Epidemiology, classification, and malignant transformation risk
Polyps are rare before the age of 30, but after 50 they are found in approximately 30% of people (more frequently in men). About half of the lesions are located in the rectosigmoid region, although the proportion of right-sided adenomas increases in elderly patients. Having a first-degree relative with colorectal cancer (CRC) increases the risk of developing neoplasms by 50%.
Based on morphology, polyps are classified as epithelial, hamartomatous, inflammatory, and nonepithelial. Adenomas (epithelial lesions) pose the greatest threat because they contain dysplasia by definition. "Advanced" adenomas carry the maximum risk of progression to cancer. Their criteria include:
- Size greater than 1 cm in diameter.
- Presence of high-grade dysplasia.
- More than 25% villous architecture in the tissue.
The risk of invasion heavily depends on the morphology of the lesion according to the Paris Classification (Paris Classification). Protruding polyps are divided into pedunculated (Ip), subpedunculated (Isp), and sessile (Is) lesions. Superficial flat lesions are divided into elevated (IIa) and depressed (IIc) lesions. Specifically, flat depressed lesions (0-IIc) carry the highest risk of malignant transformation regardless of size, while their height above the mucosa is less than 3 mm, which hinders visualization.
Particular attention is paid today to the "serrated" pathway of carcinogenesis. Sessile serrated adenomas, previously considered harmless hyperplastic polyps, exhibit microsatellite instability and gene hypermethylation. They are responsible for nearly 35% of CRC cases (predominantly in the right colon).
Hereditary polyposis syndromes
Polyps can be a manifestation of genetic disorders with an autosomal dominant inheritance pattern.
Familial adenomatous polyposis (FAP) is caused by a mutation in the APC tumor suppressor gene. Patients develop hundreds of adenomas that inevitably transform into cancer, necessitating prophylactic proctocolectomy. The syndrome is often accompanied by duodenal adenomas, osteomas, cysts, and desmoids tumors. Desmoid tumors are benign yet locally invasive masses of the mesentery or abdominal wall (the variant with numerous such tumors is referred to as Gardner syndrome).
Lynch syndrome (hereditary nonpolyposis colorectal cancer, HNPCC) is caused by mutations in DNA mismatch repair genes. Cancer develops early (at an average age of 45) and is frequently right-sided. Patients also have a high risk of endometrial and small bowel cancer.
Other conditions include Peutz-Jeghers syndrome (large intestinal hamartomas prone to bleeding and mucocutaneous pigmentation) and rare hamartomatous polyposes (Cowden and Cronkhite-Canada syndromes).
Diagnostic radiology
Today, CT colonography (virtual endoscopy) is considered the most accurate radiological method for detecting neoplasms. The physician evaluates the colon using a primary 2D approach or a 3D endoluminal fly-through. Double-contrast barium enema is considered an outdated method.
The primary task of the radiologist is to differentiate a polyp from stool residue. To achieve this, a fecal tagging technique with oral contrast is used, and the findings are assessed in 2D and 3D.
Signs that help differentiate a polyp from stool:
- Structure: A true polyp has a homogeneous soft-tissue attenuation. Stool has a variable radiodensity due to trapped internal gas (in 2D mode, a gas bubble is visible inside).
- Mobility: Stool shifts during scanning in different positions (supine and prone). A polyp remains in place (however, it is important to account for the natural mobility of the colonic mesentery using haustra and diverticula as reference points).
Flat lesions appear on 2D scans as slight wall thickening, and on 3D as contour irregularities. Computer-aided detection (CAD) systems are actively used as a "second reader" to search for them. An alternative method is magnetic resonance colonography using "dark lumen" pulse sequences, where polyps are visualized as contrast-enhancing protrusions.
Briefly: answers to topical questions
What extraintestinal manifestations are characteristic of familial adenomatous polyposis?
Familial adenomatous polyposis is characterized by lesions of the upper gastrointestinal tract and a number of other extraintestinal conditions. These include:
- Gastric adenomas and hamartomas—including fundic gland polyps.
- Duodenal adenomas—found in nearly 100% of patients, clustering around the ampulla of Vater.
- Multiple osteomas—affecting the skull and mandible.
- Epidermoid cysts—frequently located on the face.
- Congenital hypertrophy of the retinal pigment epithelium (CHRPE).
- Dental anomalies—various dental defects.
- Desmoid tumors (Desmoid disease)—benign fibromatous tumors with locally invasive growth.
What are the CT signs of malignant transformation in a colonic polyp?
Signs of malignancy in a colorectal neoplasm on conventional computed tomography include changes in the bowel wall and surrounding tissues. The following CT signs of malignancy are distinguished:
- Focal colonic wall thickening—exceeding 3 mm.
- Tumor spread—invasion into the pericolic adipose tissue.
- Lymphadenopathy—enlargement of regional lymph nodes.
The tumor structure on CT is typically homogeneous, although a heterogeneous structure is found in large adenocarcinomas, mucinous tumors, or when an abscess forms. Mucinous tumors have a marked tendency toward calcification.
Specifically which genes mutate in Lynch syndrome?
Lynch syndrome involves mutations in the DNA mismatch repair system genes (mismatch repair deficiency, MMR). Key target genes whose defects cause this hereditary cancer syndrome include:
- MLH1
- MSH2
- MSH6
- PMS1
- PMS2
- EPCAM (or EpCAM)
Mutations in these genes lead to a high level of microsatellite instability (MSI-H). Specifically, urinary tract cancer in this syndrome more frequently develops with a mutation in the MSH2 gene.
Which layers of the rectal wall are normally visualized on endorectal ultrasound?
Endorectal ultrasound normally visualizes the layered architecture of the rectal wall. From superficial to deep layers, the following structures are identified:
- Echogenic superficial mucosa (echogenic superficial mucosa).
- Hypoechoic deep mucosa (hypoechoic deep mucosa).
- Echogenic submucosa (echogenic submucosa).
- Hypoechoic muscularis propria (hypoechoic muscularis propria).
Assessment of these layers is of fundamental clinical importance for determining the depth of tumor invasion. For example, at the T1 stage, the tumor is confined to the submucosa and does not reach the muscularis propria.
What does complete patient preparation for CT colonography entail?
Patient preparation for CT colonography includes bowel cleansing, content tagging, and pharmacologic relaxation of the colon. The main stages of preparation are:
- Fecal tagging (faecal tagging)—oral administration of contrast to tag residual contents, which improves the sensitivity of the method.
- Bowel cleansing—optimal preparation is traditionally required, though the efficacy of noncathartic preparation (noncathartic CT colonography) without aggressive laxatives is being investigated.
- Spasmolysis—intravenous administration of hyoscine butylbromide (hyoscine butylbromide) to improve colonic distension.
- Colonic distension (colonic distension)—insufflation of carbon dioxide (CO2) using automated insufflators.
What malignant tumors of other organs are associated with Peutz-Jeghers syndrome?
Peutz-Jeghers syndrome is associated with an increased risk of gastrointestinal malignancies and a number of extraintestinal locations. Associated tumors of other organs include:
- Pancreatic cancer
- Ovarian cancer
- Cervical cancer
- Thyroid cancer
- Testicular cancer
- Breast cancer
Hamartomatous polyps are not precancerous per se, but patients with Peutz-Jeghers syndrome have an increased risk of gastrointestinal adenocarcinoma.
The clinical criteria for Lynch syndrome can be remembered using the "3-2-1" rule: 3 affected relatives (one of whom is a first-degree relative of the other two), 2 successive generations, 1 case diagnosed under the age of 50.
Class and exam questions
How to determine by ultrasound that rectal cancer is at an early stage (T1)?
During endorectal US, the bowel wall has a layered structure. At the T1 stage, the tumor process is limited to the echogenic submucosa and does not involve the deeper hypoechoic muscularis propria.
Why do pedunculated polyps have a more favorable prognosis after removal?
The stalk of the polyp creates a physical distance between the epithelium, where dysplasia arises, and the bowel wall. If the cancer focus does not extend beyond the resection margin, such a neoplasm is considered completely cured.
What is the difficulty in diagnosing flat lesions during CT colonography?
Their height above the mucosa is less than 3 mm. On 2D images, they appear only as a slight wall thickening, and on 3D as small protrusions. The settling of iodine-containing contrast on the surface of the lesion helps.
What else to study on this topic
- Endorectal ultrasound of the rectum: detailed layered architecture of the wall
- Genetic and molecular basis of the "serrated" pathway of carcinogenesis
- Evaluation of pedunculated polyps by Haggitt invasion levels
- Extraintestinal manifestations of FAP and MRI diagnosis of desmoid tumors
- Differential diagnosis of nonepithelial polyps (lipomas, carcinoids, GIST)
- Rare hamartomatous polyposis syndromes (Turcot, Cowden, Cronkhite-Canada syndromes)
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