## TNM Staging of Colorectal Cancer: T3 vs T4 Disease ### Understanding T Staging in Colorectal Cancer **Key Point:** T staging is based on depth of invasion through the bowel wall layers, NOT on nodal involvement or distant metastases. ### T3 Disease (Locally Advanced) - Tumour invades through the muscularis propria - Extends into the subserosa, pericolic fat, or mesorectal fat - Does NOT breach the visceral peritoneum - Does NOT invade adjacent organs - Represents the majority of "locally advanced" colorectal cancers ### T4 Disease (Locally Advanced with Organ Invasion) **T4a:** Tumour perforates the visceral peritoneum - Invades through the peritoneal surface - Exposes tumour to peritoneal cavity - Higher risk of peritoneal seeding **T4b:** Tumour directly invades adjacent structures - Bladder, prostate, uterus, abdominal wall, other bowel loops - Fistula formation (e.g., colovesical, colorectal fistula) - Requires en bloc resection of invaded organ ### Key Discriminator: T3 vs T4 | Feature | T3 | T4 | |---------|----|----|| | **Muscularis propria** | Invaded | Invaded | | **Serosa/Peritoneum** | Not breached | T4a: breached; T4b: N/A | | **Adjacent organ invasion** | No | Yes (T4b) | | **Fistula formation** | No | Yes (T4b) | | **Surgical approach** | Segmental resection | En bloc resection of invaded organ | **High-Yield:** The presence of **fistula formation** (colovesical, colorectal, colovaginal) is pathognomonic for **T4b disease** and mandates en bloc resection of the invaded organ. **Clinical Pearl:** A patient with rectosigmoid cancer presenting with pneumaturia or faecaluria has a colovesical fistula—this is T4b disease and requires anterior resection + partial cystectomy. **Mnemonic:** **FIST** = **F**istula = **I**nvasion of **S**tructures = **T4b** [cite:AJCC Cancer Staging Manual 8e; Sabiston Textbook of Surgery Ch 52]
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