Colorectal carcinoma (CRC) is the most common cancer of the gastrointestinal tract and the second most frequently diagnosed malignancy in adults.
Colorectal carcinoma is common, accounting for 15% of all newly diagnosed cancers, and tends to be a disease of the elderly, with the median age of diagnosis between 60 and 80 years of age 2, slightly younger for rectal carcinoma. There is also a slight male predilection in rectal cancers, not found for tumours elsewhere in the colon.
A number of predisposing factors have been identified, including:
- low fibre and high fat + animal protein diet
- obesity (especially in men)
- inflammatory bowel disease (IBD)
- asbestos workers
- family history of benign / malignant colorectal tumours
- history of endometrial / breast cancer
- pelvic irradiation
- colonic adenoma
- dysplasia of colon within flat mucosa
- prominent lymphoid follicular pattern
Recognised hereditary syndromes are seen in 6% of colorectal carcinomas. These include :
- familial adenomatous polyposis syndrome (FAP)
- Peutz-Jeghers syndrome
- hereditary non-polyposis colon cancer syndrome (HNPCC)
Clinical presentation is typically insidious, with altered bowel habit or iron deficiency anaemia from chronic occult blood loss. Bowel obstruction, intussusception, heavy bleeding and metastatic disease may also be the initial manifestation.
Colorectal cancers, 98% of which are adenocarcinomas, arise in the vast majority of cases from pre-existing colonic adenomas (neoplastic polyps), which progressively undergo malignant transformation as they accumulate additional mutations 2 (so-called multi-hit hypothesis).
Morphologically cancers can be :
- circumferential (apple core)
Rarely the malignant cells will widely invade the submucosa, analogous to linitis plastic of the stomach. These are typically scirrhous adenocarcinomas (signet-ring type).
In general, right sided tumours are larger and present with a mass, distant disease or iron deficiency anaemia, whereas left sided tumours present earlier with altered bowel habit.
Metastases may be widespread in advanced disease, although the liver is by far the most common site involved.
Staging : see Colon cancer staging
Colorectal cancers can be found anywhere from the caecum to the rectum, in the following distribution 2:
- recto-sigmoid : 55%
- caecum and ascending colon : 22%
- transverse colon : 11%
- descending colon : 6%
- sensitivities for polyps >1 cm:
- single contrast : 77-94%
- double contrast : 82-98%
- polyps < 1 cm : < 50% detection 3
Appearances will reflect macroscopic appearance, with lesions seen as filling defects. These need to be differentiated from residual faecal matter. Typically they appear as exophytic or sessile masses, or may be circumferential (apple core sign). Fistulas to bladder, vagina or bowel may also be demonstrated.
Rarely the stenotic segment will be long particularly with scirrhous adenocarcinomas.
CT is the modality most used for staging colorectal carcinoma, with an accuracy of only between 45-77% 4, able to asses nodes and metastases.
It is often able to diagnose tumours although it is insensitive to small masses. CT colonography is increasing in popularity as an alternative to colonoscopy.
Most colorectal carcinomas are of soft tissue density that narrow the bowel lumen 4. Ulceration in larger mass is also seen. Occasionally low-density masses with low-density lymph nodes are seen in mucinous adenocarcinoma, due to the majority of the tumour composed of extracellular mucin. Psammomatous calcifications in mucinous adenocarcinoma can also be present.
Has a staging accuracy of 73% with a 40% sensitivity for lymph node metastases 1. MR is having an increasing role to play in the staging of rectal cancer.
Treatment and prognosis
Treatment involves local control with resection in almost all cases. Adjuvant chemotherapy is reserved for stage III disease.
Overall 5 year survival rate is 40-50%, with stage at operation the single most important factor affecting prognosis.
- Duke A : 80-90%
- Duke B : 70%
- Duke C : 33%
- Duke D : 5%
Recurrence in common:
- local recurrence at line of anastomosis: and tend to occur within 2 years of diagnosis (80%) 4
- distant metastatic recurrence
Screening recommendations are contentious and vary widely from country to country. An example would be:
- for persons > 50 years of age : annual faecal occult-blood test + sigmoidoscopy / Barium enema every 3 to 5 years
- for first-degree relatives of patients with colon cancer screening should start at age 40
General imaging differential considerations on CT include
- 1. Dähnert W. Radiology review manual. Lippincott Williams & Wilkins. (2003) ISBN:0781738954. Read it at Google Books - Find it at Amazon
- 2. Kumar V, Abbas AK, Fausto N et-al. Robbins and Cotran pathologic basis of disease. W B Saunders Co. (2005) ISBN:0721601871. Read it at Google Books - Find it at Amazon
- 3. McPhee SJ, Tierney LM, Papadakis MA. Current medical diagnosis and treatment. McGraw-Hill Professional. (2007) ISBN:0071472479. Read it at Google Books - Find it at Amazon
- 4. Horton KM, Abrams RA, Fishman EK. Spiral CT of colon cancer: imaging features and role in management. Radiographics. 20 (2): 419-30. Radiographics (full text) - Pubmed citation
Synonyms & Alternative Spellings
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