel assessment, it is necessary to account for the presence of distant metastases (M stage), for which conventional CT serves as the baseline method, while FDG-PET/CT is the optimal tool for detecting occult dissemination. CT colonography, when performed correctly, approaches endoscopy in sensitivity, detecting cancer and large polyps. Ultrasound (US) can also play a certain, albeit auxiliary, role in staging.

Other neoplastic processes may be encountered on imaging:

• Appendix tumors are found in approximately 1% of appendectomies. Most often these are a carcinoid (usually small in size, becoming an incidental finding, and appendectomy is sufficient treatment) or adenocarcinoma.
• Appendiceal mucocele is associated with a high risk of malignancy (histology confirms cystadenocarcinoma in a third of cases). Due to the frequent association of mucocele with synchronous large bowel tumors, colonoscopy is strongly recommended for such patients.
• Mucinous adenocarcinomas are dangerous because residual mucin can lead to a severe complication: pseudomyxoma peritonei or retroperitoneal pseudomyxoma.
• Colonic lymphoma is an extremely rare pathology (less than 0.5% of tumors in this zone). Unlike carcinoma, it causes pronounced wall thickening and aneurysmal lumen dilation without signs of obstruction. Early lesions tend to ulcerate, which threatens perforation or intussusception.
• Secondary (metastatic) tumors reach the intestine via various pathways (including hematogenous and lymphatic). They cause a desmoplastic reaction of the serosa: the bowel wall contracts, forming a mass effect and characteristic spiculated folds (spiculated folds).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Criteria for intersphincteric space involvement and indications for abdominoperineal resection
• Specific MRI features of extramural venous invasion
• Differential diagnosis algorithm for mucinous lymphadenopathy
• Appendiceal mucocele and the risk of pseudomyxoma peritonei
• The role of FDG PET-CT in detecting occult colorectal cancer metastases

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.