Benefits of Sigmoidoscopy
Sigmoidoscopy is a colon cancer screening technique based on endoscopy. It operates similar to colonoscopy. Sigmoidoscopy is fairly cheap in comparison to colonoscopy. It can be performed in an office and does not require sedation. Sigmoidoscopy is particularly popular in the 1980s and 1990s.
Unlike colonoscopy, which allows physician to examine the whole large intestine, sigmoidoscopy only allows visualization of the rectum and the last part of the large intestine, known as the sigmoid colon. In other words, flexible sigmoidoscopy can only be used to detect cancer and polyps in one-third of the large intestine.
Despite this limitation, the use of flexible sigmoidoscopy has been associated with reducing risk of colorectal cancer incidence and mortality.Below we briefly review a few key evidences to support this observation.
1) Prostate, Lung, Colorectal and Ovarian (PLCO) trial
The PLCO cancer screening trial is a large randomized-control clinical trial that investigates the effects of cancer screening, including colon cancer screening. In this trial, 64 658 people had colon cancer screening by a 60-cm flexible sigmoidoscopy.49.5% of the participants are male. The age distribution are 33.4%, 30.7%, 22.6%, and 13.4% for age groups 55-59, 60-64,65-69, and 70-74, respectively. The fraction of patients with family history of colorectal cancer is 9.7%, and with colorectal polyp history is 7.4%. 38.7% of subjects had one or more FOBTs, and 12.9% had one or more lower GI procedures (sigmoidoscopy, colonoscopy, or barium enema) within 3 years of the start of the study.
The results of the PLCO trial demonstrated the effectiveness of sigmoidoscopy in detecting polyps, which are the precursors of colorectal cancer. 23.4% of subjects were found to have at least one polyp or mass. Risk of having polyps is less in women than in men (18.4% versus 28.3%). The rate of colorectal cancer varies between 90-470 for different age ranges. Rates of cancer among those aged 55 – 59, 60 – 64,65 – 69, and 70 – 74 years are 140, 270, 290, and 320 cases per 100,000,respectively. Rates of cancer or adenoma are 6370, 7890, 8670, and 8040 cases per 100,000, respectively.Stages of colorectal cancers can be used as a proxy for the effectivness of sigmoidoscopy on colorectal cancer mortality. Use of sigmoidoscopy lead to cancer diagnoses at early stages (stage I or stage II) for 77% of cases associated with positive sigmoidoscopy.
2) Case-control studies
Newcomb et al. (2003) performed a population-based case–control study to investigate the effects of sigmoidoscopy on colorectal cancer incidence. They found that use sigmoidoscopy reduces risk of distal colorectal cancer (cancer in the sigmoid colon and the rectume) by 76%.
Selby et al. (1992) compared the use of sigmoidoscopy in 261 patients who died of colorectal cancer with 868 control subjects matched with by age and gender. They found that only 8.8 percent of the case subjects had undergone screening by sigmoidoscopy, as compared with 24.2 percent of the controls. Taking into account other confouding factors, Selby et al. (1992) found that sigmoidoscopy reduces the risk of cancer mortality by 59%.
Disadvantages of sigmoidoscopy
The main disadvantage of flexible sigmoidoscopy is its limitation in detecting polyps and adenomas in the other side of the colon.
The other disadvantage of flexibile sigmoidoscopy is its low compliance rate. In comparison to colonoscopy, the patients often found the preparation for sigmoidoscopy easier as colonoscopy may take up 2 days for preparation. However, patients also found sigmoidoscopy more uncomfortable and embarrassing. This is mainly due to the fact that patients are sedated during colonoscopy and not aware of the procedure. Platell et al. (2002) found that only 50% of people who had sigmoidoscopy were willingly to return for re-screening after 5 years.
With the introduction of new colon cancer screening techniques such as fecal DNA and virtual colonoscopy, and the use of colonoscopy as the gold standard for colon cancer screening, it is not clear that flexible sigmoidoscopy will be used in future. Currently, Kaiser Permeante is the only health care provider that still uses sigmoidoscopy on a relatively large scale. However, it is expected that Kaiser will adopt new technologies in a near future.