Colorectal cancer 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%.
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Epidemiology
Colorectal cancer 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 cancer. There is also a slight male predilection for rectal cancers, not found in tumors elsewhere in the colon.
Risk factors
A number of predisposing factors have been identified, including:
- low fiber and high fat and animal protein diet
- obesity (especially in men)
-
inflammatory bowel disease (IBD)
- ulcerative colitis
- Crohn disease (particularly in bypassed loops/in vicinity of chronic fistula)
- asbestos exposure
- a family history of benign/malignant colorectal tumors
- history of endometrial/breast cancer
- pelvic irradiation
- ureterosigmoidostomy
- colonic adenoma
- dysplasia of colon within flat mucosa
- prominent lymphoid follicular pattern
Associations
Syndromes
Recognized hereditary syndromes are seen in 6% of colorectal cancers. These include:
-
familial adenomatous polyposis syndrome (FAP)
- Gardner syndrome variant
- Turcot syndrome variant
- Peutz-Jeghers syndrome
- hereditary non-polyposis colon cancer syndrome (HNPCC)
- juvenile polyposis syndrome
- MUTYH-associated polyposis (MAP)
Clinical presentation
Clinical presentation is typically insidious:
- altered bowel habit (constipation and/or diarrhea)
- iron-deficiency anemia (chronic occult blood loss)
However initial manifestation may be acute:
Less common presentations include:
- that of metastatic disease (e.g. respiratory symptoms from lung metastases)
- paraneoplastic syndromes (e.g. dermatomyositis)
- bacteremia or bacterial endocarditis with Streptococcus bovis (Streptococcus gallolyticus) 6
In general:
- right-sided tumors are larger and present with a mass, distant disease or iron deficiency anemia
- left-sided tumors present earlier with altered bowel habit
Pathology
Colorectal cancers, 98% of which are adenocarcinomas, arise in the vast majority of cases from pre-existing colonic adenomas (neoplastic polyps), which progressively undergo a malignant transformation as they accumulate additional mutations 2 (so-called multi-hit hypothesis).
Morphologically cancers can be:
- sessile
- exophytic
- circumferential (apple core)
- ulcerated
- desmoplastic
Rarely the malignant cells will widely invade the submucosa, analogous to linitis plastica 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.
Specific subtypes
- mucinous carcinoma of colon
- primary colorectal small cell carcinoma: extremely rare
Location
Colorectal cancers can be found anywhere from the cecum to the rectum, in the following distribution 2,5:
- rectosigmoid: 55%
- cecum and ascending colon: ~20%
- ileocecal valve: 2%
- transverse colon: ~10%
- descending colon: ~5%
Genetics
Approximately 10% of colorectal cancers have a BRAF mutation, which is more common in females, right colon colorectal cancer, advanced stage at diagnosis, and a mucinous histology 7.
Staging
See: colon cancer staging.
Radiographic features
Fluoroscopy
Barium enema
- 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 fecal 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
CT is the modality most used for staging colorectal cancer, with an accuracy of only between 45-77% 4, able to assess nodes and metastases.
It is often able to diagnose tumors although it is insensitive to small masses. CT colonography is increasing in popularity as an alternative to colonoscopy.
Most colorectal cancers are of soft tissue density that narrow the bowel lumen 4. Ulceration in larger masses is also seen. Occasionally low-density masses with low-density lymph nodes are seen in mucinous adenocarcinoma, due to the majority of the tumor composed of extracellular mucin. Psammomatous calcifications in mucinous adenocarcinoma can also be present.
Complications may also be evident, e.g. fistulae, obstruction, intussusception, perforation 4.
MRI
MRI has a staging accuracy of 73% with a 40% sensitivity for lymph node metastases 1. MRI 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 the stage at operation the single most important factor affecting prognosis.
- Dukes A: 80-90%
- Dukes B: 70%
- Dukes C: 33%
- Dukes D: 5%
BRAF-mutated colorectal cancer has a poorer prognosis with a median survival of <12 months 7.
Recurrence is common:
- local recurrence at the site of anastomosis: tend to occur within two years of diagnosis (80%) 4
- distant metastatic recurrence
The tumor marker carcinoembryonic antigen (CEA) is routinely used for detecting postoperative early recurrence and metastatic disease (especially liver disease). It is also used for monitoring response to treatment of metastatic disease.
- as with most tumor markers, it is inappropriate for screening given its poor sensitivity and specificity
- higher levels of CEA are associated with:
- higher grade tumors
- higher stage disease
- visceral metastases (especially liver metastases)
Screening recommendations
Screening recommendations are contentious and vary widely from country to country. An example would be:
- for persons >50 years of age: an annual fecal occult blood test (often a fecal immunochemical test (FIT)) and sigmoidoscopy/barium enema every 3 to 5 years
- for first-degree relatives of patients with colon cancer: screening should start at age 40
Differential diagnosis
Diverticulosis
On CT colonography, the two most useful discriminators of colorectal carcinoma and diverticular disease are absence of diverticula within the structured segment, and shouldered edges, with both features having a high negative and positive predictive value for carcinoma 8. Other features pointing to carcinoma include a shorter segment length, destroyed mucosal folds, straightening of the segment, absence of thickened fascia, and more and larger locoregional nodes.
Other imaging differential considerations on CT include:
