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1 Department of Radiology, University of Wisconsin Medical School, E3/311
Clinical Science Center, 600 Highland Ave., Madison, WI 53792-3252.
2 Department of Radiology, National Naval Medical Center, Bethesda, MD
20889.
3 Department of Radiology and Nuclear Medicine, Uniformed Services University of
the Health Sciences, Bethesda, MD 20814.
4 Department of Radiology, Walter Reed Army Medical Center, Washington, DC
20307-5001.
5 Department of Gastroenterology, Naval Medical Center-San Diego, San Diego, CA
92134-5000.
Received March 4, 2004;
accepted after revision May 25, 2004.
Supported in part by Department of Defense Advances in Medical Practice
funds and by the Society of Computed Body Tomography and Magnetic
Resonance.
Abstract
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SUBJECTS AND METHODS. Same-day CTC and optical colonoscopy were performed on 1,339 asymptomatic adults. A standard matching algorithm for polyp size and location was used. For each potential polyp detected on CTC, the level of diagnostic confidence was prospectively rated on a 3-point scale (1, least certain; 2, intermediate; and 3, most certain).
RESULTS. For CTC-detected lesions 6 mm or larger, diagnostic confidence levels of 1, 2, and 3 corresponded to matched polyps on optical colonoscopy in 33.3% (45/135), 50.0% (103/206), and 66.8% (157/235) of cases, respectively (p < 0.01). Similar trends were present for categories of lesions that measured 6–7 mm, 8–9 mm, and 10 mm or larger, rising to a match rate of 82.1% (55/67) for lesions 10 mm or larger that were diagnosed with a level-3 confidence rating. The likelihood that a matched polyp was adenomatous increased with greater levels of diagnostic confidence. Of note, level-3 confidence for lesions measuring 8–9 mm on CTC more often yielded a matching neoplasm on optical colonoscopy than level-1 or level-2 confidence for lesions measuring 10 mm or larger (60.3% [35/58] vs 20.8% [10/48]; p < 0.0001).
CONCLUSION. Greater diagnostic confidence for an individual lesion detected on CTC correlates with a significantly increased likelihood that a matching polyp will be found on optical colonoscopy and that this matched polyp will be neoplastic. Although polyp size represents the primary criterion for CTC screening algorithms, this data could help guide the decision to opt for noninvasive CTC surveillance versus optical colonoscopy for polypectomy.
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5 mm) do not need to be referred for immediate polypectomy.
However, controversy surrounds the appropriate management of
intermediate-sized lesions (6–9 mm)
[1,
2]. Although relatively little
is known about the natural history of subcentimeter polyps
[3], the existing body of data
supports the validity of noninvasive surveillance
[4,
5]. The likelihood that a lesion detected on CTC will correspond to a polyp on optical colonoscopy probably depends on a variety of factors, including size, morphology, quality of the colonic preparation, and degree of luminal distention. The purpose of this study was to assess prospectively the effect of the interpreter's overall diagnostic confidence in lesions detected on CTC on the likelihood that a matching lesion will be subsequently found on optical colonoscopy. This information could be useful for clinical decision making regarding the next appropriate step for CTC-detected lesions and could affect the development of CTC screening algorithms.
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In our colonic cleansing regimen, patients ingested split doses of 45 mL of sodium phosphate and 10 mg of bisacodyl. Patients also consumed split doses of 250 mL of dilute barium (2.1% weight/volume) and 60 mL of water-soluble iodinated contrast material (diatrizoate meglumine and diatrizoate sodium) for stool tagging and electronic fluid subtraction, as has been described previously [6]. Our CT protocol and CTC interpretation technique have also been detailed previously [1, 6]. In brief, after patient-controlled rectal insufflation of room air, breath-hold supine and prone CT acquisitions were obtained on 4- and 8-MDCT scanners (LightSpeed Plus and LightSpeed Ultra, respectively; GE Healthcare). The CT technique entailed 1.25- to 2.5-mm collimation, 13.5- to 15.0-mm/sec table speed, and 1-mm reconstruction intervals.
Prospective interpretation of CTC studies was performed by one of six radiologists using a commercially available CT colonography system (V3D Colon, version 1.2; Viatronix). The 3D endoluminal fly-through display was generally used for primary polyp detection, and the 2D images were used mainly for confirmation and problem solving. For all detected lesions prospectively identified as polyps on CTC, a level of diagnostic confidence or certainty was assigned, using a 3-point scale (1, least certain; 2, intermediate; or 3, most certain). Although assignment of confidence levels was subjective and somewhat individualized, typical features of a CTC-detected lesion with a diagnostic confidence level of 3 could include a well-circumscribed polypoid lesion clearly identifiable on both supine and prone views. Features associated with a level-1 confidence might include an ill-defined margin, internal heterogeneity, coexisting retained debris or poor luminal distention, and a lesion not well seen on one of the images. Confidence levels could also be expected to increase with larger polyp sizes. Polyp morphology was prospectively recorded as pedunculated, sessile, or flat. Lesions were measured on the 3D image using electronic calipers and were recorded by segment.
Optical colonoscopy was performed immediately after prospective CTC interpretation using standard commercial video colonoscopes. The colonoscope was advanced to the cecum and then sequentially withdrawn into more distal segments for polyp detection. Polyps were measured using a calibrated linear probe, which is more accurate than either visual or biopsy-forceps estimation [7]. Our polyp-matching algorithm required agreement between the findings on CTC and on optical colonoscopy for size (within a 50% margin of error) and location (within the same or adjacent segment) [1]. The colonoscopist completed the evaluation of a given segment before being apprised of the CTC results for the previous segment. If a polyp 5 mm or larger was detected on CTC but not on prospective optical colonoscopy, the colonoscopist closely reexamined that segment and was allowed to review the CTC images for guidance. This step provided an enhanced reference standard that surpassed optical colonoscopy alone [1].
All retrieved polyps during optical colonoscopic polypectomy were sent for histologic examination. Polyps measuring 6 mm and greater, particularly adenomas, represented the lesions of primary interest for this study. Given their lack of clinical significance, diminutive lesions (< 5 mm) were excluded from the final analysis [8].
The effect of interobserver variability was assessed by having a second radiologist interpret 100 randomly selected CTC studies. The radiologist was blinded to the results of both optical colonoscopy and the initial CTC interpretation. The same diagnostic confidence scale was applied to all lesions detected at this second interpretation.
For analysis of diagnostic confidence data, we grouped the polyps into the following categories: 6–7 mm, 8–9 mm, and 10 mm and larger. These size categories were used in our initial algorithm for primary CTC screening to help determine the next appropriate management step. Statistical analysis for significance testing was made using chi-square and Fisher's exact tests, as appropriate.
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The differences among confidence levels were statistically significant for all polyp size categories (p < 0.05). Pair-wise comparisons between two individual levels were also statistically significant (p < 0.05), except for confidence levels 1 and 2 at 8–9 mm and levels 2 and 3 at 6–7 mm. The relative separation between match rates for levels 1 and 3 increased for larger polyps (Fig. 1). For CTC-detected lesions measuring 10 mm or more, only 25% of lesions diagnosed with level-1 confidence corresponded to matching polyps on optical colonoscopy, compared with 82% of those diagnosed with level-3 confidence (p < 0.001).
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Of all 380 polyps 6 mm or larger found on optical colonoscopy, 234 (61.6%) were adenomatous, 197 of which were prospectively detected on CTC (sensitivity, 84.2%). Most CTC-detected adenomas were prospectively assigned a level-3 diagnostic confidence level, including 118 (59.9%) of 197 adenomas 6 mm or larger, 79 (71.8%) of 110 adenomas 8 mm or larger, and 44 (81.5%) of 54 adenomas 10 mm or larger. The likelihood that a CTC-detected lesion corresponded to an adenomatous polyp on optical colonoscopy and at histologic evaluation increased with higher levels of diagnostic confidence at all thresholds of polyp size (Table 1). The differences among confidence levels were statistically significant for all polyp size categories (p < 0.05). Pair-wise comparisons between individual levels were also statistically significant (p < 0.05), except for levels 1 and 2 at 8–9 mm and at 10 mm or larger, and levels 2 and 3 at 6–7 mm. The pair-wise comparison between levels 1 and 3 was highly significant for all polyp size categories (p < 0.001). As we found with the total sample of polyps, we observed greater separation between adenoma match rates for levels 1 and 3 with larger polyp sizes (Fig. 2), including increases from 9% to 60% for polyps measuring 8–9 mm and from 10% to 66% for polyps measuring 10 mm or larger.
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Important differences in CTC polyp detection can occur near a given threshold for polyp size. For instance, a lesion assigned a CTC confidence level of 3 that measures just below the 10-mm cutoff (i.e., an 8- or 9-mm lesion) would be more likely to represent a true polyp than a lesion measuring 10 mm or larger that was assigned a CTC confidence level of 1 (75.9% [44/58] vs 25.0% [5/20]; p < 0.0001). Similarly, an 8- or 9-mm lesion assigned a CTC confidence level of 3 would be more likely to represent a neoplasm (adenoma) than a CTC confidence level–1 or –2 lesion measuring 10 mm or larger (60.3% [35/58] vs 20.8% [10/48]; p < 0.0001).
For sessile, flat, and pedunculated polyp morphologies, the trend of increasing likelihood of an optical colonoscopy match for lesions diagnosed with increasing diagnostic certainty was generally evident (Table 1). The match rate for level-3 diagnostic confidence was greater than that for level-l confidence for all polyp morphologies at all size categories. For sessile lesions, the match rate progressively increased from the least to the most certain levels, and the overall differences for each size category were statistically significant (p < 0.01), except for all polyps in the 8–9 mm group. The pair-wise comparison between levels 1 and 3 for sessile morphology was statistically significant (p < 0.01) for all size categories. Multiple additional pair-wise comparisons were statistically significant for various other combinations of polyp size and morphology but were too numerous to list individually here. One interesting finding was that for CTC-detected flat lesions, the level-2 match rates for 6- to 9-mm lesions were relatively low (0–30%), whereas the level-2 match rates for similarly sized pedunculated lesions were considerably higher (40–86%). For CTC-detected pedunculated lesions measuring 10 mm or larger, 97% (29/30) matched with polyps on optical colonoscopy, and 90% (27/30) were neoplastic.
In the 100 randomly selected cases that were double-interpreted, 13 lesions 6 mm or larger were identified on the initial CTC interpretation but not on the second, and seven lesions were identified on the second interpretation but not on the first. Of these 20 "lesions" seen by just one radiologist, none was assigned a diagnostic confidence level of 3. By comparison, of the 36 lesions 6 mm or larger detected and agreed upon by both reviewers, 21 (58.3%) were assigned a diagnostic confidence level of 3. Over half of these 36 agreed-upon lesions matched adenomas on subsequent optical colonoscopy, compared with only a quarter of the 20 lesions identified by just one reviewer. Overall, the second CTC interpretation uncovered only two additional adenomas in these 100 patients. One patient already had a separate adenoma in the 6-mm-or-larger category detected prospectively by the first reviewer.
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5 mm) can be monitored at routine screening
intervals [1,
8]. The appropriate management
for CTC-detected lesions measuring 6–9 mm, however, has not been
established. Noninvasive surveillance of small lesions (< 10 mm) would seem
prudent because less than 1% of subcentimeter lesions (including both
adenomatous and nonadenomatous polyps) are histologically advanced. Most never
progress to cancer, and the typical dwell time for even a 10-mm adenoma is
possibly a decade or longer [1,
9]. Furthermore, in a
well-designed optical colonoscopy trial, follow-up of unresected subcentimeter
polyps was shown to be safe for as long as 3 years
[5].
Polyp size has served as the primary criterion by which CTC performance has
been assessed. A major reason is that polyp size serves as a rough surrogate
for histologic examination because most diminutive lesions ( 5 mm) are
nonadenomatous and most polyps 6 mm or larger are adenomatous
[1,
10]. However, polyp detection
on CTC is not always straightforward, and factors beyond polyp size alone
affect the degree of diagnostic certainty.
The diagnostic confidence level that a reviewer may assign to a given CTC finding believed to represent a polyp encompasses a complex blend of factors that cannot be easily quantified. Inherent characteristics of the detected lesion, such as its size, morphology (sessile, pedunculated, or flat), and relationship to the colonic folds all play a role. Other factors depend more on the overall study quality, such as the quality of colonic preparation and the degree of luminal distention. Reviewer experience is likely to be another important contributing factor. Our findings show that polyp size and morphology have an effect on diagnostic confidence, but we did not directly assess the relative contribution of the various remaining factors. Regardless, our findings show that there is clearly a reproducible relationship between the degree of diagnostic certainty and the likelihood of finding a matching polyp on optical colonoscopy.
This new data on diagnostic confidence could potentially play a role in primary CTC screening. For instance, noninvasive surveillance of a CTC-detected 8- or 9-mm lesion assigned a level-1 diagnostic confidence seems quite reasonable because the likelihood of finding a matching adenoma on optical colonography may be less than 10%. The likelihood of finding a matching adenoma rises to 60% for a CTC-detected 8- or 9-mm lesion assigned a level-3 diagnostic confidence; although noninvasive follow-up with CTC would still be a reasonable option, some may consider polypectomy to be appropriate for such cases. Similarly, the difference between the likelihoods of finding an adenoma match for a CTC-detected 10-mm lesion assigned a level-1 confidence rating (10%) and one assigned a level-3 confidence rating (66%) could influence management, particularly in patients with significant comorbidities. Fortunately, most polyps 8 mm or larger identified on CTC were assigned the highest level of diagnostic confidence, which bodes well for the positive predictive value in larger, more clinically significant polyps. The increased uncertainty seen with CTC-detected lesions smaller than 8 mm correlates with the decreased by-patient specificity that we have reported, which further supports surveillance of these possible lesions instead of immediate invasive polypectomy [1].
With more accumulated experience in interpreting CTC studies and continued improvement in the technique, the number of lesions assigned to the lowest level of diagnostic certainty can be expected to decrease. This decrease could result from greater specificity gained both by identifying fewer false-positive lesions and by learning to assign a higher confidence level to more likely matches.
There are limitations to our study. As we mentioned, the subjective and multifactorial nature of what constitutes a certain diagnostic confidence level prevents us from providing strict definitions at this time, but general guidelines for each category could evolve. For instance, specific language could be incorporated, such as "likely polyp" for level-3 confidence, "possible polyp" for level-2 confidence, and "unlikely polyp" for level-1 confidence. Another limitation is the use of an imperfect reference standard (optical colonoscopy). Through the use of the segment-by-segment unblinding of findings, we created an enhanced reference standard and were able to show that at least some of the false-positive findings on CTC actually represented false-negative findings on optical colonoscopy. However, there were likely additional true polyps detected on CTC that simply were not found on optical colonoscopy despite a second look. Further studies are required to assure reproducibility in CTC polyp measurement and also to compare primary 2D versus 3D polyp measurement techniques. The fact that our analysis was primarily focused on the polyp and not on the patient was also a limitation; per-patient confidence levels were not prospectively obtained.
Interobserver variability in CTC interpretation has been identified as a potential concern for implementation of a screening program [11]. However, using a primary 3D approach for polyp detection, we have shown good interobserver agreement, despite the low prevalence of significant polyps in a screening population [1].
In conclusion, the process of ultimately deciding whether a detected abnormality should be called a polyp on CTC is complex and involves a variety of factors beyond just lesion size and morphology. Despite this fact, our findings indicate that increased diagnostic confidence for an individual lesion detected on CTC correlates with a significantly increased likelihood that a matching polyp will be found on optical colonoscopy and that this matching polyp will be neoplastic. This additional information could help guide radiologists, patients, and referring physicians as to the next appropriate step when a potential polyp is detected on CTC.
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= level-1 confidence, 
= level-2 confidence, 
= level-3
confidence.