A detailed medical history with special attention to family history of colon cancer, adenomatous polyps, or inflammatory bowel disease are important factors to identifying candidates for screening. Clinical features, physical examination, laboratory and radiological tests are essential tools for a diagnosis of colon cancer.
Diagnosis of Colon Cancer
- Abdominal examination may reveal a mass.
- Per rectal examination may show bright red blood in left-sided colonic cancers or black tarry stool (melena) in right-sided colon cancers. Digital rectal examination helps in identifying rectal cancers and also the extent of cancer growth in rectum.
- Metastatic colon cancer may be associated with lymph node enlargement, liver enlargement, or jaundice.
- Blood tests may reveal iron-deficiency anemia, electrolyte imbalance or elevated levels of liver enzymes associated with colon cancer and related complications.
- A fecal occult blood test (FOBT) may be done to detect any minute blood loss in feces. These findings are of supportive value in diagnosis of colon cancer.
- Carcinoembryonic antigen (CEA) levels may be elevated in colon cancer patients. This is usually used in evaluation of recurrence in post-operative colon cancer patients. CA19-9 and CA 242 are other biomarkers that are also useful.
Scans and Scopes
- Colonoscopy is the gold standard procedure for detecting and confirming colon cancer. Colonoscopy allows direct visualization of the lesions and gives provision to remove polyps or to take biopsy samples.
- Flexible sigmoidoscopy and double contrast barium enema are other helpful investigations in diagnosis of colon cancer.
- After confirming the disease, if metastasis is suspected, pan-body computed tomography (CT) scan is done for more accurate evaluation and staging of metastatic colon cancer.
Colon Cancer Screening
Reasons for Screening
Screening is an effective tool in identifying colon cancer in the early stages or even in precancerous phases, and patients at high-risk for developing colon cancer. Patients in with intestinal polyps or other precancerous conditions and those with family history of colon cancer or intestinal polyps should undergo regular screening for colon cancer. The common screening approaches include :
- digital rectal examination (DRE)
- fecal occult blood testing
- flexible sigmoidoscopy
- double contrast barium enema
Screening Methods and Tests
A fecal occult blood test (FOBT) helps detect minute traces of blood lost in the stool. This is a simple non-invasive test that can help in indentifying individuals who may require colonoscopic evaluation for colon cancer.
Colonoscopy is the most effective screening approach currently available. The main disadvantage of colonoscopy as a screening tool is the invasive nature of the procedure and the potential for serious side effects like bowel perforation. Virtual colonoscopy with CT-colonography or MRI-colonography is a viable non-invasive alternative option for screening but the approach is not as yet standardized.
Flexible sigmoidoscopy is quick and easy procedure compared to a colonoscopy. It allows direct visualization of the sigmoid colon and descending colon to the splenic flexure, in addition to the rectum. For best results flexible sigmoidoscopy is coupled with double contrast barium enema.
Barium enema permits visualization of the entire colon and can help detection of polyps and masses in colon by the use of radiocontrast dye that highlights the colon on radiographic imaging.
Genetic testing may be done by biopsying suspicious masses, extracting nucleic acids from cells in the stool or in the blood to conduct a number of tests. The material is examined for signs of genetic alteration or known markers predisposing a person to colon cancer. This may include :
- APC protein truncating test
- MSI test for mitations in hMLH1 and hMLH2 genes
Who should undergo screening?
- Screening is highly recommended for patients and individuals with family history of hereditary forms of colon cancer.
- Colonoscopy starting at the age of 20-25 years and repeating it every 1-3 years is recommended for individuals with HNPCC family history.
- Familial adenomatous polyposis (FAP), including a family history for Turcot’s and Gardner’s syndrome warrants flexible sigmoidoscopy screening starting at the age of 10-12 years and repeated every 1-2 years until the age of 35 and repeated every 3 years thereafter.
- Screening in children with Juvenile polyposis family history should begin by 12 years of age with colonoscopy along with multiple random biopsies every 3-5 years.
- Colonoscopic screening in Peutz-Jegher’s syndrome is done every 2 years.
Staging of Colon Cancer
Colon cancer staging system that is commonly used now is the TNM classification of AJCC. Previously, Duke’s method was commonly used.
Duke’s classification was originally described in 1930 by a Scottish pathologist Cuthbert Dukes in rectal cancer but was commonly applied to cancers of the colon and rectum. The tumor is classified into Stage A, B, C, and D.
- Stage A indicates that the tumor is restricted to bowel wall.
- Stage B tumors penetrate through bowel wall, but not spread beyond bowel walls.
- Stage C tumors show spread to the regional lymph nodes.
- Stage D denotes spread to distant organs.
The Dukes classification was modified several times by different individuals, but the main drawback of this system was that the depth of tumor penetration, tumor grade and the number of positive lymph nodes were not considered in the staging.
TNM (Tumor, Node, Metastasis) Classification
The current AJCC staging system for colorectal cancer is the only staging system that is recommended now. The classification of colorectal tumors in TNM system is on the basis of the invasiveness (not size) of the primary tumor (T stage), the number of lymph nodes containing metastatic cancer (N stage), and the distant metastatic disease (M stage).
- Tis stands for carcinoma in situ which includes lesions confined to the lamina propria.
- T1 tumors invade submucosa but not beyond it.
- T2 tumors invade muscularis propria but not through it.
- T3 tumors invade through muscularis propria into the subserosa or pericolonic tissue.
- T4 tumors are those which invade adjacent structures (T4a) or perforate visceral peritoneum (T4b).
According to the current TNM classification scheme, at least 7 to 14 lymph nodes are analyzed and the N staging is done based on the number of lymph nodes that are positive for tumor.
- N0 stage denotes that all the nodes examined are negative.
- N1 indicates that metastasis to 1-3 regional lymph nodes.
- N2 stage indicates metastasis in 4 or more regional lymph nodes.
Absence of evidence for distant metastasis is M0 and presence of it is M1.
- Stage I is a T1-2 tumor with no lymph nodes or distant spread.
- Stage II is T3-4 tumor with N0 and M0.
- Stage IIA is tumor in T3.
- Stage IIB is with T4 tumor.
- Stage III is subdivided into :
- Stage IIIA (T1-2, N1, M0)
- Stage IIIB (T3-4, N1, M0)
- Stage IIIC (any T, N2, M0)
- Stage IV includes any tumor stage or node stage with distant metastasis.
Residual Tumor (R Stage)
R staging is based on the margins of resection. Tumors with negative margins after complete resection are R0 while those with microscopically positive margins are classified as R1. R2 denotes incomplete resections with grossly positive margins.