Adenomatous polyps and adenomas: Polyps are common in the large bowel (Fig. 27.1). The most significant are adenomas (i.e. benign neoplasms) and all have potential for malignant change. In general, it takes 5–10 years to progress to invasive cancer. Early removal prevents progression from adenoma to adenocarcinoma. The process by which the epithelial cells acquire increasingly severe genetic mutations is termed the adenoma–carcinoma sequence. Thus, if adenomas are found at endoscopy, all of them should be meticulously removed (Fig. 27.1c) and subjected to histological examination.

Most adenomas are typically pedunculated or sessile (stalkless), allowing easy recognition and removal by diathermy snare. The much less common flat adenomas are particularly found in the Far East. These can be small and recognition at colonoscopy requires special dye-spray techniques. Flat adenomas can be removed by injecting saline into the submucosa to ‘lift’ the flat lesion before excising the abnormal mucosa with diathermy, sometimes in several pieces.

Adenomatous lesions examined histologically display a range of epithelial abnormalities from mild to severe dysplasia, to early invasive cancer. In invasive cancer, the cellular abnormality has breached the muscularis mucosae, from where extension progressively occurs into the submucosa. As a rule, the larger the lesion, the more likely it is to be malignant: only 1% of polyps smaller than 1 cm are malignant whereas about half of those larger than 2.5 cm are malignant.

Even in apparently benign lesions, there may be discrete areas of frank malignancy; so thorough histological examination is needed. When pedunculated lesions are removed by colonoscopic snaring, it is crucial to establish whether there is stalk invasion: if it is clear of cancer, further treatment is not usually required.

Classification of colonic adenomas: Three patterns of lesion are recognised histologically: tubular adenomas, villous adenomas and tubulo-villous adenomas.

Tubular adenomas are small pedunculated or sessile lesions in which the adenoma cells retain a tubular form similar to normal colonic mucosa. Tubular adenomas have the least potential for malignant transformation.

Villous adenomas are usually sessile (no stalk) and frond-like (papilliferous) lesions which tend to secrete mucus. The epithelial component of villous adenoma is more dysplastic than tubular adenoma and there is a correspondingly greater potential for malignancy; as with tubulo-villous adenomas, this potential is proportional to size.

Tubulo-villous adenomas are intermediate between tubular and villous adenomas and comprise the majority of colonic polyps (Fig. 27.1d). Most are pedunculated, and the stalk is covered with normal colonic epithelium. The stalk probably develops by peristalsis dragging the tumour mass distally and can range from about 0.5 to 10 cm long.

Distribution of colorectal adenomas: Although adenomatous polyps can occur in any part of the large bowel, three-quarters of them arise in rectum and sigmoid colon. This exactly parallels the distribution of carcinomas and provides verification that most cancers develop from polyps.

Adenomas often arise singly (particularly villous adenoma) but more than 20% of patients with colonic polyps have multiple polyps, most often tubulo-villous. Patients with proven carcinoma often have coexisting benign adenomas (synchronous), likely to become malignant later if not removed (see Fig. 27.2). This explains why the whole colon should ideally be examined before colectomy, preferably by colonoscopy, and why long-term follow-up after treatment of large bowel cancer should include regular colonoscopy.

Symptoms and signs of colorectal polyps: Many polyps cause no symptoms, and are found incidentally on colonoscopy, barium enema or CT colonography. Symptomatic polyps typically present with rectal bleeding and sometimes iron deficiency anaemia from occult blood loss. Mucus production, especially from villous adenomas, may be so copious as to be the main presenting complaint. Very occasionally, symptomatic hypokalaemia may develop because so much potassium-containing mucus is lost. Distal lesions occasionally produce tenesmus (a painful urge to defaecate) or may prolapse through the anus. Rarely, large polyps can cause obstructive symptoms or intussusception.

Diagnosis and management of colorectal polyps: For symptomatic patients, visualisation of the colon is needed. Most colorectal investigation is somewhat invasive and uncomfortable and the benefits have to be explained to patients, whilst their dignity and privacy is respected as far as possible. In the outpatient clinic, rigid sigmoidoscopy (which actually visualises the rectum) is often performed initially, as nearly half of all polyps lie within reach of the 25 cm instrument. Flexible sigmoidoscopy, usually performed without bowel preparation or after a phosphate enema, reaches past the sigmoid and descending colon to the splenic flexure, covering 75% of the area at risk, but to view the remainder of the bowel requires colonoscopy. Well performed colonoscopy is the ‘gold standard’ investigation: it allows visualisation of the entire large bowel mucosa, and polyps may be removed at the same time. For this reason, it is the first-line investigation in many centres. However, there are disadvantages: it requires a full day’s bowel preparation and the procedure often requires sedation because of the discomfort. Furthermore, colonoscopy carries a 1 : 1000 risk of major haemorrhage or perforation. Sessile, small or flat adenomas in any location can be difficult to recognise even at colonoscopy and these potentially malignant lesions can be missed, especially if bowel preparation has been poor.

An alternative investigation growing in popularity and accuracy is CT colonography. Some studies have shown it to be virtually as sensitive for detecting polyps. It is less invasive and does not require sedation but bowel preparation is needed. Other alternative investigations include double contrast barium enema or unprepared CT scan.

Once an adenomatous polyp is found or a colorectal cancer has been treated, that patient is at risk of forming further polyps elsewhere in the large bowel and needs follow-up colonoscopies. Intervals between colonoscopies are set according to guidelines determined by the number, size and pathology of polyps at each investigation and vary between 1 and 5 years.

Ideally, people at risk would undergo colonoscopic surveillance to detect asymptomatic polyps (as well as invasive cancers) so they can be removed before undergoing malignant change. The UK NHS national bowel cancer screening programme employs faecal occult blood testing every 2 years for people between 60 and 75 years of age. This test has been shown to aid detection of about 50% of asymptomatic cancers and is predicted to reduce mortality from colorectal cancer by at least 15%. Greater reductions could undoubtedly be achieved if patient compliance could be improved, and plans are under way to supplement this programme with a single flexible sigmoidoscopy offered to people aged 55–60.

Polyposis syndromes: The most important is familial adenomatous polyposis (FAP)