Distribution of colorectal adenomas: implications for bowel cancer screening

Most colorectal cancers develop from adenomatous polyps.³ If adenomas can be identified and removed, the adenoma-carcinoma sequence is broken, and colorectal cancer may be prevented.⁴ As adenomas usually cause few, if any, symptoms, they can be detected only by searching for them in asymptomatic individuals. While colonoscopic surveillance programs for detecting adenomas are accepted for those at higher risk of developing colorectal cancer (eg, with a family history),⁵ screening for the average-risk population remains controversial.

In Australia, there are no population-based colorectal cancer screening programs, but the recent report from the Australian Health Technology Advisory Committee suggested pilot programs of faecal occult blood testing (FOBT) alone or in combination with flexible sigmoidoscopy.⁶ This combination is probably the most accepted method of screening for average-risk colorectal cancer. However, as adenomas and cancers may occur on the right side of the colon, and as not all of these bleed, they could be missed by this combination of tests.

In 1991, some private endoscopy centres in Melbourne, Victoria, developed a Bowel Cancer Prevention Program. We aimed to determine how many asymptomatic people who underwent colonosocopy as part of this program had adenomas in the proximal colon only. This would indicate the number of patients with adenomas who would be missed by flexible sigmoidoscopy screening in an Australian population.

The Bowel Cancer Prevention Program targets interested employees of major employer groups or members of participating unions and organisations. Each participant receives educational material about colorectal cancer and is asked to return a simple questionnaire. This assists an expert panel to assess and advise on individual risk of developing colorectal cancer, based on Gut Foundation⁷ and international guidelines.⁸

Asymptomatic individuals are referred for colonoscopy if:

• They are aged 40 years or over and have a self-reported family history of at least one first-degree relative with bowel cancer; or

• They are aged 50 years or over, and a doctor has requested a colonoscopy for bowel cancer screening because of the patient's marked anxiety.

• distal to the splenic flexure only;

• both distal and proximal to the splenic flexure; and

• proximal to the splenic flexure only.

Colorectal polyps were found in 270 people (24%) and were confirmed to be adenomas in 138 (12%). These 138 comprised 106 men and 32 women, with mean age 54 years (range, 40-78 years). Adenomas were found in 118 of the 907 people with a first-degree relative with bowel cancer (13%) and in 20 of the 224 people whose only risk factor was age 50 years or more (9%). This difference in adenoma rates was statistically significant (P < 0.05).

Most of the 138 patients (86%) had a single adenoma, while 14% had two or more. The adenomas ranged from 5 mm to 2 cm in diameter.

Distribution of adenomas relative to the splenic flexure is shown in the Box. The distribution did not differ significantly between people who had a family history of colorectal cancer and those who did not: in both groups, 25% had adenomas proximal to the splenic flexure only.

This theory is not supported by the results of our study. We found that in 25% of asymptomatic individuals with adenomas, the adenomas were found only in the proximal colon, beyond the reach of the flexible sigmoidoscope. This figure is consistent with other studies.⁹ A sentinel adenoma on the left side signalling proximal adenomas was found in only 14% of people; 62% of subjects had only left-sided adenomas.

In our study the distribution of adenomas was assessed from the endoscopists' reports. Experienced endoscopists are correct in determining scope-tip position 83% of the time.¹⁰ One cannot assume that a 60 cm flexible sigmoidoscope will reach the splenic flexure. Indeed, a recent report suggests that it reaches 60 cm in only 34% of cases, and far less often in women than in men, with the problem worsening with increasing patient age.¹¹

There are no other Australian data on the incidence or distribution of adenomatous polyps in the colon and rectum. Collett et al showed that distal colorectal cancer or adenomas predicted proximal neoplasia in 30% of subjects, but their study was based on colonoscopy only after a positive result from flexible sigmoidoscopy.¹² Their study design did not allow them to accurately determine the distribution of adenomas in the colon and rectum.

Colonoscopy remains the final diagnostic pathway for any positive result for a bowel cancer screening test. Some international experts recommend use of colonoscopy as the optimum screening tool to prevent the development of colorectal cancer.¹³ However, it has not been embraced as the primary screening modality because of concerns about cost, compliance and risk. The risks of colonoscopy include those related to sedation, as well as the procedure, such as perforation (reported at 1 in 2000¹⁴) and bleeding complicating polypectomy. Complication rates are usually derived from hospital populations, which include many ill patients who have significant comorbidity, and it may be that the risk is considerably less in the far healthier "screening" population. No significant sedation- or procedure-related complications or deaths occurred in our comparatively small series of colonoscopies.

The proportion of men to women in the group with the adenomas was about 2:1. This probably reflects our target population rather than a true difference in the incidence of adenomas. However, men are significantly more likely to develop colorectal cancer than women.¹⁵ Unfortunately, most reported case series of adenomas have studied populations heavily biased with men. Larger studies examining the influence of sex on adenoma frequency and distribution within the colon and rectum are needed.

Well-designed overseas studies have confirmed that programs based on annual FOBT followed by colonoscopy for those with positive FOBT results significantly reduce anticipated mortality from colorectal cancer.^16,17 Adding flexible sigmoidoscopy to FOBT to detect proximal adenomas seems sensible but adds significantly to cost. Indeed, a recent Australian economic study using computer modelling clearly showed that only FOBT or five- or 10-yearly colonoscopy are cost-effective bowel-cancer screening methods; flexible sigmoidoscopy alone or in combination with FOBT was not cost-effective.¹⁸ The distribution of adenomas found in our study adds weight to the recent suggestion that it is time to consider adding full colonoscopy to the menu of options available to general practitioners when discussing bowel cancer screening and prevention with their patients.¹⁹

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Reprints: Dr M G Korman, Department of Gastroenterology, Monash Medical Centre, 246 Clayton Road, Clayton, VIC 3168.
m.kormanATshcn.com.au

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