Validation of mitotic cell quantification via microscopy and multiple whole-slide scanners

Abstract

Background The establishment of whole-slide imaging (WSI) as a medical diagnostic device allows that pathologists may evaluate mitotic activity with this new technology. Furthermore, the image digitalization provides an opportunity to develop algorithms for automatic quantifications, ideally leading to improved reproducibility as compared to the naked eye examination by pathologists. In order to implement them effectively, accuracy of mitotic figure detection using WSI should be investigated. In this study, we aimed to measure pathologist performance in detecting mitotic figures (MFs) using multiple platforms (multiple scanners) and compare the results with those obtained using a brightfield microscope. Methods Four slides of canine oral melanoma were prepared and digitized using 4 WSI scanners. In these slides, 40 regions of interest (ROIs) were demarcated, and five observers identified the MFs using different viewing modes: microscopy and WSI. We evaluated the inter- and intra-observer agreements between modes with Cohen’s Kappa and determined “true” MFs with a consensus panel. We then assessed the accuracy (agreement with truth) using the average of sensitivity and specificity. Results In the 40 ROIs, 155 candidate MFs were detected by five pathologists; 74 of them were determined to be true MFs. Inter- and intra-observer agreement was mostly “substantial” or greater (Kappa = 0.594–0.939). Accuracy was between 0.632 and 0.843 across all readers and modes. After averaging over readers for each modality, we found that mitosis detection accuracy for 3 of the 4 WSI scanners was significantly less than that of the microscope ( p = 0.002, 0.012, and 0.001). Conclusions This study is the first to compare WSIs and microscopy in detecting MFs at the level of individual cells. Our results suggest that WSI can be used for mitotic cell detection and offers similar reproducibility to the microscope, with slightly less accuracy.

Mitotic Figures—Normal, Atypical, and Imposters: A Guide to Identification

“…3,32,53 Grade assignments via LM or WSI has not yielded consistent results. 13,32,54 One study found that the modality used (LM vs WSI) did not have significant effects on tumor grading derived from the MC for breast cancer in humans, 3 while another study found that the grades obtained by using MC derived from WSI versus LM were different for sarcomas and neuroendocrine tumors in humans. 32 Grading canine cutaneous MCT with a 2-tier system using WSI was inferior as compared to LM; however, pathologists lacked long-term experience with WSI.…”

“…Comparisons between WSI and LM in terms of identifying individual structures (including MF) has revealed conflicting results. 54,61 Comparing the ability to identify or classify individual structures as MF or MLF with LM and WSI is technically challenging. 54,61 Tabata et al 54 used a light microscope mounted with a camera and specific software to compare LM to WSI from 4 different scanners.…”

Counting mitoses: SI(ze) matters!

“…It still takes no account of cell size, the presence of intratumoral stroma separating tumor cells which impacts tumor cell density, the number of cells present in the defined area, inter-observer variation in the assessment of what is recognized by an individual pathologist as a mitotic figure, or the thickness of the section. These issues have surfaced recently as artificial intelligence (AI) tools begin to be used to count mitoses in digital whole slide images [16][17][18].…”

“…However, digital pathology systems do have the capacity to measure areas with considerable accuracy, without the requirement for calculation. For those using such systems, automated counting of cells identified by immunohistochemistry, or mitoses identified by deep learning methods, it is already possible to express these values per mm 2 [16][17][18]34]. This makes it even more important to avoid the use of non-standardized units and to switch to mitotic counts per mm 2 .…”

“…Planimetric methods have been used for many years in research, but simple measurements of tumor depth and distance from margins of excision are now performed on digital images providing more accurate results. More sophisticated computational methods can also be applied to assess mitotic counts and the evidence is that these are as good or better than direct observation using the microscope, and are improving with time [17,32,[35][36][37]. Comparative studies remain important to avoid potential grade migration [31].…”