What is the classification of a colorectal tumor that invades through the muscularis propria into the subserosa, specifically a T1 (tumor invading the submucosa), T2 (tumor invading the muscularis propria), T3 (tumor invading through the muscularis propria into the subserosa), or T4 (tumor invading the visceral peritoneum or other organs) lesion?

From the Guidelines

A colorectal tumor that invades through the muscularis propria into the subserosa is classified as a T3 lesion. According to the UICC TNM staging (8th edition) classification for colon and rectal cancer, as described in the study by 1, the T staging specifically describes the depth of tumor invasion through the layers of the bowel wall. The key points to consider are:

• T1 lesions are confined to the submucosa
• T2 lesions invade into but not through the muscularis propria
• T3 lesions extend through the muscularis propria into the subserosa or into non-peritonealized pericolic or perirectal tissues
• T4 lesions directly invade other organs or structures and/or perforate the visceral peritoneum The distinction between these stages is important for treatment planning and prognosis, with deeper invasion generally associated with higher risk of lymph node involvement and distant metastasis, as noted in the study by 1. Therefore, based on the provided evidence, the correct answer is (c) T3 lesion. Some key points from the study by 1 include the use of endoscopic rectal ultrasound (ERUS) and pelvic magnetic resonance imaging (MRI) for defining treatment and locoregional clinical staging, but the primary classification of the tumor is based on the TNM staging system. The study by 1 provides a comprehensive overview of the diagnosis, treatment, and follow-up of rectal cancer, and the TNM staging system is a crucial component of this process.

From the Research

Colorectal Tumor Staging

• The TNM classification system is used to determine the prognosis of cancer patients, with the T category reflecting the depth of tumor invasion 2.
• A tumor that invades through the muscularis propria into the subserosa is classified as T3 2.
• The T3 category can be further subdivided into T3a (≤1 mm), T3b (>1−15 mm), and T3c (>15 mm) based on the depth of invasion beyond the outer border of the muscularis propria 2.
• T2 lesions are defined as tumors that invade the muscularis propria, but do not extend into the subserosa 3, 4.
• T1 lesions are defined as tumors that invade the submucosa, but do not extend into the muscularis propria.
• T4 lesions are defined as tumors that invade other organs or structures.

Depth of Invasion and Prognosis

• The depth of invasion beyond the outer border of the muscularis propria into the subserosa or into nonperitonealized pericolic tissue is an important prognostic factor in colorectal cancer 2.
• The 5-year distant metastasis and disease-free survival rates decrease significantly with increasing depth of invasion 2.
• Tumors extending to the outer muscularis propria (pT2b) are associated with older patient age, larger tumor size, higher likelihood of lymphovascular invasion, and higher lymph node stage compared to tumors limited to the inner muscle layer (pT2a) 3.

Answer to the Question

• Based on the evidence, a colorectal tumor that invades through the muscularis propria into the subserosa is a T3 lesion 2.