- Email: [email protected]
The difference in colon polyp size before and after removal
The difference in colon polyp size before and after removal Thomas G. Morales, MD, Richard E. Sampliner, MD, Harinder S. Garewal, MD, PhD M. Brian Fennerty, MD, Mikel Aickin, PhD Tucson, Arizona
Background: Accurate knowledge of polyp size is important in assessing cancer risk in both clinical studies and individual patients. We sought to determine if a difference exists between the endoscopic estimation of colon polyp size and the actual measurement after removal. Methods: We measured polyps in a systematic fashion. Using open biopsy forceps as a guide, the largest diameter of 31 pedunculated polyps was estimated endoscopically. The polyp was then removed by snare polypectomy and directly measured by a technician who was blinded to the endoscopic estimate. Each polyp was also measured after formalin fixation by a pathologist who was blinded to previous measurements. Results: There was a significant difference between the endoscopic estimates and the postpolypectomy measurements. Endoscopic estimates on average were 1.6 mm greater than the postpolypectomy measurements (p < 0.05), representing an 18% difference. Twenty-three of the 31 (74%) endoscopic estimates were larger than the postpolypectomy measurements. There was not a significant difference between the postpolypectomy and postfixation measurements. Conclusions: The size of polyps measured endoscopically is significantly larger on average than postpolypectomy measurements. This is most likely due to factors involved in the removal of the in vivo polyp. (Gastrointest Endosc 1996;43:25-8.)
It is generally accepted t h a t most colorectal cancers arise from adenomatous polyps. 1 This has led to the practice of removing all polyps found during colonoscopic examination. There is now evidence to support the concept t h a t removal of adenomatous polyps reduces the incidence of colorectal cancer. 2 Patients with larger polyps m a y be at greater risk for the development of cancer than those with polyps 5 m m or smaller.3, 4 Therefore, accurate assessment of polyp size is important in assessing cancer risk. There have Received August 19, 1994. For revision November 8, 1994. Accepted April 25, 1995. From the Arizona Health Sciences Center and Tucson Veterans"Affairs Medical Center, Department of Internal Medicine, Sections of Gastroenterology, Hematology-Oncology, and Family and Community Medicine, Tucson, Arizona. Reprint requests: Thomas G. Morales, MD, Arizona Health Sciences Center, Gastroenterology, Tucson, AZ 85724. 37/1/66490
VOLUME 43, NO. 1, 1996
been no uniform recommendations in the literature as to whether the endoscopic estimate of polyp size or the postpolypectomy measurement should be used in assessing cancer risk. In addition, it is not clear whether there is a substantial difference between these measurements. We have previously reported on the inaccuracy of endoscopic polyp measurement using an artificial colon polyp model. 5 Both our study and another study measuring steel ball bearings in a colon model 6 showed t h a t endoscopic measurement tends to underestimate the actual size of an object. However, this finding is at variance with the clinical experience of m a n y gastroenterologists, whose impression is t h a t the size is often overestimated endoscopically,7 especially when compared to the postpolypectomy pathologic measurement. Because polyp size estimation is important in the setting of clinical studies and cancer risk assessment, we have extended our observations to h u m a n GASTROINTESTINAL ENDOSCOPY 25
Table 1. Endoscopic estimates and post-polypectomy measurements of colon polyp size Polyp Endoscopic Postpolypectomy EstimatePostfixation estimate measurement measurement measurement No. difference (ram) (mm) (ram) (ram) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
10 8 8 20 10 7 6 8 9 6 8 7 8 9 6 14
9 7 6 15 7 7 4 10 9 4 9 5 7 6 5 12
+1 +1 +2 +5 +3 0 +2 -2 0 +2 -1 +2 +1 +3 +1 +2
17
12
i0
+2
7 5 6 7 6 8 7 9 3 7 6 10 5 9 7.3 -* 0.5*
+1 -1 +2 +3 +2 0 -1 +2 +5 0 +1 +5 +2 +4 +1.6 _+0.3*
18 8 19 4 20 8 21 10 22 8 23 8 24 6 25 11 26 8 27 7 28 7 29 15 30 7 31 13 Mean 8.9 -+ 0.6*
10 5 6 15 7 5 7 10 10 5 7 6 6 5 6 12 10 6 4 6 7 5 7 8 6 2 7 4 10 7 10 7.1 _+0.5*
*Standard error of measurement.
subjects. We c o m p a r e the size of polyps e s t i m a t e d i n v i v o at the time of colonoscopy v e r s u s e x v i v o after removal, both before a n d after fixation.
METHODS Using a video colonscope (Olympus CFIT100L, Olympus America Inc., Melville, N.Y.) the endoscopic estimation of colon polyp size versus the postpolypectomy measurement was evaluated for each. The largest diameter of 31 pedunculated polyps in 25 patients was estimated endoscopica]ly prior to removal by snare polypectomy. Six endoscopists participated in the study, each of whom had performed over 200 colonoscopies (two gastroenterology attendings and four fellows). One endoscopist (TGM) performed 55% of the polypectomies. Each endoscopist heard a 5-minute talk on the standardized endoscopic measurement technique to be used in the study. The endoscopic measurement was made by placing biopsy forceps with an open diameter of 8 mm (Radial Jaw TM biopsy forceps, Microvasive Corp., Watertown, Mass.) on the polyp as a guide. The biopsy forceps were opened and then withdrawn in the open position toward the 26
GASTROINTESTINAL ENDOSCOPY
endoscope tip as far as possible such that both cups of the forceps were still fully visualized. Next the endoscope tip was advanced so that the open forceps were against the polyp at its widest diameter. After subsequent snare polypectomy, polyps were retrieved using a tripronged polyp grasper (Tripod retrieval forceps, Olympus America Inc., Melville, N.Y.) without bringing the polyp through the endoscopic channel. A technician blinded to the endoscopic estimate then directly measured the largest diameter of the excised polyp with a ruler to the nearest millimeter after removal but prior to fixation. In addition, each polyp was measured by ruler after formalin f~xation (10% formalin solution) by a pathologist who was blinded to the previous measurements. Measurements were analyzed using the paired-sample t test analysis.
RESULTS The m e a n age of the 25 p a t i e n t s in the s t u d y was 64 y e a r s (range, 34 to 78). T h e r e were 21 m e n a n d 4 women, the difference reflecting the large proportion of patients from the V e t e r a n s ' Medical Center. Of the 31 polyps r e m o v e d in these patients, 29 were from the rectosigmoid region a n d 2 were from more proximal locations. The histologic diagnoses of these polyps were as follows: 13 t u b u l a r a d e n o m a s , 12 tubulovillous adenomas, 2 h y p e r p l a s t i c polyps, 1 villous aden o m a , 1 juvenile polyp, a n d 1 h a m a r t o m a t o u s polyp. The m e a n endoscopic estimate of polyp size was 8.9 m m a n d the m e a n postpolypectomy m e a s u r e m e n t was 7.3 m m (Table 1). The m e a n difference was 1.6 mm, 18% (p < 0.05). The 90% confidence interval for the m e a n difference was 1.0 to 2.1 mm. T w e n t y - t h r e e of 31 (74%) of the endoscopic estimates of polyp size were l a r g e r t h a n the postpolypectomy m e a s u r e m e n t . Nine of the 31 endoscopic e s t i m a t e s of polyp size were 1 cm or larger a n d five postpolypectomy meas u r e m e n t s were 1 cm or larger. T h e r e was a g r e a t e r m e a n difference in size for polyps 1 cm or l a r g e r t h a n for those 1 cm or smaller, 2.4 versus 1.4 m m (Fig. 1). Finally, no significant difference was seen b e t w e e n the m e a n post-polypectomy a n d post-fixation meas u r e m e n t s , 7.3 a n d 7.1 m m , respectively (Table 1).
DISCUSSION We h a v e d e m o n s t r a t e d that, on average, the endoscopic e s t i m a t i o n of colon polyp size in h u m a n subjects is significantly l a r g e r t h a n the m e a s u r e m e n t after removal. T h e r e m a y b e several reasons for this difference. First, the endoscopic m e a s u r e m e n t of the polyps was m a d e u s i n g biopsy forceps as a guide. The biopsy forceps did not always open in the largest d i a m e t e r of the polyp as t h e y exited the endoscope c h a n n e l (Fig. 2), which necessitated a more subjective e s t i m a t e t h a n w h e n the forceps opened in the a p p r o p r i a t e orientation. For those polyps m u c h l a r g e r t h a n 8 ram, the est i m a t e of polyp size required an e s t i m a t e of how m a n y multiples larger t h a n the forceps the polyp was VOLUME 43, NO. 1, 1996
20-
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o
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Post-Polypectomy Measurement ( m m )
Figure 1. Graph depicting the difference between the endoscopic estimate of polyp size and the postpolypectomy measurement (mm). Open circle represents size of polyp. Solid line represents the line of equality.
(Fig. 2). Furthermore, the largest diameter was not completely in view for some of the larger polyps. These factors may explain why there was a larger difference in those polyps 1 cm or larger in size. Optical problems m a y also introduce error to endoscopic measurements. It is well known that the distance of the endoscope from an object affects its measurement.S, 9 Objects farther from the endoscope tip appear smaller than when moved directly in front of the tip. We attempted to minimize this error by keeping the biopsy forceps withdrawn to the endoscope tip in the open position and then placing the endoscope tip and forceps directly onto the polyp (Fig. 2). Other authors 9, 10 have evaluated the error introduced to endoscopic measurements by the use of wide-angle lenses in endoscopes, which create image distortion in the periphery of the field such that objects in the periphery will appear compressed. This compression results in an apparent decrease in the size of objects in the periphery. Even if open biopsy forceps are used as a guide, there would be unequal distortion because the forceps are in the center of the field and at least a portion of the polyp lies in the periphery. The systematic error introduced by this '%arrel distortion" would be expected to be more significant for larger polyps. However, this error should be expected to cause an underestimation of size rather than an overestimation. We considered the possible effects related to polyp removal that may cause shrinkage of tissue and thus a smaller postpolypectomy measurement. As a polyp is cauterized there is shrinkage of tissue, but if the polyp is pedunculated the stalk should be the area most affected. Some transmission of heat to the bulk of the polyp would be expected, however, and this V O L U M E 43, NO. 1, 1996
Figure 2. Endoscopic photograph of polyp size estimation using open biopsy forceps. Note that the forceps are withdrawn to the endoscope tip and then placed onto the polyp. Also note that for this particular polyp the forceps did not open in the greatest diameter, thus requiring a more subjective estimation of polyp size.
could lead to a certain amount of desiccation and shrinkage. In our study only pedunculated polyps were included to minimize this effect. A greater amount of shrinkage might be expected with the snare polypectomy of a sessile polyp, since a larger area of the polyp would be cauterized. Another factor that would be expected to significantly affect the size of the polyp before and after removal is vascular collapse. In the in vivo setting there is arterial flow to polyps. The pressure generated by this flow is lost on removal, when the arterial vessels collapse. This collapse and subsequent loss of pressure then leads to a decrease in the size of the polyps as measured after endoscopic removal. Polyp graspers may compress a polyp somewhat during removal, but this would not be expected to induce a permanent deformation. If a polyp is brought through the endoscope channel, either with graspers or by suction, this may tear or lacerate the polyp and alter the largest diameter. In our study, the polyps were not brought through the channel of the endoscope to avoid this problem. Finally, it is generally thought that formalin fLxation m a y cause some shrinkage of the specimen; however, in our study the prefLxation and postfixation measurements were similar. This suggests that shrinkage caused by formalin fLxation is unlikely to be a significant factor in the difference between the endoscopic estimate and the pathologic measurement. The findings of our study are at variance with reports using colon models (including our own previous study), which have shown that endoscopic measurements tend to underestimate the size of an object. The most obvious reason for this would be the factors involved in removal of the in vivo polyp, such as GASTROINTESTINAL ENDOSCOPY
27
shrinkage due to cautery and vascular collapse. In the colon model studies, the artificial objects were not removed and therefore were not subject to shrinkage or damage. The endoscopic underestimation in size in these studies was based on the directly measured size of the artificial polyp before or after placement in a colon model, 5, 6 thereby reflecting endoscopic measurement error. To validate the accuracy of the endoscopic measurements of colon polyps in h u m a n subjects one would need to measure a polyp endoscopically and then somehow measure that polyp directly while still in vivo, such as during a colectomy. As this is obviously impractical, one must understand the limitations of endoscopic measurements. Although the endoscopic estimation of object size may underestimate its true size, as shown in colon model studies, our study in human subjects shows that when polyps are measured after removal they are considerably smaller than the endoscopic estimate. This difference may be explained by endoscopic measurement error, optical distortion, or shrinkage during removal. It is important for endoscopists to be aware of these issues when interpreting data from clinical trials and when assessing cancer risk in individual patients. Most clinical studies, including the National Polyp Study, 11 stratify polyps based on size using 1 cm as a cutoff for "large" polyps. In our study there were nine polyps that were endoscopically estimated to be 1 cm or larger in size. However, only five polyps were in fact 1 cm or larger by measurement after removal. This further illustrates the significant difference between these two measurements and the potential impact on polyp classification and patient cancer risk stratification. These data emphasize the need for systematic and uniform measurement of polyps, particularly in the setting of clinical studies. Outside the research
28 G A S T R O I N T E S T I N A L E N D O S C O P Y
setting the significance of this measurement difference may be less apparent because although patients with larger polyps m a y have a potential increase in cancer risk, all polyps are usually removed and appropriate surveillance examinations performed regardless of their size. Finally, we propose that a standardized approach to the endoscopic estimation of polyp size with open biopsy forceps as described in this study be used for polyp size classification rather than the postpolypectomy measurement. Such standardization would decrease the potential for misclassification of patients in clinical trials based on polyp size. REFERENCES 1. Mute T, Bussy HJR, Morson BC. The evolution of cancer of the colon and rectum. Cancer 1975;36:2251-70. 2. Winawer SJ, Zauber AG, He MN, et al. Prevention of colorectal cancer by colonoscopic polypectomy. N Engl J Med 1993;329: 1977-81. 3. Panish JF. Management of patients with polypoid lesions of the colon: current concepts and controversies, Am J Gastroenterol 1979;71:315-24. 4. Atkins WS, Morson BC, Cuzick J. Long-term risk of colorectal cancer after excision of rectosigmoid adenomas. N Engl J Med 1992;326:658-62. 5. Fennerty MB, Davidson J, Emerson SS, et al. Are all endoscopic measurements of colonic polyps reliable? Am J Gastroenterol 1993;88:496-500. 6. Marguiles C, Krevsky B, Catalano MF. How accurate are endoscopic estimates of size? Gastrointest Endosc 1994;40: 174-7. 7. Waye JD. The accuracy of endoscopic estimates: What is large? What is small? Am J Gastroenterol 1993;88:483-4. 8. Okabe H, Ohida M, Okada N, et al. A new disk method for endoscopic determination of gastric ulcer area. Gastrointest Endosc 1986;32:20-4. 9. Vakil N, Smith W, Bourgeios K, Everbach EC, and Knyrim K. Endoscopic measurement of lesion size: improved accuracy with image processing. Gastrointest Endosc 1994;40:178-83. 10. Sonnenberg A, Giger M, Kern L, et al. How reliable is determination of ulcer size by endoscopy? BMJ 1979;2:1322-4. 11. Winawer SJ, Zauber AG, O'Brien MJ, et al. Randomized comparison of surveillance intervals after colonoscopic removal of newly diagnosed adenomatous polyps. N Engl J Med 1993;328: 901-6.
VOLUME 43, NO. 1, 1996
Background: Accurate knowledge of polyp size is important in assessing cancer risk in both clinical studies and individual patients. We sought to determine if a difference exists between the endoscopic estimation of colon polyp size and the actual measurement after removal. Methods: We measured polyps in a systematic fashion. Using open biopsy forceps as a guide, the largest diameter of 31 pedunculated polyps was estimated endoscopically. The polyp was then removed by snare polypectomy and directly measured by a technician who was blinded to the endoscopic estimate. Each polyp was also measured after formalin fixation by a pathologist who was blinded to previous measurements. Results: There was a significant difference between the endoscopic estimates and the postpolypectomy measurements. Endoscopic estimates on average were 1.6 mm greater than the postpolypectomy measurements (p < 0.05), representing an 18% difference. Twenty-three of the 31 (74%) endoscopic estimates were larger than the postpolypectomy measurements. There was not a significant difference between the postpolypectomy and postfixation measurements. Conclusions: The size of polyps measured endoscopically is significantly larger on average than postpolypectomy measurements. This is most likely due to factors involved in the removal of the in vivo polyp. (Gastrointest Endosc 1996;43:25-8.)
It is generally accepted t h a t most colorectal cancers arise from adenomatous polyps. 1 This has led to the practice of removing all polyps found during colonoscopic examination. There is now evidence to support the concept t h a t removal of adenomatous polyps reduces the incidence of colorectal cancer. 2 Patients with larger polyps m a y be at greater risk for the development of cancer than those with polyps 5 m m or smaller.3, 4 Therefore, accurate assessment of polyp size is important in assessing cancer risk. There have Received August 19, 1994. For revision November 8, 1994. Accepted April 25, 1995. From the Arizona Health Sciences Center and Tucson Veterans"Affairs Medical Center, Department of Internal Medicine, Sections of Gastroenterology, Hematology-Oncology, and Family and Community Medicine, Tucson, Arizona. Reprint requests: Thomas G. Morales, MD, Arizona Health Sciences Center, Gastroenterology, Tucson, AZ 85724. 37/1/66490
VOLUME 43, NO. 1, 1996
been no uniform recommendations in the literature as to whether the endoscopic estimate of polyp size or the postpolypectomy measurement should be used in assessing cancer risk. In addition, it is not clear whether there is a substantial difference between these measurements. We have previously reported on the inaccuracy of endoscopic polyp measurement using an artificial colon polyp model. 5 Both our study and another study measuring steel ball bearings in a colon model 6 showed t h a t endoscopic measurement tends to underestimate the actual size of an object. However, this finding is at variance with the clinical experience of m a n y gastroenterologists, whose impression is t h a t the size is often overestimated endoscopically,7 especially when compared to the postpolypectomy pathologic measurement. Because polyp size estimation is important in the setting of clinical studies and cancer risk assessment, we have extended our observations to h u m a n GASTROINTESTINAL ENDOSCOPY 25
Table 1. Endoscopic estimates and post-polypectomy measurements of colon polyp size Polyp Endoscopic Postpolypectomy EstimatePostfixation estimate measurement measurement measurement No. difference (ram) (mm) (ram) (ram) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
10 8 8 20 10 7 6 8 9 6 8 7 8 9 6 14
9 7 6 15 7 7 4 10 9 4 9 5 7 6 5 12
+1 +1 +2 +5 +3 0 +2 -2 0 +2 -1 +2 +1 +3 +1 +2
17
12
i0
+2
7 5 6 7 6 8 7 9 3 7 6 10 5 9 7.3 -* 0.5*
+1 -1 +2 +3 +2 0 -1 +2 +5 0 +1 +5 +2 +4 +1.6 _+0.3*
18 8 19 4 20 8 21 10 22 8 23 8 24 6 25 11 26 8 27 7 28 7 29 15 30 7 31 13 Mean 8.9 -+ 0.6*
10 5 6 15 7 5 7 10 10 5 7 6 6 5 6 12 10 6 4 6 7 5 7 8 6 2 7 4 10 7 10 7.1 _+0.5*
*Standard error of measurement.
subjects. We c o m p a r e the size of polyps e s t i m a t e d i n v i v o at the time of colonoscopy v e r s u s e x v i v o after removal, both before a n d after fixation.
METHODS Using a video colonscope (Olympus CFIT100L, Olympus America Inc., Melville, N.Y.) the endoscopic estimation of colon polyp size versus the postpolypectomy measurement was evaluated for each. The largest diameter of 31 pedunculated polyps in 25 patients was estimated endoscopica]ly prior to removal by snare polypectomy. Six endoscopists participated in the study, each of whom had performed over 200 colonoscopies (two gastroenterology attendings and four fellows). One endoscopist (TGM) performed 55% of the polypectomies. Each endoscopist heard a 5-minute talk on the standardized endoscopic measurement technique to be used in the study. The endoscopic measurement was made by placing biopsy forceps with an open diameter of 8 mm (Radial Jaw TM biopsy forceps, Microvasive Corp., Watertown, Mass.) on the polyp as a guide. The biopsy forceps were opened and then withdrawn in the open position toward the 26
GASTROINTESTINAL ENDOSCOPY
endoscope tip as far as possible such that both cups of the forceps were still fully visualized. Next the endoscope tip was advanced so that the open forceps were against the polyp at its widest diameter. After subsequent snare polypectomy, polyps were retrieved using a tripronged polyp grasper (Tripod retrieval forceps, Olympus America Inc., Melville, N.Y.) without bringing the polyp through the endoscopic channel. A technician blinded to the endoscopic estimate then directly measured the largest diameter of the excised polyp with a ruler to the nearest millimeter after removal but prior to fixation. In addition, each polyp was measured by ruler after formalin f~xation (10% formalin solution) by a pathologist who was blinded to the previous measurements. Measurements were analyzed using the paired-sample t test analysis.
RESULTS The m e a n age of the 25 p a t i e n t s in the s t u d y was 64 y e a r s (range, 34 to 78). T h e r e were 21 m e n a n d 4 women, the difference reflecting the large proportion of patients from the V e t e r a n s ' Medical Center. Of the 31 polyps r e m o v e d in these patients, 29 were from the rectosigmoid region a n d 2 were from more proximal locations. The histologic diagnoses of these polyps were as follows: 13 t u b u l a r a d e n o m a s , 12 tubulovillous adenomas, 2 h y p e r p l a s t i c polyps, 1 villous aden o m a , 1 juvenile polyp, a n d 1 h a m a r t o m a t o u s polyp. The m e a n endoscopic estimate of polyp size was 8.9 m m a n d the m e a n postpolypectomy m e a s u r e m e n t was 7.3 m m (Table 1). The m e a n difference was 1.6 mm, 18% (p < 0.05). The 90% confidence interval for the m e a n difference was 1.0 to 2.1 mm. T w e n t y - t h r e e of 31 (74%) of the endoscopic estimates of polyp size were l a r g e r t h a n the postpolypectomy m e a s u r e m e n t . Nine of the 31 endoscopic e s t i m a t e s of polyp size were 1 cm or larger a n d five postpolypectomy meas u r e m e n t s were 1 cm or larger. T h e r e was a g r e a t e r m e a n difference in size for polyps 1 cm or l a r g e r t h a n for those 1 cm or smaller, 2.4 versus 1.4 m m (Fig. 1). Finally, no significant difference was seen b e t w e e n the m e a n post-polypectomy a n d post-fixation meas u r e m e n t s , 7.3 a n d 7.1 m m , respectively (Table 1).
DISCUSSION We h a v e d e m o n s t r a t e d that, on average, the endoscopic e s t i m a t i o n of colon polyp size in h u m a n subjects is significantly l a r g e r t h a n the m e a s u r e m e n t after removal. T h e r e m a y b e several reasons for this difference. First, the endoscopic m e a s u r e m e n t of the polyps was m a d e u s i n g biopsy forceps as a guide. The biopsy forceps did not always open in the largest d i a m e t e r of the polyp as t h e y exited the endoscope c h a n n e l (Fig. 2), which necessitated a more subjective e s t i m a t e t h a n w h e n the forceps opened in the a p p r o p r i a t e orientation. For those polyps m u c h l a r g e r t h a n 8 ram, the est i m a t e of polyp size required an e s t i m a t e of how m a n y multiples larger t h a n the forceps the polyp was VOLUME 43, NO. 1, 1996
20-
o
15 o
i I •
10
~";
o •
!
'
o
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o
lb
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Post-Polypectomy Measurement ( m m )
Figure 1. Graph depicting the difference between the endoscopic estimate of polyp size and the postpolypectomy measurement (mm). Open circle represents size of polyp. Solid line represents the line of equality.
(Fig. 2). Furthermore, the largest diameter was not completely in view for some of the larger polyps. These factors may explain why there was a larger difference in those polyps 1 cm or larger in size. Optical problems m a y also introduce error to endoscopic measurements. It is well known that the distance of the endoscope from an object affects its measurement.S, 9 Objects farther from the endoscope tip appear smaller than when moved directly in front of the tip. We attempted to minimize this error by keeping the biopsy forceps withdrawn to the endoscope tip in the open position and then placing the endoscope tip and forceps directly onto the polyp (Fig. 2). Other authors 9, 10 have evaluated the error introduced to endoscopic measurements by the use of wide-angle lenses in endoscopes, which create image distortion in the periphery of the field such that objects in the periphery will appear compressed. This compression results in an apparent decrease in the size of objects in the periphery. Even if open biopsy forceps are used as a guide, there would be unequal distortion because the forceps are in the center of the field and at least a portion of the polyp lies in the periphery. The systematic error introduced by this '%arrel distortion" would be expected to be more significant for larger polyps. However, this error should be expected to cause an underestimation of size rather than an overestimation. We considered the possible effects related to polyp removal that may cause shrinkage of tissue and thus a smaller postpolypectomy measurement. As a polyp is cauterized there is shrinkage of tissue, but if the polyp is pedunculated the stalk should be the area most affected. Some transmission of heat to the bulk of the polyp would be expected, however, and this V O L U M E 43, NO. 1, 1996
Figure 2. Endoscopic photograph of polyp size estimation using open biopsy forceps. Note that the forceps are withdrawn to the endoscope tip and then placed onto the polyp. Also note that for this particular polyp the forceps did not open in the greatest diameter, thus requiring a more subjective estimation of polyp size.
could lead to a certain amount of desiccation and shrinkage. In our study only pedunculated polyps were included to minimize this effect. A greater amount of shrinkage might be expected with the snare polypectomy of a sessile polyp, since a larger area of the polyp would be cauterized. Another factor that would be expected to significantly affect the size of the polyp before and after removal is vascular collapse. In the in vivo setting there is arterial flow to polyps. The pressure generated by this flow is lost on removal, when the arterial vessels collapse. This collapse and subsequent loss of pressure then leads to a decrease in the size of the polyps as measured after endoscopic removal. Polyp graspers may compress a polyp somewhat during removal, but this would not be expected to induce a permanent deformation. If a polyp is brought through the endoscope channel, either with graspers or by suction, this may tear or lacerate the polyp and alter the largest diameter. In our study, the polyps were not brought through the channel of the endoscope to avoid this problem. Finally, it is generally thought that formalin fLxation m a y cause some shrinkage of the specimen; however, in our study the prefLxation and postfixation measurements were similar. This suggests that shrinkage caused by formalin fLxation is unlikely to be a significant factor in the difference between the endoscopic estimate and the pathologic measurement. The findings of our study are at variance with reports using colon models (including our own previous study), which have shown that endoscopic measurements tend to underestimate the size of an object. The most obvious reason for this would be the factors involved in removal of the in vivo polyp, such as GASTROINTESTINAL ENDOSCOPY
27
shrinkage due to cautery and vascular collapse. In the colon model studies, the artificial objects were not removed and therefore were not subject to shrinkage or damage. The endoscopic underestimation in size in these studies was based on the directly measured size of the artificial polyp before or after placement in a colon model, 5, 6 thereby reflecting endoscopic measurement error. To validate the accuracy of the endoscopic measurements of colon polyps in h u m a n subjects one would need to measure a polyp endoscopically and then somehow measure that polyp directly while still in vivo, such as during a colectomy. As this is obviously impractical, one must understand the limitations of endoscopic measurements. Although the endoscopic estimation of object size may underestimate its true size, as shown in colon model studies, our study in human subjects shows that when polyps are measured after removal they are considerably smaller than the endoscopic estimate. This difference may be explained by endoscopic measurement error, optical distortion, or shrinkage during removal. It is important for endoscopists to be aware of these issues when interpreting data from clinical trials and when assessing cancer risk in individual patients. Most clinical studies, including the National Polyp Study, 11 stratify polyps based on size using 1 cm as a cutoff for "large" polyps. In our study there were nine polyps that were endoscopically estimated to be 1 cm or larger in size. However, only five polyps were in fact 1 cm or larger by measurement after removal. This further illustrates the significant difference between these two measurements and the potential impact on polyp classification and patient cancer risk stratification. These data emphasize the need for systematic and uniform measurement of polyps, particularly in the setting of clinical studies. Outside the research
28 G A S T R O I N T E S T I N A L E N D O S C O P Y
setting the significance of this measurement difference may be less apparent because although patients with larger polyps m a y have a potential increase in cancer risk, all polyps are usually removed and appropriate surveillance examinations performed regardless of their size. Finally, we propose that a standardized approach to the endoscopic estimation of polyp size with open biopsy forceps as described in this study be used for polyp size classification rather than the postpolypectomy measurement. Such standardization would decrease the potential for misclassification of patients in clinical trials based on polyp size. REFERENCES 1. Mute T, Bussy HJR, Morson BC. The evolution of cancer of the colon and rectum. Cancer 1975;36:2251-70. 2. Winawer SJ, Zauber AG, He MN, et al. Prevention of colorectal cancer by colonoscopic polypectomy. N Engl J Med 1993;329: 1977-81. 3. Panish JF. Management of patients with polypoid lesions of the colon: current concepts and controversies, Am J Gastroenterol 1979;71:315-24. 4. Atkins WS, Morson BC, Cuzick J. Long-term risk of colorectal cancer after excision of rectosigmoid adenomas. N Engl J Med 1992;326:658-62. 5. Fennerty MB, Davidson J, Emerson SS, et al. Are all endoscopic measurements of colonic polyps reliable? Am J Gastroenterol 1993;88:496-500. 6. Marguiles C, Krevsky B, Catalano MF. How accurate are endoscopic estimates of size? Gastrointest Endosc 1994;40: 174-7. 7. Waye JD. The accuracy of endoscopic estimates: What is large? What is small? Am J Gastroenterol 1993;88:483-4. 8. Okabe H, Ohida M, Okada N, et al. A new disk method for endoscopic determination of gastric ulcer area. Gastrointest Endosc 1986;32:20-4. 9. Vakil N, Smith W, Bourgeios K, Everbach EC, and Knyrim K. Endoscopic measurement of lesion size: improved accuracy with image processing. Gastrointest Endosc 1994;40:178-83. 10. Sonnenberg A, Giger M, Kern L, et al. How reliable is determination of ulcer size by endoscopy? BMJ 1979;2:1322-4. 11. Winawer SJ, Zauber AG, O'Brien MJ, et al. Randomized comparison of surveillance intervals after colonoscopic removal of newly diagnosed adenomatous polyps. N Engl J Med 1993;328: 901-6.
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