Three methods have been shown to be effective in the primary diagnosis of colorectal cancer: endoscopy, double contrast barium enema, and Computed Tomography (CT) pneumocolon. The success of each method depends on adequate bowel preparation.

Where colorectal cancer is suspected clinically, the whole of the large bowel should be examined.

Currently, the evidence relating to comparisons between colonoscopy and double contrast barium enema comes from only one randomised trial⁹² and cohort studies containing elements of bias.⁹³ The trial showed that there was no significant difference in accuracy between double contrast barium enema combined with flexible sigmoidoscopy and colonoscopy in the detection of cancers or polyps equal to or greater than 9 mm in size. Evidence level 1+,2

There is currently no clear consensus as to which should be the investigation of choice for the diagnosis of colorectal cancer.⁹⁴

The diagnosis of colorectal cancer on purely clinical grounds is unreliable. Since the prognosis depends to a large extent on the stage of the disease at the time of diagnosis, early investigation of symptoms and signs is appropriate.

6.1 Endoscopy

Relevant endoscopic techniques include rigid sigmoidoscopy, flexible sigmoidscopy and colonoscopy. Flexible sigmoidoscopy is more effective than rigid sigmoidoscopy for visualising the rectum and distal colon.^{95, 96} Evidence level 4

Colonoscopy is an extremely sensitive diagnostic test for colorectal cancer and has the major advantages of allowing both biopsy and polypectomy and does not involve exposure to ionising radiation.⁹⁷ It also has some disadvantages: in a variable proportion of cases (5-30%) the caecum is not reached,⁹⁸ intravenous (IV) sedation is nearly always required, the localisation of tumour can be inaccurate,⁹⁹ and there is a small but significant risk of complications.^{100, 101} The procedure-related mortality is approximately 1 in 5,000 for colonoscopy and 1 in 50,000 for sigmoidoscopy and for double contrast barium enema. Evidence level 3,4

Colonoscopy is recommended as a very sensitive method of diagnosing colorectal cancer, enabling biopsy and polypectomy.

6.2 Double contrast barium enema

Double contrast barium enema is a sensitive, safe, well tolerated method not requiring sedation, and has a high completion rate.^{102, 103, 104} In addition, it is widely available throughout Scotland. Disadvantages include the radiation dose and reduced accuracy in the presence of sigmoid diverticular disease.⁹⁸ The radiation dose can be significantly reduced by modern digital technology. Evidence level 2+,4

Accuracy is significantly increased when double contrast barium enema is combined with flexible sigmoidoscopy.¹⁰⁵ Evidence level 1+

Double contrast barium enema may be employed as a sensitive, safe alternative to colonoscopy.

Where the sigmoid colon is not well visualised, eg in the presence of severe diverticular disease, double contrast barium enema should be combined with flexible sigmoidoscopy.

In all cases the rectum should be visualised by rigid or flexible sigmoidoscopy.

6.3 Computed tomography pneumocolon

Computed Tomography (CT) pneumocolon is a sensitive method for the detection of colorectal cancer, but not for polyps less than 10 mm.¹⁰⁶ Evidence level 2+

CT pneumocolon is well tolerated by patients and as it provides information outside the colon and rectum it can be used for staging malignant disease (local invasion, liver metastases, lymph node spread etc.).⁹⁸ It is not possible to detect or exclude tumours in normally sized lymph nodes. The technique is particularly useful in frail, immobile and elderly patients. The radiation dose with modern equipment and best practice can be comparable with conventional barium enema radiation dose levels.¹⁰⁷ In the UK, expertise with, and access to fast CT scanners are limited at present, but the increasing use of multi-slice CT and more sophisticated computer software is likely to increase the clinical application of this technique. Recent evidence in support of magnetic resonance colography is encouraging¹⁰⁸ but lack of access and available expertise makes widespread use of this technique in Scotland impractical at present. Evidence level 4

Where the radiological expertise and equipment exist, a CT pneumocolon is recommended as a sensitive test for colorectal cancer.