Colorectal carcinoma (CRC) is the most common cancer of the gastrointestinal tract and the second most frequently diagnosed malignancy in adults. CT and MRI are the modalities most frequently used for staging. Surgical resection may be curative although five-year survival rate is 40-50%.
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 for rectal cancers, not found in tumours elsewhere in the colon.
A number of predisposing factors have been identified, including:
- low fibre and high fat and animal protein diet
- obesity (especially in men)
- inflammatory bowel disease (IBD)
- asbestos workers
- a 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. Positive blood cultures or bacterial endocarditis with Streptococcus bovis is strongly suggestive of underlying colorectal cancer 6.
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.
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).
Metastases may be widespread in advanced disease, although the liver is by far the most common site involved.
Colorectal cancers can be found anywhere from the caecum to the rectum, in the following distribution 2,5:
- recto-sigmoid: 55%
- caecum and ascending colon: ~20%
- ileocaecal valve: 2%
- transverse colon: ~10%
- descending colon: ~5%
See: colon cancer staging.
- 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 maybe 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 assess 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: tend to occur within two years of diagnosis (80%) 4
- distant metastatic recurrence
The tumour marker CEA is routinely used for detecting post operative early recurrence and metastatic disease (especially liver disease). It is also used for monitoring response to treatment of metastatic disease
- as with most tumour markers, it is inappropriate for screening given it poor sensitivity and specificity
- higher levels of CEA are associated with:
- higher grade tumours
- higher stage disease
- visceral metastases (especially liver metastases)
Screening recommendations are contentious and vary widely from country to country. An example would be:
- for persons >50 years of age: an annual faecal occult blood test and 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
- 5. Eisenberg RL. Gastrointestinal radiology. Lippincott Williams & Wilkins. (2003) ISBN:0781737060. Read it at Google Books - Find it at Amazon
- 6. Boleij A, Schaeps RM, Tjalsma H. Association between Streptococcus bovis and colon cancer. J. Clin. Microbiol. 2009;47 (2): 516. doi:10.1128/JCM.01755-08 - Free text at pubmed - Pubmed citation