Artificial intelligence-assisted detection and classification of colorectal polyps under colonoscopy: a systematic review and meta-analysis

Introduction

Colorectal cancer (CRC), is the third most common cancer worldwide and poses a considerable threat to public health due to its high mortality (1). Colorectal adenoma (CRA) and serrated polyps have been proven to be precancerous lesions of CRC. Colonoscopy is performed for the detection and resection of these lesions and been demonstrated to reduce the incidence and mortality of CRC (2,3). A large US cohort study (4) showed that the mortality rate of CRC was reduced by approximately 70% by colonoscopy screening and on-demand therapeutics. There is evidence suggesting that the adenoma detection rate (ADR) can indicate the colonoscopy quality and that ADR is inversely proportional to postcolonoscopy CRC risk (5,6). However, due to operator-dependent limitations, polyps smaller than 5 mm may be missed at colonoscopy with an overall missed diagnosis rate for adenomas as high as 27% (7-9). Colorectal polyps can be divided into neoplastic and nonneoplastic polyps and require different treatment strategies. Therefore, there is an urgent need to reduce the miss rate of polyps and improve the accuracy of polyp pathology evaluation under endoscopy.

Artificial intelligence (AI) emerged as a scientific discipline in 1956, but what is now shown to people is more of a technology, which refers to systems with the ability to reason, discover meaning, generalize, or learn from past experience, thus able to perform tasks normally requiring human interaction (10). At present, artificial intelligence has been applied to many aspects of human life, such as transportation, entertainment, trade, medical care and so on. In contemporary society, Artificial intelligence has been gradually applied to the field of digestive endoscopy. Notably, it has been employed in the detection and classification of colorectal polyps. However, sensitivity and specificity differences have been reported in the results of AI-assisted colorectal polyp diagnosis (11-13). Although there have been meta-analysis articles on the diagnostic performance of AI-assisted colonoscopy for colorectal polyps, most articles only focus on the detection of adenomas or only study one type of AI system (14-19), and the original research articles are constantly updated. Thus, we aimed to systematically review and meta-analyze the diagnostic quality of AI-based technologies in both the detection and characterization of colorectal polyps combining with updated articles. This review has been registered on PROSPERO: Diagnostic performance of artificial intelligence in the detection and classification of colorectal polyp: a systematic review and meta-analysis; ID: CRD42021256884. We present the following article in accordance with the PRISMA reporting checklist (available at https://dx.doi.org/10.21037/atm-21-5081).

Methods

Search strategy

We searched all published articles evaluating the diagnostic performance of AI-assisted detection and classification of colorectal polyps in PubMed, Web of science, Embase, and Cochrane Library until April 2021. The search strategy was based on the following keywords: {[“artificial intelligence”] OR [“convolutional neural networks”] OR [“deep learning”] OR [“computer-aided”]} AND {[“colonoscopy”] OR [“endoscopy”]} AND {[“colon”] OR [“colonic”] OR [“colorectal”]} AND {[“polyp”] OR [“polyps”] OR [“adenoma”] OR [“adenomas”]}.

Inclusion and exclusion criteria

The inclusion criteria were as follows: (I) studies reporting AI-assisted detection and classification of colorectal polyps in international publications; (II) studies containing patients, endoscopic images, or videos receiving AI-assisted diagnosis of colorectal polyps with definite diagnostic results; (III) studies whose diagnostic methods included AI-assisted diagnosis (including detection and classification of colorectal polyps) without restrictions of algorithms, with those studies reporting the classification of colorectal polyps being based on histopathological diagnosis; and (IV) studies providing accurate diagnostic data.

The exclusion criteria were as follows: (I) non-English language studies; (II) case-reports, reviews, meeting abstracts, comments, letters, systematic reviews, or study protocols; (III) studies with an irrelevant subject; (IV) studies with incomplete data; and (V) studies with a small sample size.

Study selection and data extraction

Study selection and data extraction were completed independently by 2 investigators (Wang and Mo). Based on the inclusion and exclusion criteria, the candidate articles were screened by reviewing their titles and abstracts at first. Relevant studies were then further evaluated through a reading of the full text. Finally, search results were cross-checked by 2 investigators, and the discrepancies were resolved by a third investigator (Zhong).

Data extracted from studies were placed onto a standard spreadsheet template using Microsoft Excel. For each study, the following data were extracted: the first author’s name, publication year, country where the study was conducted, data source, type of study (detection or classification of colorectal polyps), type of observation (image and video verification or real-time monitoring), AI algorithms, test objects, sample group, and original data reflecting the diagnostic performance [i.e., true positive (TP), false positive (FP), true negative (TN), and false negative (FN)]. For studies involving multiple AI structure verification, the method for merging all structures was applied to raw data processing. For studies verifying the same AI system in different databases, the method for merging all databases was used for raw data processing until the original data were complete. For the studies splitting the same database into different subdatabases, only the original data of the original database were included. For studies that listed the original data of colorectal polyps diagnosed by experts and nonexperts, the data of experts and nonexperts were entered separately before being included in the meta-analysis. For the studies listing the diagnosis from experts and nonexperts one by one, the data of experts and nonexperts were added separately before being included in the meta-analysis.

Quality assessment

RevMan 5.4 (Cochran Training) was used to assess the quality of all included literature, and the risk of bias was evaluated by 2 investigators (Wang and Mo) independently adopting Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2) criteria. For each item, an evaluation of “yes” or “unclear” or “no” was given, and each item was classified as “high risk” or “unclear” or “low risk”. In terms of the applicability evaluation, each item was classified as “high concern” or “unclear concern” or “low concern”.

Objective and outcome indicators

The objective of this study was to explore the diagnostic performance of AI-assisted detection and classification of colorectal polyps. The outcome indicators consisted of pooled sensitivity, pooled specificity, pooled positive likelihood ratio (PLR), pooled negative likelihood ratio (NLR), pooled diagnosis odds ratio (DOR), summary receiver operating characteristic curves (SROC), and the area under the curve (AUC), all which were calculated based on TP, FP, TN and FN.

Statistical analysis

Statistical analysis was performed by Stata 13.0 (StataCorp). The heterogeneity caused by a threshold effect was tested by Spearman correlation analysis, and the heterogeneity caused by a non-threshold effect was tested by Cochran-Q and I² value, where <50% was low and >50% was high; the fixed effects model and the random effects model were used to merge respectively. Four grid tables for AI-assisted detection and classification of colorectal polyps were listed, and the sensitivity (SEN), specificity (SPE), PLR, NLR, DOR, and their 95% confidence interval (95% CI) were calculated. The probabilities before and after the test were observed through Bayesian analysis, and the changes of positive and negative results were evaluated. The sensitivity analysis of our study was to eliminate studies with low quality or different efficacy evaluation criteria, and then conducted merger analysis to compare with the merger effect before elimination, so as to explore the impact of the elimination study on the merger effect. If there was no significant change in the amount of merger effect before and after elimination, the result was relatively stable. If there was a large difference or even an opposite conclusion, it indicated that the stability of the results was poor. Furthermore, we drew the SROC curve, calculated the AUC, and evaluated the diagnostic value. The AUC values were interpreted as follows: no diagnostic value if AUC <0.5, low diagnostic value if 0.5≤ AUC <0.7, high diagnostic value if 0.7≤ AUC <0.9, and extremely high diagnostic value if AUC >0.9. Finally, the publication bias of the included studies was quantitatively assessed by bias analysis.

Results

Literature screening

According to the above retrieval strategy, a total of 709 articles were identified from the databases (PubMed 210, Web of Science 100, Embase 326, Cochrane Library 73). In addition, 5 articles were obtained after screening the published articles of related systematic reviews and meta-analysis, which totaled 714 records. After 284 articles were excluded as duplicates and 372 articles were excluded on the basis of titles and abstracts, 15 studies on polyps detection (9,12,13,20-31), 10 studies on polyps classification (32-41), and 1 (42) which was a combination of both were identified as being appropriate for full-text review. The process of literature screening and inclusion is shown in Figure 1.

Figure 1 The process of literature screening and inclusion.

The basic characteristics of the included literature

Studies in this systematic review included 15 preclinical studies on polyps detection (9,12,13,20-31), 10 preclinical studies on polyps classification (32-41), and 1 (42) which was a combination of both. In terms of the detection of polyps, 5 studies exploring the (9,23,26-28) the performance of real-time AI-assisted detection reported no TN. There were 4 studies that (13,20,22,30) used pictures or videos with polyps to verify AI-assisted detection performance, in which the reported number of TN was 0. However, the other trials all reported TN. In terms of the classification of polyps, except for a (37) real-time AI-assisted classification study, all AI-assisted classification studies used pictures or videos. A further 5 studies (33,35-38) compared the diagnostic performance of AI, experts, and non-experts, while 1 study (40) only compared the diagnostic performance of AI with that of experts. Among all the literature, only the studies of Jia (22) and Patel (39) assessed the diagnostic performance of convolutional neural network (CNN) systems with different structures, which was a kind of feedforward neural networks with depth structure including convolution calculation, and also one of the representative algorithms of deep learning. The basic characteristics and diagnostic characteristics of the included literature are shown in Table 1 and Table 2, respectively.

Results of literature quality evaluation

Among the 26 included articles, the overall quality of the research was medium. Nine studies (20,22,29,32,34-36,38,40) were classified as “high risk” in terms of patient selection due to the lack of indication of whether the included cases or polyp images were continuous and randomized and due to the exclusion criteria of the inappropriate cases. One study (40) was rated as “high risk” in terms of flow and timing because not all endoscopic images were included in the outcome analysis. Four studies (35-38) were listed as “high concern” in terms of patient selection, mainly because enlarged endoscopic images were included in the studies. One study (12) was “high concern” in terms of the reference standard because the existence of polyps was confirmed by different endoscopists. The quality evaluation results of the included literature are shown in Figure 2.

Figure 2 Literature quality evaluation map.

Meta-analysis

Meta-analysis of AI-assisted detection of colorectal polyps

A total of 16 studies reported the performance of AI-assisted detection of colorectal polyps. The TN was set to 0 in studies reporting no TN. For the pooled analysis of 16 studies, the heterogeneity (I²) of the Sen was 99.85 (P<0.01), and the Sen was 0.95 (95% CI: 0.89–0.98), as shown in Figure 3. In terms of literature analysis, the 19% probability after the test was calculated from the probability before test and PLR [1] in the positive test results, while the 97% probabilities before and after the test were calculated from the pretest probability and NLR (114.31) in the negative test results (Figure 4). The AUC under the SROC curve was estimated to be 0.79 (95% CI: 0.79–0.82), as shown in Figure 5. Moreover, the publication bias of included literature was quantitatively analyzed, and the results are shown in Figure 6 (P=0.07>0.05) and suggested no significant publication bias.

Figure 3 Meta-analysis of the sensitivity and specificity of AI-assisted polyp detection. AI, artificial intelligence.

Figure 4 Bayesian analysis of posttest probability and pretest probability (polyp detection).

Figure 5 SROC curve of AI-assisted polyp detection. SROC, summary receiver operating characteristic curve; AI, artificial intelligence.

Figure 6 Funnel plot of included literature (polyp detection).

AI-assisted detection of colorectal polyps: a subgroup meta-analysis of studies with TN

A total of 7 studies with TN reported the performance of AI-assisted detection of colorectal polyps. In the pooled analysis of the 7 studies, the heterogeneity (I²) of the sensitivity was 99.95 (P<0.01), and the sensitivity was 0.88 (95% CI: 0.81–0.92). The heterogeneity (I²) of the specificity was 99.99 (P<0.01), and the specificity was 0.95 (95% CI: 0.94–0.96), as shown in Figure 7. In the SROC curve, the AUC was 0.97 (95% CI: 0.95–0.98), as shown in Figure 8.

Figure 7 Meta-analysis of the sensitivity and specificity of AI-assisted polyp detection (including TN subgroup). AI, artificial intelligence; TN, true negative.

Figure 8 SROC curve of AI-assisted polyp detection (including TN subgroup). SROC, summary receiver operating characteristic curve; AI, artificial intelligence; TN, true negative.

Meta-analysis of AI-assisted classification of colorectal polyps

A total of 11 studies reported the performance of AI-assisted classification of colorectal polyps for distinguishing neoplastic and nonneoplastic polyps. The heterogeneity (I²) of the sensitivity was 99.37 (P<0.01), and the heterogeneity (I²) of the specificity was 99.17 (P<0.01). The sensitivity was 0.92 (95% CI 0.88–0.95), and the specificity was 0.82 (95% CI: 0.71–0.89). The PLR was 5.0 (95% CI: 3.1–8.2), and the NLR was 0.10 (95% CI: 0.06–0.15). The DOR was 51 (95% CI: 22–117), as shown in Figure 9. In terms of literature analysis, the 57% of the posttest probability was calculated from the pretest probability and PLR [5] in the positive test results, while the 2% of the posttest probability was calculated from the pretest probability and NLR (0.09) in the negative test results (Figure 10). In the SROC curve, the AUC was 0.94 (95% CI: 0.92–0.96), as shown in Figure 11. The publication bias of included literature was quantitatively analyzed, and the results are shown in Figure 12 (P=0.13>0.05) and suggested no significant publication bias.

Figure 9 Meta-analysis on sensitivity and specificity of AI-assisted polyp classification. AI, artificial intelligence.

Figure 10 Bayesian analysis of posttest probability and pretest probability (polyp classification).

Figure 11 SROC curve of AI-assisted endoscopic polyp classification. SROC, summary receiver operating characteristic curve; AI, artificial intelligence.

Figure 12 Funnel plot of included literature (polyp classification).

AI-assisted classification of colorectal polyps: a subgroup meta-analysis of diminutive polyps (≤5 mm)

A total of 8 studies reported the performance of AI-assisted classification of diminutive polyps (≤5 mm). The heterogeneity (I²) of the sensitivity was 69.22 (P<0.01), and the heterogeneity (I²) of the specificity was 96.86 (P<0.01). The sensitivity was 0.95 (95% CI: 0.94–0.97), and the specificity was 0.88 (95% CI: 0.74–0.95). The PLR was 8.2 (95% CI: 3.5–19.3), the NLR was 0.05 (95% CI: 0.04–0.07), and DOR was 155 (95% CI: 60–400), as shown in Figure 13. The AUC under SROC curve was estimated to be 0.97 (95% CI: 0.95–0.98), as shown in Figure 14.

Figure 13 Meta-analysis of the sensitivity and specificity of AI-assisted diminutive polyp classification. AI, artificial intelligence.

Figure 14 SROC curve of AI-assisted classification of diminutive polyps. SROC, summary receiver operating characteristic curve; AI, artificial intelligence.

AI-assisted classification of colorectal polyps: a subgroup meta-analysis of magnification endoscopy

A total of 4 studies reported the performance of AI-assisted classification of colorectal polyps under magnification endoscopy. The heterogeneity (I²) of the sensitivity was 89.49 (P<0.01), and the heterogeneity (I²) of the specificity was 93.28 (P<0.01). The sensitivity was 0.94 (95% CI: 0.92–0.96), and the specificity was 0.95 (95% CI: 0.80–0.99). The PLR was 17.4 (95% CI: 4.4–69.3), the NLR was 0.06 (95% CI: 0.04–0.09), and the DOR was 293 (95% CI: 51–1,673), as shown in Figure 15. The AUC under the SROC curve was estimated to be 0.97 (95% CI: 0.95–0.98), as shown in Figure 16.

Figure 15 Meta-analysis of the sensitivity and specificity of the AI-assisted magnification endoscopy subgroup. AI, artificial intelligence.

Figure 16 SROC curve of the AI-assisted magnification endoscopy subgroup. SROC, summary receiver operating characteristic curve; AI, artificial intelligence.

Discussion

AI technology has been applied in many areas of clinical diagnosis and treatment, including intelligent inspection, diagnosis, treatment, monitoring, and prevention, with the common purpose of improving the quality of medical health (43). In the diagnosis and treatment of colorectal polyps under colonoscopy, the current applications of AI mainly include polyp detection and classification (44-46). The former mainly aims at improving the detection rate for polyps and adenomas, while the latter mainly focuses on the classification of neoplastic polyps and nonneoplastic polyps, with the goal of improving the quality of colonoscopy and the accuracy of endoscopists (especially young endoscopists). For the classification of colorectal polyps, AI is usually used to capture the local features of polyps involving texture, shape and color from the endoscopic target area, and summarize the hidden features in the image. The local features and hidden features are fused into AI data analysis to classify the images of neoplastic polyps and non-neoplastic polyps.

We conducted a meta-analysis to examine the current status of diagnostic performance for AI-assisted technologies in the detection and classification of colorectal polyps. We found several machine learning methods being applied for polyp detection and characterization in numerous studies. In terms of the detection of colorectal polyps, although the meta-analysis showed no prominent publication bias in the included literature, the heterogeneity was statistically significant, which may be relevant to the absence of TN in some studies. Our results highlight a high diagnostic accuracy of AI-assisted polyp detection, with a sensitivity of 95% and an AUC of 0.79. Results concerning the reliability of specificity were suspect, as there was no reported TN in some studies. Thus, we performed a subgroup analysis in studies reporting TN, and results demonstrated a sensitivity of 88% and a specificity of 95% with an AUC of 0.97, indicating a missed diagnosis rate and a misdiagnosis rate of 12% and 5%, respectively. These outcomes demonstrated good results for AI techniques in detecting polyps. Our results suggested an increase of 10% in ADR in patients with the use of AI for polyp detection compared with patients who achieved standard colonoscopy.

Various of factors may contribute to the lack of applicability of the AI techniques in clinical practice. A considerable proportion of research into AI-assisted polyp detection and has been carried out in China and Japan, but differences in polyp biology and tumorigenesis may limit the application of findings in endoscopic practice. Furthermore, only AI technologies that enable real-time detection have clinical application value in endoscopy. However, most recent studies used endoscopic high-quality images or videos to train and verify the performance of AI-assisted detection, which might have led to an overestimation of the AI’s detection performance. Meanwhile, several published clinical studies (23,24,35) have shown that for real-time detection, the AI may be affected by the quality of intestinal preparation, intestinal mucosal folds or other intestinal diseases, and foreign bodies, resulting in false positives. Therefore, a further development of AI diagnostic models is needed to reduce interference factors in real-time detection.

Results evaluating the classification performance of AI in colorectal polyps showed no significant publication bias in the included literature. More importantly, our meta-analysis demonstrated a high diagnostic accuracy of AI-assisted polyp classification with a sensitivity of 92% and a specificity of 82%, indicating a missed diagnosis rate of 8% and a misdiagnosis rate of 18%. The pooled PLR was 5.0, suggesting that the probability of correctly classifying colorectal polyps was 5 times more than that of misclassifying. Moreover, the pooled NLR was 0.10, revealing that the probability of incorrect classification is 0.1 times higher than that of correct classification. DOR, the diagnostic odds ratio, indicated the strength of the association between the diagnostic results of tests and diseases. Our study yielded a pooled DOR of 51, indicating the high diagnostic value of AI-assisted detection and classification in polyps. Additionally, Bayesian test analysis showed that the overall correct diagnostic rate of endoscopy increased by 37% and the overall false diagnostic rate decrease d by 18% with the use of AI. The AUC of the SROC curve was 0.94, which confirmed the high value of AI in the classification of colorectal polyps. Considering the obvious heterogeneity of included studies, which may be related to differences in the size of polyps, we performed a subgroup analysis of diminutive polyps (≤5 mm). The results showed a lower heterogeneity than before, and no significant publication bias in the included literature. The sensitivity of 95% and a specificity of 88% indicated a missed diagnosis rate and misdiagnosis rate of 5% and 12%, respectively. Meanwhile, an AUC of 0.97 suggested that AI-assisted classification of diminutive polyps also has high auxiliary diagnostic value.

In the comparison of the diagnostic performance of AI, endoscopic experts, and nonexperts in the classification of colorectal polyps, a previously published meta-analysis (14) had shown the diagnostic performance of AI to be equivalent to that of endoscopic experts and significantly better than that of nonexperts. Moreover, the AUC obtained from our meta-analysis showed that AI had an extremely high diagnostic performance in the classification of polyps, while current studies comparing the classification performance of AI with experts and nonexperts seem to require further investigation.

The subgroup analysis of different types of endoscopies produced a sensitivity of 94% and a specificity of 95%, indicating a missed diagnosis rate and misdiagnosis rate of 6% and 5%, respectively. The AUC was estimated to be 0.97, suggesting a high auxiliary diagnostic value of AI-assisted classification under magnification endoscopy. Cell endoscopy is currently used in clinic, and research into AI for polyp classification and evaluation of infiltration depth under cell endoscopy may intensify substantially in the near future.

Two inevitable limitations to our study should be acknowledged. First, due to the differences in AI systems, a large degree of heterogeneity was found among the included study groups, and thus the results should be further scrutinized. Second, a few of the including studies did not clarify the specific types of endoscopies, and the specificity and sensitivity of AI for different types of endoscopes could not be further analyzed.

In conclusion, our study demonstrated the high clinical value of AI in the detection and classification of colorectal polyps, suggest that AI may be used as a novel auxiliary diagnostic method in the upcoming years. Looking to the future, AI-assisted diagnosis should be developed to be more accurate and rapid, which will be more conducive to the real-time detection and classification of colorectal polyps and the evaluation infiltration depth. Only in this way can the application of AI in endoscopy improve the detection rate and classification accuracy of colorectal polyps and lighten the workload of endoscopists, and promote the diversified and balanced development of medical resources.

Acknowledgments

Funding: Special funding for this study was received from the Guangdong Provincial Hospital of Traditional Chinese Medicine (No. YN10101914) and the Guangzhou University of Chinese Medicine “Double First -Class” and High-level University Discipline Collaborative Innovation Team (No. 2021xk58).

Footnote

Reporting Checklist: The authors have completed the PRISMA reporting checklist. Available at https://dx.doi.org/10.21037/atm-21-5081

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://dx.doi.org/10.21037/atm-21-5081). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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