A 58-year-old man from rural India presents with a 6-month history of progressive abdominal pain, alternating constipation and diarrhoea, and weight loss of 8 kg. On examination, a palpable mass is felt in the left lower abdomen. Colonoscopy reveals a stricturing lesion at the sigmoid colon with shouldering at the margins. Biopsy shows infiltrating adenocarcinoma with marked desmoplastic stromal reaction. CT abdomen shows local invasion into the bladder wall. What is the most likely TNM stage of this tumour?

Explanation

## Clinical Analysis **Key Point:** TNM staging in colorectal cancer requires assessment of depth of invasion (T), lymph node involvement (N), and distant metastases (M). ### T-Stage Determination The tumour has invaded the bladder wall, which is a structure beyond the muscularis propria. This defines **T4a** (tumour penetrates the visceral peritoneum or invades adjacent structures). The desmoplastic stromal reaction and stricturing pattern are classic features of invasive adenocarcinoma. ### N and M Status - **N0:** No mention of regional lymph node involvement on imaging or pathology - **M0:** No distant metastases documented (local invasion to bladder is T4a, not M1) ### Stage Classification T4a + N0 + M0 = **Stage IVA** (AJCC 8th edition) **High-Yield:** Stage IVA indicates locally advanced disease with invasion of adjacent organs but WITHOUT distant metastases. This carries a 5-year survival of ~10–15%. ### Clinical Pearl The palpable left lower abdominal mass, stricturing lesion with shouldering (classic "napkin-ring" appearance on imaging), and desmoplastic reaction are hallmarks of advanced colorectal adenocarcinoma with poor prognosis. Invasion into bladder makes this T4a regardless of lymph node status. **Mnemonic: T4 Invasion** — **T**umour breaches **T**he **T**wo layers (visceral peritoneum or adjacent organ invasion). [cite:AJCC Cancer Staging Manual 8e]