Describe Colorectal Carcinoma in detail.

The Concept & the Adenoma-carcinoma Sequence

Colorectal carcinoma is a common adenocarcinoma of the colon and rectum, and its defining biological principle is that it develops through the adenoma-carcinoma sequence: a normal mucosa acquires mutations (classically APC, then KRAS, then p53) and progresses normal epithelium → adenomatous polyp → dysplasia → invasive carcinoma over years. This slow, stepwise evolution through a detectable, removable precursor (the polyp) is exactly what makes screening and polypectomy so effective at preventing the cancer.

Risk Factors

Risk rises with age (>50), a diet low in fibre and high in red/processed meat, obesity and smoking. Important high-risk groups are those with inflammatory bowel disease (especially ulcerative colitis), a family history, and the hereditary syndromes — familial adenomatous polyposis (FAP, APC gene) and hereditary non-polyposis colorectal cancer (HNPCC/Lynch syndrome).

Clinical Features — Site Determines the Picture

The presentation depends on where the tumour is, because the calibre and contents of the bowel differ:

Investigation & Staging

Colonoscopy with biopsy is the diagnostic investigation (CT colonography is an alternative). Staging then guides treatment: CT of chest, abdomen and pelvis for distant disease, and MRI of the pelvis for rectal tumours (to assess the circumferential margin and plan therapy). Carcinoembryonic antigen (CEA) is not diagnostic but is a useful baseline and follow-up marker of recurrence. Staging uses Dukes' (A confined to the wall, B through the wall, C nodal spread, D distant metastases) and the TNM system.

Spread

Spread is direct (into adjacent structures), lymphatic (to regional nodes), blood-borne via the portal vein to the liver (the commonest site of metastasis), and transcoelomic across the peritoneum.

Management

• Surgery is the mainstay of cure — resection of the tumour-bearing segment with its regional lymph nodes (right or left hemicolectomy, sigmoid colectomy, high anterior resection). Low rectal cancers may need an abdominoperineal resection with a permanent colostomy, whereas higher rectal tumours allow a sphincter-preserving anterior resection with total mesorectal excision (TME).
• Chemotherapy is added as adjuvant therapy for node-positive (Dukes' C) disease.
• Radiotherapy (usually neoadjuvant) is used for rectal cancer to reduce local recurrence.
• Screening (faecal immunochemical testing and colonoscopy) detects early cancers and removes precursor polyps.

Hereditary Syndromes in More Depth

The two hereditary syndromes are worth expanding because they explain many young colorectal cancers. FAP (APC gene) produces thousands of polyps and near-certain cancer, managed by prophylactic colectomy. HNPCC / Lynch syndrome arises from DNA mismatch-repair gene defects, produces relatively few polyps but a high lifetime risk of colorectal (typically right-sided) and other cancers (endometrial, ovarian, gastric). Recognising a suggestive family history triggers genetic assessment and intensive surveillance.

Principles of Cancer Surgery

Curative surgery follows oncological principles: the tumour-bearing segment is removed with an adequate margin together with its draining lymph nodes along the supplying vessels, which are ligated at their origin. For rectal cancer, total mesorectal excision (TME) — removing the rectum within its intact mesorectal envelope — is the key advance that dramatically reduced local recurrence. Whether the sphincter can be preserved (anterior resection) or must be sacrificed (abdominoperineal resection with a permanent colostomy) depends on how low the tumour lies.

Screening & Prognosis

Because the disease evolves slowly through a removable polyp, screening is highly effective — faecal immunochemical testing (fit) of the population and colonoscopy in higher-risk groups detect early cancers and remove precursor adenomas. Prognosis correlates strongly with stage at diagnosis (Dukes'/TNM), which is why earlier detection saves lives. Even patients with resectable liver metastases may be cured, so metastatic disease is assessed for surgery rather than assumed incurable.

• Diagnose by colonoscopy + biopsy; stage with CT (cap) + MRI pelvis for rectal cancer; CEA for follow-up.
• Dukes': A (in wall), B (through wall), C (nodes), D (distant metastases); liver is the commonest metastatic site.
• Adjuvant chemotherapy for node-positive disease; neoadjuvant radiotherapy for rectal cancer.

The adenoma-carcinoma sequence is what makes screening effective.

• Adenocarcinoma arising via the adenoma-carcinoma sequence (APC→KRAS→p53) — the basis for screening/polypectomy.
• Risk: age, low-fibre/red-meat diet, IBD, FAP and HNPCC/Lynch, family history.
• Right colon → anaemia/mass (late); left colon → obstruction/change in bowel habit; rectum → bleeding, tenesmus.
• Colonoscopy + biopsy diagnose; stage with CT ± MRI pelvis (rectal); CEA for follow-up; Dukes'/TNM; liver commonest metastasis.
• Surgery = segmental resection + nodes (± APR/TME for rectum); adjuvant chemo for node-positive; neoadjuvant radiotherapy for rectal cancer.