From the Guidelines
Tubular adenomas and tubulovillous adenomas are considered benign growths, but they have the potential to transform into colorectal cancer over time if left untreated. These adenomas are non-cancerous lesions that develop from the glandular epithelial cells lining the colon. Tubular adenomas are characterized by tubular glands, while tubulovillous adenomas contain a mixture of tubular and villous components (finger-like projections) 1. Although these polyps are benign, the risk of malignant transformation increases with the size of the adenoma (particularly those larger than 1 cm), the presence of villous components (tubulovillous and villous adenomas have higher malignant potential than purely tubular ones), and the degree of dysplasia.
Key Factors Influencing Malignant Transformation
- Size of the adenoma: larger adenomas (≥10 mm) have a higher risk of malignant transformation 1
- Presence of villous components: tubulovillous and villous adenomas have a higher malignant potential than purely tubular ones 1
- Degree of dysplasia: high-grade dysplasia increases the risk of malignant transformation 1
Surveillance Recommendations
- The US Multi-Society Task Force recommends surveillance colonoscopy at 3-year intervals for patients with adenomas ≥10 mm, adenomas with tubulovillous or villous histology, or adenomas with high-grade dysplasia 1
- The British Society of Gastroenterology recommends surveillance colonoscopy at 3-year intervals for patients with high-risk findings on index colonoscopy, and suggests that people with premalignant polyps but no high-risk findings on index colonoscopy should not undergo colonoscopic surveillance, but should be strongly encouraged to participate in their national bowel screening programme when invited 1
Importance of Complete Removal and Follow-up
- Complete removal of adenomas during colonoscopy is crucial to prevent malignant transformation 1
- Appropriate follow-up surveillance based on the number, size, and histology of the polyps found is essential to detect any potential malignant transformation early 1
From the Research
Benignity of Tubular Adenomas and Tubulovillous Adenomas
- Tubular adenomas are generally considered benign, but they can show molecular features characteristic of tubulovillous adenomas, which have a higher malignant potential 2.
- The presence of minor villous changes in tubular adenomas can correlate with advanced molecular features, such as KRAS mutation, MGMT loss, and p53 overexpression 2.
- Tubulovillous adenomas have a higher incidence of invasive adenocarcinoma, especially in patients with multiple adenomas, family history of colorectal cancer, and adenomas greater than 4 cm in diameter 3.
- Preoperative biopsy examination has no role in diagnosing malignant transformation in tubulovillous adenomas, and recurrence can occur frequently after standard local resection 4.
Molecular Features and Malignant Potential
- The Wnt pathway gene expression profile can differentiate between tubular and villous adenomas, with successive increases in the expression of ctnnb1 (beta-catenin) from normal to tubular adenomas to villous adenomas 5.
- Genes involved in canonical and non-canonical Wnt signaling show increased expression in villous adenomas, suggesting changes in Wnt signaling regulation during the progression from normal mucosa to tubular adenomas to villous adenomas 5.
- The expression of Myc in adenoma stroma suggests a dynamic signaling network within adenomas between mucosal and stromal elements 5.
Clinical Implications
- The clinical impression on examination is more accurate than biopsy in assessing the presence of malignancy in tubulovillous adenomas 3.
- Recurrence after local treatment of both benign and malignant tumors is high, and is significantly related to the size of the adenoma, location, and type of treatment 3.
- The overall 5-year actuarial survival rate for patients with tubulovillous adenomas is 85%, but the recurrence rate can be as high as 26.7% after local resection and 6.7% after segmental colectomy 4, 3.