Colorectal Polyps and Carcinoma

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 to 10 years to progress to invasive cancer. Early removal prevents the 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 (see 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 found particularly in the Far East. These can be small and meticulous colonoscopy increases detection rates. Recognition may require special dye-spray techniques. Flat adenomas can be removed by endoscopic mucosal resection, 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 low- to high-grade dysplasia, to early invasive cancer. In invasive cancer, the cellular abnormality has breached the muscularis mucosa, 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 the stalk is clear of cancer, further treatment is not usually required. The Haggitt classification for stalked and the Kikuchi classification for flat cancers give indications of their malignant potential.

Classification of Colonic Adenomas

Three patterns of lesion are recognised histologically: tubular adenomas, villous adenomas and tubulovillous 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 adenoma s 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 tubulovillous adenomas, this potential is proportional to size.

Tubulovillous adenomas are intermediate between tubular and villous and make up most colonic polyps (see Fig. 27.1D ). Most are low-grade, 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. The Paris classification helps determine their potential for malignancy.

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 tubulovillous. Patients with proven carcinoma often have coexisting benign adenomas ( synchronous ), likely to become malignant later if not removed ( 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 computed tomography (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 entire colon is needed. Most colorectal investigation is somewhat invasive and uncomfortable and the benefits have to be explained to patients, whilst respecting their dignity and privacy as far as possible. In the outpatient clinic, rigid sigmoidoscopy (which actually visualises the rectum not the sigmoid) is often performed initially, as nearly half of all polyps lie within 15 cm, the realistic reach of the instrument. Flexible sigmoidoscopy , usually performed with 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 virtually all 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, but if positive, colonoscopy should follow. It is less invasive and does not require sedation but bowel preparation is needed. Other alternative investigations include double contrast barium enema or rarely, unprepared CT scan, although this is unreliable for detecting polyps, it may be useful in excluding cancers in the frail and elderly.

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, everyone at risk (by age or predisposing factors) would undergo colonoscopic surveillance (screening) to detect asymptomatic polyps (as well as invasive cancers) so polyps can be removed before undergoing malignant change. The UK National Health Service national bowel cancer screening programme uses faecal occult blood (FOB) testing every 2 years for people between 60 and 74years of age and more recently, bowel scope screening at 55 years. FOB testing aids detection of about 50% of asymptomatic cancers and is predicted to reduce mortality from colorectal cancer by at least 15%. FOB is to be superseded by the faecal immunochemical test, which is more sensitive. Greater reductions will be achieved if patient compliance can be improved, and by the more recent single flexible sigmoidoscopy offered to people aged 55 years.

Epidemiology of Colorectal Carcinoma

Table 27.1 shows that colorectal cancer is the third most common cause of death from cancer in the developed world; one in 20 people in the United Kingdom suffers from it at some point in their lives and almost a third arise in the rectum. The disease is rare before the age of 50 years (except in inherited colorectal cancer syndromes, see later) but common after the age of 60 years. There is little difference in incidence between the sexes.

Apart from increasing age, diet seems to be an important factor. Since colorectal cancer is more common in developed countries, the Western low-fibre, high-fat diet may increase risk. High intake of red meat and alcohol may increase risk, whilst fish, fibre, and antioxidants in fruit and vegetables may reduce it. Other protective agents may include aspirin and resistant dietary starch.

Ulcerative colitis and Crohn disease, chronic inflammatory conditions of the large bowel (see Ch. 28 ), carry independent risk of bowel neoplasia. After 10 years of active disease, the cancer risk rises by 1% each year.

Finally, some inherited genetic conditions give rise to colorectal cancer. This explains the higher risk in first-degree relatives of patients with early-onset cancers, and why it is important to ask about family history of bowel or other potentially inherited cancers in patients presenting with bowel symptoms.